CROSSLINKING PROCESS
WIPO Patent Application WO/
A process for crosslinking a poly(alkylene carbonate) (PAC) in the presence of a metal ion, said process comprising: a) adding a crosslinking agent to said poly(alkylene carbonate) and b) crosslinking the resulting product of step a); so as to form a crosslinked PAC in which the crosslinks formed involve the complexation of the metal ion to said PAC and said crosslinking agent.
Inventors:
SOLER, Carlos Barreto (Bakkevegen 25, Porsgrunn, N-3940, NO)
FREDRIKSEN, Siw Bodil (Tyrisvingen 2, Skien, N-3744, NO)
Application Number:
Publication Date:
07/17/2014
Filing Date:
01/10/2014
Export Citation:
NORNER IP AS (Asdalstrand 291, Stathelle, N-3960, NO)
International Classes:
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DATABASE WPI Week 200872 Thomson Scientific, London, GB; AN
XP, & CN 101 104 681 A (HENAN TIANGUAN GROUP CO LTD) 16 January -01-16)
Attorney, Agent or Firm:
CAMPBELL, Neil (Dehns, St Bride's House10 Salisbury Square, London EC4Y 8JD, GB)
1 . A process for crosslinking a poly(alkylene carbonate) (PAC) in the presence of a metal ion, said process comprising:
a) adding a crosslinking agent to said polyf alkylene carbonate) and b) crosslinking the resulting product of step a);
so as to form a crossl inked PAC in which the crosslinks formed involve the complexation of the metal ion to said PAC and said crosslinking agent.
2. A process for crosslinking a polyfalkylene carbonate) in the presence of a metal ion, said process comprising:
a) adding a crosslinking agent to said poly(alkyiene carbonate) and b) crosslinking the resulting product of step a);
so as to form a crosslinkcd PAC in which the crosslinks formed do not involve covalent bonds between the crosslinking agent and the polyi alkylene carbonate).
3. A process as claimed in claim I or 2, wherein said crosslinked PAC comprises at least 0.001 wt% of metal ions, preferably at least 0.01 wt% of metal ions, such as at least 0. 1 wt% metal ions.
4. A process as claimed in claim I to 3, wherein the metal ions are Zn, Co, Mn, Mg, Al, Zr or Ca ions, preferably Zn, Mg or Zr.
5. A process as claimed in any of claims 1 to 4, wherein the PAC is polypropylene carbonate (PPC), polyethylene carbonate (PEC), polycyclohehexene- propylene carbonate ( PCHC-PPC), ethylene carbonate (PEC), polycyclohehexene- ethylene carbonate (PCHC-PEC), polycyclohexene carbonate ( PCHC) or a mixture of two or more of these PAC's.
6. A process as claimed in any of claims 1 to 5, wherein the PAC is produced in the polymerization of CO & and one or more epoxides selected from the group of ethylene oxide, propylene oxide and cyclohexene oxide.
7. A process as claimed in any of claims I to 6, wherein the crosslinking agent comprises a multifunctional acid or anhydride such as maleic anhydride, glutaric acid, adipic acid, oxalic acid, citric acid, glutamic acid, lactic acid, maleic acid, fumaric acid, acrylic acid, methacrylic acid, pyromellitic anhydride, trimellitic anhydride, mellitic anhydride, benzene tricarboxylic acid
or the crosslinking agent is a multifunctional polymeric acid or anhydride such as copolymers comprising maleic anhydride, maleic acid, fumaric acid, acrylic acid, methacrylic acid.
8. A process as claimed in any of claims I to 6, wherein the crosslinking agent is po iyicthyl e ne-c o- ac ry I i c acid), poly( maleic acid-co-acryi ic acid), poly-acrylic acid, polymethacrylic acid, copolymers of acryiic and methacrylic acid with allyl ( meth)acrylatc, crotonic acid, ( meth )acrylamide, styrene sulfonic acid, ethylene, isobutylene.
9. A process as claimed in claim 8 wherein the crosslinking agent is maleic anhydride, maleic acid, acrylic acid, methacrylic acid or mixtures thereof.
10. A process as claimed in claim 9, wherein the crosslinking agent is maleic anhydride, acrylic acid or mixtures thereof.
1 1 . A process as claimed in any of claims I to 10, w herein the crosslinking step occurs by a thermal or UV method or involves the presence of a peroxide.
12. A process as claimed in any preceding claim for crosslinking a polyf alkylene carbonate) in the presence of metal ions, said process comprising the steps of (i) adding a crossl inking agent to the polyf alkylene carbonate) in the pr (ii) heating t
(ii) crosslinking the polyf alkylene carbonate); and
(iv) swell ing the crossl inked polyf alkylene carbonate).
13. Crosslinked PAC obtained by a process as claimed in any of claims I to 12..
14. A crosslinked PAC as claimed in claim 13 having a thermal decomposition temperature higher than the temperature of the corresponding non crosslinked PAC.
15. A crosslinked PAC as claimed in claim 13 or 14, wherein the gel content of said PAC is at least 1%, e.g. at least 40%.
16. A crosslinked polyl(alkylene carbonate), preferably as claimed in claim 13, swollen in a solvent e.g. carbon dioxide in supercritical state.
17. A foamed article or aerogel comprising 20wt % or higher content of a foamed crosslinked PAC, preferably as claimed in claim 13, using for the foaming process supercritical carbon dioxide.
18. An article comprising the crosslinked polyf alkylene carbonate) as claimed in any of claims 13 to 17.
Description:
Crosslinking Process
Field of the Invention The present invention relates to processes for crosslinking poly(alkylene carbonates) (PAC). In particular, the invention relates to the use of an external cross-linking agent for cross-linking. The invention also relates to the crosslinked polyf alkylene carbonates) produced by such processes and to use thereof in various applications, e.g. in the manufacture of articles. In particular, the invention relates to cross-linked polyf alkylene carbonates) with measurable gel content and articles made therefrom.
Background Until recently, polycarbonates have had l imited commercial application. They have been used as sacrifice polymers in the electronics industry but in few other applications. Other applications of these polymers have been limited by their relative thermal instability and mechanical properties.
The present inventors have realised that these polyf alkylene carbonates) (PACs) offer environmentally friendly potential. The use of carbon dioxide in the formation of polyf alkylene carbonates) provides a useful sink for carbon dioxide and therefore these polymers offer an environmentally friendly alternative to fossil fuel based materials such as a poiyolefin. There are therefore significant benefits to using PACs industrially.
As noted above, commercial applications of some polyf alkylene carbonates) are limited by their low Tg. Furthermore, onset of thermal decomposition of these polymers occurs at rather low temperatures, e.g. at 150 - 180°C for polyf propylene carbonate) (PPC). These two properties severely limit the processability of PACs on a commercial scale. Methods for improving the properties of PACS so as to enhance the applicability of PACs are therefore sought. Crosslinking the PAC is one route which has been investigated and, to date, some progress has been achieved. Tao et al. reported in J. Polym. Sci. Part A: Polym Chem. 44, - 5336, that crosslinked PAC's (specifically poly(propylene carbonate)) can be prepared by incorporating small amounts of epoxides bearing unsaturations, such as allyl glycidyl ether, into the polymer chain. The inclusion of unsaturated anhydrides is an alternative method which was reported by Song et al. in J. Appl. Polym. Sci., , . In these methods, the inclusion of the additional type of monomer was found to increase thermal stability and improve mechanical properties. These methods therefore use a reactive monomer to provide a cross- linkable moiety pending from or in the polymer backbone itself. The use of diepoxides is described by Cyriac et al in Polym. Chem., 0-956.
An alternative method is described in US 6248860 in which crosslinking occurs via cross-linkable groups on pending substituents on the polymer backbone, or via multifunctional cross-linkable chain end groups.
Although offering improv ements in processability, some of these methods inv olv e harsh reaction conditions which hav e led to poor quality polymeric products. Furthermore, the inclusion of various types of unsaturated comonomers inev itably leads to an alteration in the core structure of the polymer. Additional steps to carry out chain end functionalisation of the aliphatic polycarbonate further adds complexity and costs to the polymer manufacturing. Such modifications may cause difficulties in finding effective catalysts systems suitable for the polymerisation steps. These additional challenges reduce the potential for effective scale-up of these processes.
There remains, therefore, a need for new methods to produce significant quantities of crosslinked PAC's, which do not possess these drawbacks. In particular, new processes which possess improvements in cost and efficiency arc desired. Processes which may enable production of PAC's with higher glass transition temperature (Tg) values and/or enhanced thermal stability over those known in the art are needed. These processes preferably avoid the inclusion of reactive monomers in the polymer backbone or in the polymer side chains.
Moreover, the inv ention preferably avoids the use of reactive end capping groups.
The present inventors have surprisingly established that effective
crosslinking of PAC's can be achieved by the use of an external cross-linking agent to a polvlalkylcnc carbonate) and crosslinking the resulting mixture. It is envisaged that the crosslinking agent forms, together with metal ions, a species capable of crosslinking the PAC chains. The resulting materials exhibit beneficial properties, in particular in terms of rigidity and thermal stability. Moreover, the cross-linked PACs of the invention may be insoluble in traditional PAC solvents such as dichloromethane and tetrahydrofuran (DCM, THF). In fact, it has been observed that in some cases the PACs can swell in the presence of these "solvents" to provide a material akin to a gel.
Moreover, in contrast to other heavily cross I inked PACs which are thermosetting, the cross-linked materials of the present invention can be thermoplastic.
The gel formation and swelling ability of the PACs of the invention opens new fields for application as, for example, gels for controlled release of substances or for acoustic and thermal insulation, or as gel polymer electrolyte for lithium-ion batteries. Gels may also be used in combination with liquids or gases for example to affect the system viscosity.
Summary of the invention Thus, viewed from a first aspect the invention provides a process for crosslinking a polyf alkylene carbonate) in the presence of a metal ion, said process comprising:
a) adding a crosslinking agent to said polyialkylene carbonate) and b) crosslinking the resulting product of step a);
so as to form a cross! inked PAC in which the crosslinks formed involve the complexation of the metal ion to said PAC and said crosslinking agent.
Viewed from another aspect the invention provides a process for crosslinking a polyialkylene carbonate) in the presence of a metal ion, said process comprising: a) adding a crosslinking agent to said polyialkylene carbonate) and b) crosslinking the resulting product of step a); so as to form a cross! inked PAC in which the crosslinks formed do not involve covalent bonds between the crosslinking agent and the polyialkylene carbonate).
Viewed from another aspect the invention prov ides a process for crosslinking a polyi alkylene carbonate) in the presence of a metal ion, said process comprising: a) adding a crosslinking agent to said polyialkylene carbonate) and b) crosslinking the resulting product of step a);
so as to form a crossl inked PAC in w hich the crosslinks formed w ould not be formed in t he absence of a metal ion.
Optionally and preferably, the polyi alkylene carbonate) comprises at least 0.001 wt% of metal ions.
Viewed from another aspect the inv ention provides a process for crosslinking a polyf alkylene carbonate) in the presence of metal ions, said process comprising the steps of
(i) adding a crosslinking agent to a polyi alkylene carbonate);
(ii) heating the resulting mixture: and
(ii) crosslinking the polyf alkylene carbonate).
Viewed from another aspect the inv ention provides a process for crosslinking a polyf alkylene carbonate) in the presence of metal ions, said process comprising the steps of
(i) adding a crosslinking agent to a polyf alkylene carbonate);
(ii) heating t and
(ii) crosslinking the polyfalkylene carbonate) in the presence of an initiator.
Viewed from another aspect the inv ention prov ides a process for crosslinking a polyfalkylene carbonate) in the presence of metal ions, said process comprising the steps of
(i) adding a crossl inking agent to the polyf alkylene carbonate);
(ii) heating t
(ii) crosslinking the polyf alkylene carbonate); and
(iv) swelling the crosslinked polyf alkylene carbonate).
Viewed from another aspect, the inv ention provides the crosslinked polyfalkylene carbonate) or swelled crosslinked polyf alkylene carbonate) obtained by or obtainable by the process as hereinbefore described. Optionally and preferably, the crosslinkcd poiyi alkyicnc carbonate) has a gel content of at least 1% or higher such as at least 40%.
Articles made from the optionally swelled crosslinkcd poiyialkyicnc carbonate) as hereinbefore described are also provided by the invention.
Detailed Description of the Invention
Poly(alkylene carbonate) The processes of the invention involve the crossiinking of a poiyialkyicnc carbonate) (PAC). The term polyf alkylene carbonate) is used to indicate that the polycarbonates of this invention are free of aromatic groups in the main backbone of the polymer. They can however, contain cyclic, non aromatic groups in the backbone. These cyclic groups can be saturated or unsaturated. The poiy(alkylene carbonates) of the invention are not therefore based on bisphenol-A type products. The PAC's are otherwise broadly defined.
The backbone of the PACs of the invention contains 0-C( =())-() linkages along with a non aromatic l inker between those linkages.
The backbone of the polymer can however, carry a w ide variety of substituents (side chains) including aromatic side groups and various functional side groups.
The PAC is preferably one formed from the polymerisation of carbon dioxide with a cyclic ether or perhaps from the ring opening of a cyclic carbonate. The term cyclic ether is used here to cover not only epoxides ( 3-membered cyclic ethers) but also larger cyclic ethers such as those based on 4-6 membcred rings or more. Preferably, the cycl ic ether is an epoxide such as alkylenc based epoxide.
For example, the reaction of the four membcred ring ether oxirane w ith carbon dioxide gives polytrimethylene carbonate ( Darensbourg, D. J. Inorg. Chem. 765-10780).
Suitable (non epoxy) cyclic ether monomers are therefore of formula (II)
where a is 0-2 and R5 is the same as Ri below. The number of R5 groups which may be present may be the same as the number of carbon atoms in the ring of the cyclic ether (e.g. up to 5). Preferably however, only 1 such group is present, if at all.
Alternatively, PACs can be formed during the ring opening of a cyclic carbonate w ith a variety of catalysts as described in e.g. Suriano Polym. Chem., 8-533; Endo et al. Journal of Polymer Science Part A: Polymer
Chemistry ), .
As long as the backbone of the PAC does not contain an aromatic group within the backbone then any method can be used to form the PACs of the invention.
It is most preferred however if the PACs are obtained through the polymerisation of a cyclic ether w ith carbon dioxide and especially through the polymerisation of carbon dioxide and an epoxide. Preferably, the epoxide of use in the invent ion is of formula (I)
wherein Ri to R4 are each in C1-10 alkyl optionally interrupted by one or more hctcroatoms selected from O or N; C.& 10-alkenyl optionally interrupted by one or more hctcroatoms selected from O and N; C6-10- or
R ; and R3 taken together can form a non aromatic, cyclic group having 4 to 8 atoms in the ring, said ring optionally comprising one or more hctcroatoms selected from O or N; said non aromatic cyclic group or any of Ri to R.\ being optionally substituted by one or more alkyl groups, C 2-10-alkenyl groups, C6- 10-aryl groups, -OC1-6 alkyl groups or OH.
It is preferred if at least one, preferably at least two of R i to R4 are hydrogen. Ideally, the carbon atoms attached to the epoxide should also be bonded directly to a hydrogen atom. In a highly preferred embodiment, three of Ri to R4 are hydrogen, and one is an alkyl group, preferably a methyl group, thus forming propylene oxide, or all 4 are hydrogen (thus forming ethylene oxide).
When not hydrogen, it is preferred if substitucnts R 1 to R4 arc Ci-6-aikyl or C2-6 -alkenyl groups, I f an alkenyl group is present, the double bond should preferably not conjugate the epoxide. Any alkenyl group should preferably contain at least 3 carbon atoms and the double bond should be at least beta to the epoxide carbon. Alternatively, it is also preferred that one of the substituents is an alkoxy group (thus forming for example glycidyl ethers).
If R2 and R3 arc taken together, they preferably form a 5 or 6 membercd ring with the carbon atoms to which they are attached, especially a carbocyclic ring. That ring can be saturated or monounsaturated, preferably saturated. A 6-membered ring is in particular preferred.
In formula (I), it is preferred if no heteroatoms other than the O of the epoxide are present. It is also preferred if compounds of formula (I) arc free of alkenyl groups. It is generally preferred therefore if the epoxide is free from any groups which could partake in a cross-linking reaction.
A preferred monomer is therefore of formula (II)
where Rr and R v arc independently hydrogen, C I -6 alkyl, phenyl or Rr R V taken together form a 5 or 6 saturated or monounsaturated carbocyclic ring, preferably where one or both of Rr and Rv arc hydrogen. Rr is hydrogen and Ry methyl. Preferred epoxide monomers include limoncne oxide, styrcne oxide, propylene oxide, ethylene oxide or cyclohexene oxide, i.e. the compound
The use of propylene oxide is especially preferred.
It will be appreciated that the polymerisation reaction takes place in the presence of carbon dioxide. In an ideal scenario, the PAC of the invention is one such as
where n is 0 to 4 and R is a side chain such as defined above for R, to R4.
the invention may be:
It will be appreciated, however, that when the epoxide and the carbon dioxide are polymerised, the structure of the polymer which forms may not be a perfectly alternating ABABA type polymer as depicted here. The invention encompasses the polymer which forms when these two monomers are polymerised. The polymer regioregularity may be described by the "head to tail" ratio as used in the conventional sense for polyalkylene carbonates and determined as described e.g. in Lcdnor et al. J. Chem. Soc. Chem. Commun. 9.
Further, the polymer chains may also include blocks of e.g. epoxide monomer residues as is well known. It is very common for ether linkages to be prcscnt in PACs. It is preferred if the content of polymer chains containing ether linkages is less than 15 wt%, preferably less than 10 wt%. The ether content can be determined by 1 H NMR e. g. as described in Luinstra, G. Polymer Reviews, 48: 192- 219, 2008 .
It is of course possible for a mixture of cycl ic ether monomers to be used to produce the polyf alkylene carbonate) used in the invention, especially ones free of any groups that could partake in a cross-linking reaction (such as alkenyl groups).
It is also possible for other monomers to be present in the PACs of the invention. For example, a di functional or poly functional epoxide can be present during the polymerisation reaction in addition to the monomers described above. Typically this will be added in small amounts, e.g. less than 1 wt% of the reaction mixture as a whole, preferably less than 0.1 wt% of the reaction mixture.
It is also within the scope of the invention for other monomers to be used in the manufacture of the PAC. For example, the use of lactone monomers is envisaged. Lactone monomers of interest include β-propiolactone, γ-butyrolactone, δ-vaierolactone, ε-caprolactone. The use of lactones typically results in the formation of block polymers (see Lluiscr et at Macromolecules, ), pp 1 132 I 139, Lu and Huang, Journal of Polymer Science Part A: Polymer Chemistry, Volume 43 (12) , 2005; Hwang et al. Macromolecules 10- 8212).
Other alternative monomers include anhydrides such as maleic anhydride. Some monomers can therefore provide cross-linkable units into the backbone/side chains of the polymer. In addition the side chains of the PAC can contain groups such as alkenyl groups w hich can be crosslinked.
It is highly preferred how ever if the PACs of use in the invention are free of groups w hich can be used in a conventional crosslinking reaction such as vinyl groups or epoxide groups. It is an important feature of the invention that the crosslinking is effected by the addition of a separate crosslinking agent that can connect the ligands that coordinate with a metal ion present in the PAC.
As noted above, PACs can have varying degrees of regularity, in particular depending on the nature of the catalyst used to manufacture the polymer. In some embodiments on the invention the PACs may be highly regioregular. Where an epoxide monomer is not symmetrical, it is possible for the addition of each monomer to proceed in differing fashion. The invention covers all different regioregularities.
Other PACs of interest are formed in the presence of chain transfer agents. When chain transfer agents are present in the polymerization or added durin or when coupling agents arc added as terminating agents or in a post reactor stage, these substances can give raise to special polymer architectures.
Chain transfer agents of interest include water and di or poly functional molecules like alcohols, amines, thiols, carboxylic acids, sulphonic acids, phophinic acids and other chain transfer agents that are well known in the art The structures can have two or more branches depending on the functionality of the chain transfer coupling terminating agent. The PAC may for example have the form PAC-O- PAC. PAC-O-R-O-PAC. PAC-S-R-S-PAC, HN-R-NH-PAC.
These materials are not considered crosslinked as required by the present invention as the linkages formed are covalent. It is a feature of the invention that the crosslinking reaction envisaged by the claimed process does not involve a significant amount of an actual covalent linkage between the PAC chains and the crosslinking agent.
It is preferred if maleic anhydride is not a comonomer used in the formation of the PACs of the invention. The invention preferably does not involve the terpolymerisation of epoxide, carbon dioxide and maleic anhydride. Whilst maleic anhydride is a possible cross-linking agent, this is preferably not added as a monomer during the polymerisation. Rather it can be added once the PAC has been formed.
Preferably however, the PAC is formed from the polymerisation of carbon dioxide and epoxide(s) of formula (I) only. In particular, the PAC is polypropylene carbonate (PPC), polyethylene carbonate (PEC), polycyclohehexene-propylene carbonate ( PCHC-PPC), ethylene carbonate (PEC) or polycyclohexene carbonate ( PCTiC), most especially polypropylene carbonate. Preferably, the PAC can also be a polymer formed by the polymerisation of CO? and at least two different epoxide monomers carrying no substituents or only alkyl substituents, in particular by the polymerisation of CCK and two or more of ethylene oxide, propylene oxide and cyclohexene oxide.
Several catalyst systems are known that catalyze the copolymerisation reaction of epoxides and C02. The polymerisation can be catalysed by known catalysts, especially Zn based catalysts, Mg based catalysts or Co based catalysts such as cobalt salen catalysts. The use of zinc and magnesium catalysis, e.g.
heterogeneous or homogeneous mono- or multinuclear Zn catalysis is preferred. Most preferably these carboxylates are zinc g!utarates, e. g. as described in
US4789727 and in Ree et al. J. Pol. Sci. Part A. : Polymer Chemistry Vol. 37,
(1999) or other zinc based catalysts e. g. such as macrocycl ic zinc complexes as described in WO. Magnesium catalysts are typically macrocyclic magnesium catalysts such as described in WO. Cobalt based catalysts are typically cobalt salen catalysts as described in WO and in Cyriac et al. Macromol. -7801. Catalysts may need a cocatalyst as is well known in the art, e. g. as described in WO.
Other well known catalysts for PAC formation are based on homogeneous reaction systems and include porphyrin systems such as DM A P. The use of phenoxide catalysts is also a possibility as well as the use of β-diiminate catalysts. As noted above, Co based salen systems are also of interest. A comprehensive discussion of available catalysts can be found in Coord Chem Rev (2011), Klaus et al. and in Kcmber et al. Chem. Commun. 1-163. The skilled man is capable of choosing an appropriate catalyst. The catalyst will preferably contain a metal.
Catalysts may need a cocatalyst as is well known in the art.
The procedures required to polymerise the monomers to form PACs are well known and arc described in the literature. PACs are also commercially available products e.g. from Empower Materials.
The PACs of the invention can be amorphous or semicrystalline. Typically they are amorphous. Preferably they will hav e a glass transition temperature (Tg) of at least 0°C, preferably at least 10 °C, such as at least 15°C, such as at least 20°C. It will be appreciated that the Tg will depend heavily on the nature of the PAC in question. The number average molecular weight Mn of the PAC may be at least 1500 g/mol, preferably at least 2000 g/mol. The use of higher Mn PACs is preferred in this invention. Values of at least 10,000, preferably at least 20,000 are therefore preferred. It is envisaged that by using a higher Mn, this may enhance gel content during the cross-linking react ion. Mn values of higher than 50,000, preferably at least 75,000 g/mol are also favoured. Mn can be measured by GPC.
The Mw Mn of the PAC is preferably at least 1.1, such as at least 2, preferably at least 3.
The skilled man will appreciate that PACs can be end capped. That means that a different group (i.e. one not formed during a polymerisation reaction) can be attached to the end of the polymer chain, for example an ester group. The presence of reactive end-capping groups (i.e. those capable of undergoing a cross-linking reaction such as those containing a vinyl link.) could increase the yield of gel discussed below. The invention therefore encompasses PACs that are end-capped with reactive groups or which are not end capped at all or are end capped with non reactive groups.
It is preferred if the PACs of the invention arc not capped with a group that can undergo a cross-linking reaction. Some commercial PACs may already carry end groups such as esters but these are not capable of undergoing a cross-linking reaction. Non reactive caps like glutarate, adipate and acetate are not believed to have negative effects on the cross-linking reaction and hence in the formation of gels.
The end group of a PAC polymer may be reactive and it is known to cap some PACs with maleic anhydride, for instance. Ideal ly, the PAC of the present invention will not be end capped with maleic anhydride.
In particular, the cross-linking reaction described in detail below can be effected in absence of reactive end groups like unsaturated groups ( maleate or fumarate), isocyanides etc.
It will be appreciated that the formation of the PAC may give rise to a well known cycl ic carbonate impurity. For example, during the formation of polypropylene carbonate, propylene carbonate may be formed as a by-product. That is the compound
It is common to try to remove this by-product but the present inventors have realised that it acts as a plasticiser and can offer some beneficial properties. It may therefore be necessary to reduce the content of this impurity but not necessarily remove it completely. Preferably, the amount of carbonate impurity in the PAC of the inv ention (i.e. relative to the weight of the PAC ) is less than 10 wt%, preferably less than 7 wt%, e.g. 6 wt% or less. In some embodiments, its content can be reduced to less than 2 wt%, especially less than 1 wt%. In many polymers the amounts of carbonate impurity are too low to be detected. It is an option however, if there is at least I wt% of the carbonate impurity in the PAC, e.g. at least 2 wt%. The carbonate impurity is preferably propylene carbonate but obviously the nature of the impurity depends on the nature of the PAC being formed.
PACs suitable for use in the inv ention can be purchased commercially, e.g. under the trade name QPAC.
It is not uncommon that residues of other small molecules are also present in the PAC used in the inventions, for example epoxide monomer residues or solvent residues. Typically, such volatiles are present in concentrations lower than 1 wt% percent. Metal ions
As noted in detail below, it is believed that the presence of metal ions in the reaction enables the cross-linking reaction of this invention to take place. The metal ion is crucial to the formation of the actual crosslinking species. In a highly preferred embodiment, the PAC's used in the invention preferably comprise at least 0.001 wt%, e.g. at least 0.005 wt%, preferably at least 0.01 wt% of metal ions. This figure is based on the content of metal ions in the PAC/metal compound mixture. Ideally the crossiinked PAC comprises 0.0001 wt%, e.g. at least 0.0005 wt%, preferably at least 0.001 wt% of metal ions. Values of 0.01 wt% or more arc also envisaged. In other embodiments, the level of metal ions can be even higher such as 0.05wt% or more, e.g. 0.1 wt% or more, even 0.5 wt% or more.
The metal ions are present as part of a compound. In theory, the PAC's could comprise up to 40 wt% of metal compounds should a PAC be mixed with extraordinary amounts of metal compounds. Typically, however the PAC should contain less than 5 wt%, such as less than 3 wt%, e.g. less than 2.5 wt% such 2 wt% or less of the metal compounds (based on the amount of metal compounds and PAC in total). The minimum amount of metal compound is preferably 0.001 wt%, e.g. at least 0.005 wt%, preferably at least 0.01 wt% of metal compound. This amount is based on the metal compound in question not simply the metal ion itself.
The metal ions may originate from residual catalyst present in the PAC following its preparation, or from metal compounds added after polymerisation is complete.
Alternatively viewed the metal ion content of the PAC metal ion blend is preferably at least 50 ppm, preferably at least 100 ppm, e.g. at least 500 ppm, especially at least 900 ppm. In most embodiments of the invention the metal ion content of the PAC will be at least 1500 ppm. In particular, the crosslinkcd PAC preferably contains at least 50 ppm, preferably at least 100 ppm, e.g. at least 500 ppm, especially at least 900 ppm of metal ions. In most embodiments of the invention the metal ion content of the PAC will be at least 1500 ppm.
It will be appreciated that it is generally accepted practice to remove as much metal ions as possible after PAC synthesis. These are generally considered an impurity. There are good reasons for the metal ion removal. In some cases active catalyst residues are detrimental to thermal stability. In some cases the catalyst residues may catalyze the degradation of the PAC via a chain unzipping mechanism.
In PACs prepared using cobalt catalysts, the catalyst typically must be removed if a colourless product is wanted. It is therefore conventional to remove catalyst residues. However, it may be useful in the present invention to leave those residues in the PAC or control their removal such that some residues still remain. Many commercially available PACs arc heavily treated to ensure metal residue removal however, and it may therefore be necessary to add metal residues to such polymers to carry out the preferred embodiment of this invention.
When added separately, the metal may be in any form, e.g. any metal salt such as a metal chloride, fluoride, bromide iodide, nitrate, sulphide, sulphate, carbonate, silicate, aiuminosilicate, phosphate, borate, selenide, oxide or telluride. in particular, the oxide of the metal can be used or especially an organometallic compound containing the metal ion such as a carboxylate, e.g. glut a rate. It is also possible to use a mixture of two or more metal compounds.
It is envisaged that the metal ion becomes non covalently associated with the PAC polymer chain. It is ideal therefore if the metal ion is added in the form of a complex thereof with a ligand. It may also be that the ligand makes non covalent interactions with the PAC.
It is particularly preferred if the metal compound is a carboxylate such as a monocarboxylate, dicarboxylate, tricarboxylate, or any polyfunctionai carboxylate. In general, a ligand which is capable of making complexes with the metal and which is capable of making of secondary interactions ( hydrogen bonding, etc) with the PAC is envisaged here. In a further embodiment, metal ions might be added as part of the crosslinking agent, e.g. as a salt of the crosslinking agent. This is an especially preferred embodiment.
in all cases, the metal residues are preferably transition metal or group 11 to IV metal residues.
Preferable transition metal residues include those of the first row in the Periodic Table, such as Zn, Mn, Ti, Cu, Fe or Co. Preferred Group 11 to IV metals include Zn, Ca, Mn, Ti, Mg, Zr and Al. Other interesting metals arc zirconium, cadmium, silver, gold, platinum, iron and palladium. M ixtures of metal ions can of course be employed, in a particularly preferred embodiment, the metal residue is a Zn, Zr, Ca, Mn, Mg, A I or Co residue, especial ly Zn, Mg and Zr.
The metal ion can be added to the PAC if the concentration of metal ion is lower than the required loading to achieve crosslinking. The metal added could be di fferent than the metal in the catalyst used for PAC formation, e.g. adding Zr (which is not a catalyst residue) is an excellent route for crosslinking. Most especially, the residues derive from catalyst which remains in the PAC after its synthesis, most especially Zn or Mg residues which are present. The valency of the metal is preferably its most stable valency. This should be at least 2+.
It is envisaged that after a fairly typically polymerisation process using a Zn gluturate catalyst, the Zn gluturate content is around 1.0 wt% in the crude polymer product. The level of residue required in a PAC to ensure cross- 1 inking varies depending on the nature of the metal residues and the type of ligand around it.
Preferably metal ion levels should be at least 500 ppm, preferably at least 1000 ppm. It is envisaged that after a fairly typical polymerisation process using a dizinc or dimagnesium macrocyclic ligand like in Kember 2009, the zinc Mg catalyst content is less than 1,0 wt %.
Without wishing to be limited by theory, it has been found that the presence of metal ions in the PAC enables a successful cross-l inking reaction to take place. For example, it might be that the PAC, metal ions and cross-linking agent interact in some fashion, e.g. v ia complexation of the metal ion by the cross-linking agent and formation of coordination linkages between the metal ion and the carbonyl groups in the PAC. In the absence of metal ions, cross-linking is much less successful.
The PAC's used in the processed of the invention preferably comprise 0.01 to 3 wt% of metal ions, preferably 0.05 to 2.5 wt%, more preferably 0.1 to 2.0 wt% before contact w ith the cross-linking agent.
Cross-linking agent
The cross-l inking reaction in the present invention involves the addition of a separate cross-linking reagent. The cross-linking cannot simply be effected by internally reacting side groups on the PAC polymer or using reactive end groups rather the invention requires the use of an external crossl inking agent. It will be appreciated however that the crosslinking reaction may rely upon the presence of components such as metal ions present in the PAC before crosslinker addition.
Any crossl inking agent is suitable for use in the processes of the invention. Typically, it will be an organic cross-linker. The crosslinker preferably contains groups that will coordinate to metal ions to form complexes such as carboxylic acids, amines or hydroxy! groups. The crosslinking agent is preferably a low molecular weight organic compound comprising at least one, preferably at least two, carboxylic acid, amine or hydroxy! groups and hav ing a molecular weight of less than 200 g/niol.
Cross-linking agents of use in the invention are preferably multifunctional, such as bifunctional. Preferably, the crosslinking agents useable in the invention comprise at least one double bond and at least two acid moieties, e.g. acids. Suitable crosslinking agents include polycarboxyiic compounds I i ke anhydrides (which term includes dianhydrides like pyromellitic dianhydride), saturated and unsaturated carboxylic acids compounds, polyacids, unsaturated acryiates and polyacryiic acids, polyurethanes and mixtures with coupling agents like isocyanides, acyl ha I ides and mixtures thereof.
Crosslinking agents can be considered "low" coordinating or "high" coordinating. Low coordinating agents are capable of coordinating/establishing linkages with one or just a few metal ions. Examples are maieic anhydride, acrylic acid, glutaric acid, adipic acid, oxalic acid, citric acid, and glutamic acid.
High coordination agents are those capable of coordinating/establishing l inkages with a large number of metal ions. In some cases, this type of ligand is capable of generating gel contents &40%, in the crosslinked PAC. H igh coordination ligands can be produced using low coordinating ligands as building blocks.
Low coordinating agents typically contain fewer than 15 carbon atoms, e.g. fewer than 10 carbon atoms. Crosslinking agents of interest include acryl ic acid, methacry!ic acid, maieic acid, fumaric acid, maieic anhydride, acryl ic acid, methacrylie acid, styrenc sulfonic acid, mono and dial lyl pyromei lytatc (mono and diestcrs of ally! alcohol- pyromellitic dianhydride), pyromellitic anhydride and mixtures thereof. In general, unsaturated organic acids and anhydrides are favoured.
Preferred crosslinking agents arc those containing at least two carboxylic acid groups (e.g. disguised carboxylic acid groups such as in anhydrides) or those containing at least two carboxyl groups and a carbon carbon double bond.
Particularly preferable crosslinking agents are anhydrides, such as maieic anhydride, pyromellitic anhydride, trimell itic anhydride, mcllitic anhydride or acids l ike malcic acid and fumaric acid, acrylic and mctliacryl ic acids, styrcne sulfonic acid, crotonic acid and mixtures thereof.
High coordinating ligands are often polymers such as po!y(ethylene-co- acrylic acid), poly( malcic acid-co-actylic acid), poly-acrylic acid, and
polymethacrylic acid, copolymers of acrylic and mctliacrylic acid with ally! ( meth)aerylatc, ( meth)aerylamidc, styrcne sulfonic acid, ethylene, isobutylene, crotonic acid) and its isomers and so on.
In general, the high coordinating crossl inking agents are di and
polycarboxylic acids and their anhydrides capable to coordinate with two or more metal ions, it is an advantage if the molecules bear a second type of functionality such as an unsaturation, hydroxy!, amine or epoxy functionality in order to increase the connectivity.
High coordinating l igands can be produced in situ by the coupl ing of two or more low coordinating ligands or the bridging of two or more of low coordinating ligands and a bridging molecule (or initiator) as is well known, such as HO-R-OH (diol), HS-R-SH (dithiol), H.,N-R-NH , (diamine), =-R-= (diene)
A single crosslinking agent can be added or a mixture of crosslinking agents can be added. In particular, a mixture of di and po!yfunctional molecules can be added. Where a mixture of crosslinking agents is used it is preferred if one is at least trifunctional.
Preferred crossl inking agents are in situ produced products of the polymerization of two or more of malcic anhydride, ethylene, fumaric acid, acrylic acid and mctliacrylic acid using an activator such as peroxide or UV irradiation. Preferred options also include poly(ethylcne-co-acrylic acid), polyf malcic acid-co- acrylic acid), poly-acrylic acid, polymers from malcic anhydride, homopolymers, copolymers and modified oligomers and polymers of malcic acid, and poly (acrylic acid co-ally I methacrylate).
Other options are styrcne sulfonic acid, crotonic acid, vinyl phosphonic acid. These polymeric ligands can be generated in situ perhaps by polymerisation of the monomers forming the polymeric crosslinking agent. Addition of the monomers necessary to the PAC followed by initiation may allow simultaneous crosslinking agent generation and crosslinking. As noted above, in some embodiments, the crossl inking agent can be added as a salt in order to provide the metal ion necessary for the invention. Suitable salts involve ammonium or the metal ions discussed above.
Addition of the crosslinking agent
The cross-linking agent may be added to the PAC after polymerisation, for example during or after work-up of the polymerisation reaction. It is important to remember that the metal ions in the PAC are involved in the crosslinking reaction. in a further embodiment, it is envisaged that the metal ions required and the crosslinking agent required are added as a single compound such as a salt of a crosslinking agent. Metal ions of interest are those discussed above. Suitable such materials are salts of the carboxylic acids discussed above.
After termination of polymerisation reaction, the crude product typically containing metal ions is preferably milled to form a fine powder. Milling preferably occurs at a low temperature, i.e. less than 0 °C. Particle sizes may be 0,05 mm- 1 cm. The crosslinking agent is preferably added at this stage generated at this stage. Also other methods for generating a fine powder of low particle sizes may be used like pellctisation and for example underwater pelletisation.
The cross-l inking agent can be added in the presence of any suitable solvent, such as organic solvents. Howev er, it is preferred if the cross-linking agent or the reagents for the in-situ generation of the crossl inking agent is present in water w hen contacted with the PAC. This method for adding or generating the crosslinking agent to the poly(aikylene carbonate) offers the advantage that only w ater is added while no organic solvent is used, enhancing the environmental efficiency of the ov erall process of the inv ention. Also, it is believed that in the presence of w ater, the process suppresses any end capping reaction w hich might otherwise occur. Maleic anhydride for example, can react with the end group on the PAC chain to form an ester. In water howev er, ester formation is minimized.
The crossl inking agent can also be added to a PAC with low or almost negligible amount of metal residues (e.g. 4-40 w t ppni ). In this case the metal compounds (i.e. metal oxides or carboxylates) should be added and/or form part of t he crosslinking agent. The incorporation of the crossl inking agent might be done in the solution of a PAC in an organic solvent and then precipitated in an antisolvent, alternatively the polymer solution or dispersion can be dried to recover the solid PAC based compound. The incorporation of the crosslinking agent can also be done by solid mixing and subsequent hot-pressing or preferably by standard melt compounding.
The incorporation of the components of the crosslinking agent in the PAC can also be done in one or more of the stages described in the prev ious paragraphs.
Moreover, if the metal residues are extracted (i.e. after the solid-liquid work up in methanol and other organic polar highly coordinating solvents) no significant amounts of gel are observed. This fact shows the main role that the metal residues play in the invention described here. If the cross-linking reaction was effected through an end-capping linker, no such decrease in the amount of gel could be observed simply because PAC chains arc insoluble in polar solvents and could not be extracted. The amount of gel formed should be nearly the same after such extraction.
Crosslinking activation by heat The resulting mixture comprising PAC, metal ions and cross-linking agent can then be heated, e.g. to more than 30°C such as e.g. up to 180°C. This may take place in the presence of peroxides.
It is also believ ed that such hot stage may also encourage an interaction between the PAC, the cross-linking agent and metal ions that arc present in the PAC. Without wishing to be bound by theory, it is thought that the organic ligands bind to the metal ions, w hich arc intimately linked to the poiy(alkyiene carbonate) itself, via metal-ion coordination. Also, as described below, it is believed that the activation step allows the formation of higher order ligands i.e. from monomeric carboxylic acids to polymeric ones like polyacrylic acid coordinated to the metal ion
Amount of crosslinking agent The amount of cross-l inking agent added can vary over wide limits.
Typically, there will be more PAC than cross-linking agent in terms of weight. Thus, the weight ratio might be 1 : 1 to 1 :30, such as 1 : 1 to 1 :20, preferably 1 : 1 to 1 : 10, cross-l inking agent to PAC, e.g. 1 : 1.5 to 1 :3. This is determined on the basis of the amount of crosslinking agent added.
The crosslinking agent content of the whole mixture at this stage may be at least 0.01 wt%, such as at least 0.1 wt%, preferably at least 0.5 wt%, preferably 1 to 10 wt%, such as I to 5 wt%. Formation of "high" coordinating ligands from unsaturated "low" coordinating ligands
"High" coordinating ligands can be built from the polymerization / crosslinking of unsaturated organic compounds. As is well known, this type of reaction can be encouraged by the addition of a free radical initiator or, if possible, the reaction can be initiated simply using heat or irradiation. A combination of both of these options is preferred here.
Any initiator can be used in the process of the invention. Initiators based on peroxides or azides are known in the art and can be used for example.
It is preferred if a peroxide initiator is used. A wide variety of peroxides are known for this purpose and the use of any of these is envisaged. The use of benzoyl peroxide is convenient as this compound is readily available commercially. Suitable solvents for the dissolution of any initiator added and any other necessary reaction conditions required to ensure effective crosslinking, such as the use of an inert nitrogen atmosphere and heat to initiate the reaction, will be appreciated by those skilled in the art, and may be used, w here appropriate.
The amount of initiator added can vary over wide limits but if too much is added then the acid initiator residues degrade the polymer and if too little initiator is added no gel formation is observed. Assuming however, an appropriate amount of initiator is added, the results suggest that the concentration of the initiator does not have a significant bearing on the gel formed after cross-linking other than on its hardness and swelling degree. Amounts of at least 0.009 wt% based on the weight of the PAC/crosslinking agent material are effect ive for example.
The initiator can be prov ided in an inert solvent, typically a hydrocarbon solvent such as a C6-10 hydrocarbon solvent.
The polymerization reaction of low coordinating ligands containing double bonds can be initiated by heating, e.g. to above the decomposition temperature of the initiator in question, such as at least 60°C, preferably at least 100°C. The reaction mixture may alternatively be initiated by irradiation. When irradiation is used an additional UV absorber could be added to the PAC to ease the crosslinking process. Once initiated, a crosslinking reaction occurs to form the crosslinked PAC of the invention.
The reactions with peroxides and other initiators leave some residual compounds which in some cases may be undesirable for the properties of the material. Optionally, after cross-linking, the cross-linked polymer can be subjected to various treatments to remove undesirable components.
Properties of the crosslinked PACs
The crosslinked PAC's produced by the process of the invention possess improved thermal stability, enhanced mechanical properties and increased Tg values over conventional non-crosslinkcd materials. In particular, the polymers of the invention can exhibit thermoplastic character despite their crosslinking. Thus the process of the invention gives rise to PACs with broader applicability and wider processing windows.
Viewed from a further aspect the invention provides a thermoplastic cross- linked poiyf alkylenc carbonate).
The thermal stability of the PACs of the invention is important. Thus, the thermal stability of the crosslinked PACs is equal or preferably higher than a PACs worked up after a traditional work up in organic solvents and aqueous HCl or by the removal of catalyst residues by filtratio . Tg values arc also important. Tg values for PACs of the invention equal or preferably higher than a PAC with a similar level of plasticizcr worked up after a traditional work up in organic solvents and aqueous HQ.
The crosslinked PAC produced by the process of the invention preferably has a gel content of at least 1 wt%, preferably at least 20 wt%, such as at least 40 wt%, more preferably at least 50 wt%. The gel content here is a measure of the wt% of the PAC which is insoluble in dichloromethane. Dichloromethane typically dissolves PAC polymers and we show below that it dissolves non cross-linked PPC and PPC which has been contacted with the crosslinking agent but not cross-linked. When the crossl inked polymers are contacted with dichloromethane however, the polymer swells. When the DCM is removed by filtration and drying, the remaining residue is that w hich is insoluble in DCM and represents the gel content of the polymer. This test was carried out at 30°C in the presence of an excess of DCM.
Preferably the cross-linked PAC is (at least partially) insoluble in other organic solvents too. Some crosslinked PACs may swell w hen placed in contact with dichloromethane. These swel lablc PAC will also swell when contacted with other solvents conventionally believed to dissolve PACs. Such solvents include THF, chloroform, ethyl acetate, DMSO, NMP, aceonitrile, diglyme, MEK, acetone and so on. It is therefore preferred if the PAC of the invention are at least partially insoluble in solvents capable of dissolving non cross-linked PACs.
The swelling procedure might also be possible with mixtures of solvents such as mixtures of solvents known to dissolve non cross-linked PACs with other solvents known not to dissolve PACs. Alternatively, missible water/solvent mixtures might be used. Examples are acetone w ater or THF w ater or ethyl acetate/water.
The crosslinked PAC obtained after swelling and removing the "solvent" can be regarded as a gel, especially an aerogel. This is a swollen PAC according to the invention. The gels of the invention, e.g. the aerogels have very low bulk density, such approximately 0,02g /cm3. Typically gel ranges are therefore 0.01 to 0.1 g/cm3. The formation of cross-linked PACs with such low bulk densities is new and these materials form a still yet further aspect of the invention. In a highly preferred embodiment, the gel structure is produced after a solvent extraction in a supercritical fluid, especially supercritical CO?.
It has been observed that the uptake of dichloromethane by the crosslinked PAC increases with higher initiator content. This is important because the swelling represents lower density in the swollen gel/aerogel. It is also provides a higher capacity for absorption where an absorbent material is targeted.
This is a measure therefore of swelling degree. Also, the hardness of the gel which forms increases with increasing initiator content. It is possible therefore to tailor the hardness properties and swelling (solvent uptake) of the polymers of the invention by controlling the initiator content.
The swelling behaviour can also be increased or decreased depending on the type of PAC. amounts and type of crossiinking agent and strength of the interactions metal-ligand-PAC . Applications
The crosslinked PAC's produced by the process of the invention are suitable for use in a variety of applications. They might be used in moulding applications or for film forming or in lamination. In particular, they arc well suited for use in the preparation of articles such as bottles, containers and films. These may offer enhanced resistance towards solvents. Alternatively, they could be utilised in circuit boards as sacrifice polymers. They could also be used as gel polymer electrolyte for lithium-ion batteries.
The swelling properties highlighted above also render them suitable for use in a range of novel applications. As noted above, after contact with some solvents the some crosslinked PACs swell. The solvent could be one of the typical PAC solvents like DCM, acetone, ester and ethers, but also those causing polymer swelling like carbon dioxide in supercritical condition. Once the solvent is removed and the PAC dried, the PAC appears to have formed a type of aerogel. An Aerogel is a low density foam which can be used in applications as mechanic, acoustic or thermal insulator. Thus, viewed from another aspect the invention provides an aerogel comprising a crosslinked PAC. The use of these materials is envisaged as carriers for a compound to be released such as drags, aromas, pesticides and agrochemicals. It is noted that some PACs of the invention have a glass transition temperature close to body temperature. This feature can be used to initiate the release of a drug, perhaps only when it is implanted or during fever episodes. The crosslinked PACs may also find application in the remediation of chemical and oil spills and hence generally as absorbents and adsorbents. Aerogels could also be applied as filtration/adsorption media.
Other applications of interest include the formation of foams, films, coatings and cap liners. The crosslinked materials of the invention might be coated onto substrates such as plastics, metals, textiles, cardboard and paper.
The invention will now be described with reference to the following non l imiting examples and figures.
Figure 1 is a theoretical depiction of the crosslinking process. Zn is coordinated to the PAC after the polymerisation reaction. On addition of the cross- linking agent, this becomes associated with the Zn and upon cross-linking, the polymer chains are crosslinking via reaction of the cross-linker. The links are believed to rely on the non cov alent interaction of the metal ion with the polymer. For this reason, the crosslinked PAC of the invention is believed to be thermoplastic.
Figure 2. Isothermal TGA characterization in nitrogen of crude PPC and its crosslinked modification in example 3.
Analytical methods:
The 1 H NMR analysis of PPC in order to determine ether linkage content has been outlined by Luinstra. G. Luinstra Polymer Reviews, 48: 192 - 219, 2008)
Determination of metal residue content:
PAC sample are calcined to ashes in an oven at 650°C. The ashes are dissolved in acid and the metal quantification is done by atomic absorption. The estimated detection limit depending on the Zn concentration is : 0.25ppm Zn (for low concentrations [Zn]&100ppm), 5.0ppm Zn (for medium concentration
100&[Zn]&2500) and 50 ppm (for high concentration [Zn]&2500 ) wtppm. GPC (Molecular weight and molecular weight distribution, Mw and MWD): The molecular-weight distribution was determined by size-exclusion
chromatography (SEC) in an Agilent. PL-GPC 50 equipped with a refractive index detector and calibrated with narrow polystyrene standards. The determinations were performed in TH F as the eluent at 40 °C. It was used a sample preparation system PL-SP260VS with 1 μ glass fiber filter to remove the particles in suspension from composite samples. TGA (onset of decomposition): TGA was measured on a PerkinEimer TGA analyzer according to ISO 1 1 358. the analyses were run up to 550°C in nitrogen with a heating rate of 20°C/min or alternatively at 5°C/min.
TGA (Isothermal): TGA was measured on a TA TGA analyzer according to ISO 1 1358. The analyses were run during one hour at 200°C in nitrogen. The samples typically reach the target temperature approximately after l Ominiites heating from room temperature
DSC (Tg): DSC was measured on a Netzsch 204-F 1 instrument according to ISO! 1357-2 for determination of glass transition temperature: 1. -10 to 140°C, cooling: 140 to -10°CC, 2. -10 to 140°C. Heating/cooling rate was 20°C/min. The ev aluation of the glass transition temperature is performed at the second heating segment. The sample specimens are pellets. Example 1: Preparation of the Crossttnkable PPC
Polymerisation:
Propylene oxide (332 g, 5.71 mol) and zinc glutarate (1,0 g. 5, 1 mmol) were placed in a 2 dm3 stainless steel autoclave reactor with an impeller. CO2 was added to the reactor and the temperature was raised to 60 °C. The pressure was maintained at 30 bars by replenishing CO2 at regular intervals throughout the reaction time of 40 hrs. The reaction was terminated by venting C(&& and remov ing excess monomer by distillation. The crude polymer was recovered as a solid that contained 1,5 wt % zinc glutarate. The propylene carbonate content was estimated as 4 wt% from FTIR. The product was dissolved in dichloromethane, diluted hydrochloric acid was added, the aqueous phase was removed , and the polymer was precipitated by the addition of methanol. The poiy( propylene carbonate) was dried to yield a white solid that was analysed by GPC, FT-I R, ? NMR. Mw; 443000; Mw/Mn = 4,7; ether linkages: 11 mo! %), TGA (onset)=232 (5°C/min)
8 g of crude PPC prepared as described in the previous paragraph was mil led and transferred to a 250ml jacketed glass reactor which contained 100 ml of an aqueous solution of maleic anhydride 0,4 wt/vol%. The reactor was locked, the mechanical stirring was adjusted to 400 r.p.m and the temperature was set to 60°C for 30 minutes and afterwards increased to 90°C for 3 hours. The aqueous solution was removed and repla this operation was repeated two times in order to remove acid residues. Agglomerates of ~3 mm were obtained. These particles were dried for 48h at 35 °C under N2 flushing. Propylene carbonate was removed.
Example 2: Preparation of Crosslinked PPC
0.3 g of the particles of example 1 were placed in a 100 ml beaker with 3 ml of peroxide solution. The peroxide employed was benzoyl peroxide purchased from Sigma Aldrich. Solutions of peroxide were prepared as 0, 0.3, 0.45 and 0,6 wt/vol% in heptane. Samples with each of the peroxide solutions were prepared. The samples were cured at 110°C for 2 hours and then degassed overnight at 35 °C. Characterisation
The preliminary characterization of the crosslinked PPC is done by swelling trials and determination of gel residues. In the swelling trials, the insolubility of the polymer in DCM is a sign of crosslinking. The formation of well defined gels ( independent of the morphology of the containing recipient ) is a sign of the formation of a chemically crosslinked gel. In the same way, the leaching of the cross-linking products in DCM and quantification of the insoluble residues will give a quantification of the extent of the conversion or gel%.
Swelling trials
In the swelling trials, 20 to 30 mg of cured polymer was placed in a 4 ml v ial with a magnetic stirrer. 3 ml of dichloromethane (DCM) - which is a very good solvent for poiyf alkylene carbonates) was added and the magnetic stirring at 30 °C was maintained for 20 h. Samples subjected to the crosslinking procedure were evaluated together with reference samples of crude polymer, crosslinkabic untreated polymer from example I and polymer heat treated in example 2 but in the absence of the peroxide. The results can be observed in Table 1.
Seconds after the addition of DCM, all the samples which had been crosslinked with peroxide swelled forming large chemical gels floating in the DCM and not sticking to the vial walls. All the reference samples did not form gels and were completely dissolved without forming discrete particles. The formation of chemical gels is proof of the generation of chemical crosslinking.
Gel formation docs not occur in the raw polymer, is not generated after the malcic acid addition and, in this case, is not a result of the thermal program that is implemented in example 2 (in the absence of peroxide). It is purely induced after the thermal treatment in presence of peroxides.
The wet gels showed a DCM uptake increasing from 48 to 64 g DCM /g of gel residue. That figure increased w ith the concentration of peroxide (Table 1). The qualitative hardness of the swollen wet gels was also increased with the concentration of crosslinking agent (Table 1).
Gel Residues
The determination of gel residues was a continuation of the swelling trials. The liquid phase was removed in the samples after 20 h of extraction of soluble PPC in DCM. After that, the samples were dried for 20 h at 35°C and then over two more hours at 90°C in order to remove al l the DCM. Consequently the weight of the polymer residues was determined gravimetrically and the gel% calculated. The results show that approximately 60% of the initial PPC was converted into a gel residue (Table 1). This value was basically insensitive to concentration of peroxide. In the samples corresponding to 0.45 and 0.60% of peroxide, the original morphology of the polymer before the crosslinking was conserved.
The FTIR absorption at 1635 cm-1 shows a decreased intensity in the crossl inked and thermally treated samples. The absorption at 1635 comes from the
unsatii rations in the MA. The decrease of this absorption reflects the fact that the free radical crossl inking on the MA proceeded. The films hot-pressed for this analysis were all thermoplastics. This fact is relevant as evidence that the crosslinking is promoted by metal coordination.
Example 3 Preparation of Crosstinked PPC with enhanced thermal stability
Copolymerisation:
Propylene oxide (332 g, 5,7 1 mol) and zinc glutaratc (1,2 g, 6, 1 mmol ) were placed in a 2 dm3 stainless steel autoclave reactor. CO.) was added to the reactor and the temperature was raised to 60 °C. The pressure was maintained at 20 bar by replenishing CO.) at regular intervals throughout the reaction time of 40 hrs. The react ion was terminated by venting CO2 and removing excess monomer by distillation. The crude polymer was recovered as a solid that contained 2,3 wt % zinc glutaratc. The propylene carbonate content was estimated as 4 wt% from FTIR. The product was dissolved in dichloromethane, diluted hydrochloric acid was added, the aqueous phase was removed and the polymer was precipitated by the addition of methanol. The polyf propylene carbonate) was dried to yield a white solid that was analysed by GPC : Mn; 194000; Crosslinkable PPC
8 g of crude PPC prepared as described in the previous step was worked up as in the Example I . Additionally, the agglomerates were cryomilled again to particle sizes lower than 1 mm.
Crosslinked PPC
3g of the particles from the previous step were placed in a 100 ml beaker with 30 ml of peroxide solution 0,6 wt/vol% in heptane. The peroxide employed was the same as in the Example 2. The samples were cured at 110°C for 2 hours and then degassed overnight at 35 °C. 1 mm thick plates were hot-pressed at 150°C from the powder and characterized. The isothermal TGA in N2 was measured and gel content and TGA was characterized as in the example 2. The gel content in the crosslinked sample was 61%. The results of the isothermal TGA characterizations are shown in the Figure 2 and indicate that the crosslinked PPC has a superior thermal stability at 200°C than the starting crude PPC
Example 4 swelling in various solvents Preparation of PPC
Propylene oxide (140 g) and zinc glutarate (2 g, ) were placed in a 2 dm3 stainless steel autoclave reactor. CO.; was added to the reactor and the temperature was raised to 60 °C. The pressure was maintained at 40 bar by replenishing CO& at regular intervals throughout the reaction time of 8 hrs. The reaction was terminated by venting CO? and removing excess monomer by distil lation. The crude polymer was recovered as a solid (36g ) that contained approximately 5,5 wt % zinc glutarate. The propylene carbonate content was estimated as 4 wt% from FTI R. A l Og sample was dissolved in dichloromethane, diluted hyd