Epoxide cyclic carbonate polymers

During polymer chain growth, a back-biting process can lead to cyclic carbonate formation. In general, this process is more facile for aliphatic epoxides than for alicyclic epoxides and when the growing polymer chain dissociates from the metal center (Scheme 3). 

Carbon dioxide can effectively be added to the epoxide ring of GVE to produce the corresponding cyclic carbonate, OVE. Quaternary ammonium salt catalysts showed good catalytic activity even at atmospheric pressure of carbon dioxide. Since the blends of poly(OVE-co-AN) and SAN showed good miscibility, catalytic fixation of carbon dioxide to polymer blends via cyclic carbonate could be one of choice for the reduction and utilization of the greenhouse gcis. 

Polymer supported crown ether/metal salt Prep, of cyclic carbonates from epoxides and CO2 ... 

A-Heterocyclic carbenes (NHCs), used as organocatalysts, have received great interest due to their unique reactivity and selectivity observed in many different types of organic reactions (for selected recent reviews see [250-261]. More recently, NHC-mediated reactions have also been employed for polymer synthesis ([262-264] for selected reviews see [20, 265-267]), especially in the ROP of heterocyclic monomers, such as lactides [268-272], lactones [273-276], epoxides [277-279], cyclic carbonates [280], cyclic siloxanes [281, 282], and A-carboxyl-anhydrides [283, 284]. NHC-mediated step-growth polymerization has also been reported [285-287]. 

Following these results, Darensbourg et al. have continued the research and used other bifunctional Cr(salen) complexes as catalysts for polycarbonate synthesis. They observed that when a monofunctional Cr(salen) complex (5) was used to catalyze the reaction between epoxide and CO2, the product formed was cyclic carbonate. However, when a bifunctional Cr(salen) catalyst (6) was used, 79% selectivity towards the polycarbonate was obtained at 70 °C. The reason for this difference lies in the structure of the bifunctional catalyst, which provides steric hindrance in the epoxide ring-opening process to form the cyclic carbonate. Therefore, it can be inferred that spatial requirements in the active site of the metal catalyst determine the selectivity for the kinetic polymer product over the thermodynamically more stable cyclic carbonate product. 

Other researchers [69, 70] have shown that polymer-immobilized nano-gold particles had unprecedented catalytic activity for activation of carbon dioxide and high turnover frequency (TOF) for the synthesis of cyclic carbonates. Moreover, Xiang et al. [69] have reported a novel and convenient route for the direct synthesis of cyclic carbonates that avoids the preliminary synthesis and isolation of intermediate alkene oxide, coupling the two sequential reactions of epoxidation of alkene and cycloaddition of CO to epoxide into one pot. It is still not clear at this stage about the reaction mechanism. Shi et al. [70] proposed that the activation of... 

Figure 8.18 Possible mechanism for (a) cycli2ation reactions of epoxides and (b) one-pot synthesis of cyclic carbonate from epoxide and CO over polymer-supported nano-gold...

PTMC synthesis is realised either by copolymerisation of epoxides with carbon dioxide or by the ring-opening polymerisation (ROP) of cyclic carbonate monomers. It is also possible to obtain aliphatic carbonates via polycondensation of dialkyl or diphenyl carbonate or chlorophormates and aliphatic diols. However, polycondensation usually leads to polymers with rather low molar masses (Hyon et al., 1997). Besides, side reactions often occur during polycondensation (Jerome and Lecomte, 2008). 

Copolymers of lactide with other cychc monomers such as e-caprolactone can be prepared using similar reaction conditions (Fig. 6.8). These monomers can be used to prepare random copolymers or block polymers because of the end growth polymerization mechanism. Cyclic carbonates, epoxides and morpohinediones have also been copolymerized with lactide. 

The carboxylation of epoxides (6.20) has been known since 1943 but has been exploited only much more recently. It may afford either cyclic carbonates or polymers, depending on the catalyst used [124-126] and the reaction conditions [127-138]. 

The use of carbon dioxide in the synthesis of functional molecules is of considerable interest. An example is the industrially important reaction of epoxides with carbon dioxide to give cyclic carbonates. Also, functionalization of acetylenes and dienes with carbon dioxide on transition metal catalysts gives rise to the formation of cyclic lactones or dicarboxylic acids. The activation of carbon dioxide by metal complexes was reviewed in 1983 . Reactions of carbon dioxide with carbon-carbon bond formation catalyzed by transition metal complexes was reviewed in 1988 ", heterogenous catalytic reactions of carbon dioxide were reviewed in 1995, and the use of carbon dioxide as comonomers for functional polymers was reviewed in 2005. ... 

Vinyl-functional alkylene carbonates can also be prepared from the corresponding epoxides in a manner similar to the commercial manufacture of ethylene and PCs via CO2 insertion. The most notable examples of this technology are the syntheses of 4-vinyl-1,3-dioxolan-2-one (vinyl ethylene carbonate, VEC) (5, Scheme 24) from 3,4-epoxy-1-butene or 4-phenyl-5-vinyl-l,3-dioxolan-2-one (6, Scheme 24) from analogous aromatic derivative l-phenyl-2-vinyl oxirane. Although the homopolymerization of both vinyl monomers produced polymers in relatively low yield, copolymerizations effectively provided cyclic carbonate-containing copolymers. It was found that VEC can be copolymerized with readily available vinyl monomers, such as styrene, alkyl acrylates and methacrylates, and vinyl esters.With the exception of styrene, the authors found that VEC will undergo free-radical solution or emulsion copolymerization to produce polymeric species with a pendant five-membered alkylene carbonate functionality that can be further cross-linked by reaction with amines. Polymerizations of 4-phenyl-5-vinyl-l,3-dioxolan-2-one also provided cyclic carbonate-containing copolymers. 

In many cases, reactive double bonds were introduced into the cyclic carbonate stmcture for subsequent cross-linking, epoxidation, or addition reactions.Polymers of six-membered cyclic carbonate with pendant allyl ether group... 

It is clear from the numerous accounts in literature that DMCs can efficiently catalyze the copolymerization of CO2 and epoxides. DMCs can however also be used to develop systems that selectively catalyze the CO2 cycloaddition rather than the copolymerization (Scheme 1.4) as is illustrated by the work of Dharman et al. [20]. By itself, a Zn-Co-DMC is an efficient catalyst for the copolymerization reaction. However, the addition of a quaternary ammonium salt to the reaction mixture switches the selectivity of the catalytic system toward the exclusive formation of the cyclic carbonate. The quaternary ammonium ion plays two important roles in the catalytic system it accelerates the diffusion of CO2 into the reaction mixture and it favors a backbiting mechanism. As such, it hinders the growth of the polymer chain and it enables the selective cyclic carbonate production. Although most zinc-containing catalysts for this reaction are very sensitive toward water, Wei et al. have shown that, for example, the combination of Zn-Co-DMC with CTAB (cetyltrimethylammonium bromide) could even use water-contaminated epoxides as an epoxide feed [21]. 

Control of moleculair weight in polymerization reaction is a central subject of synthetic polymer chemistry in fundamental as well as practical aspects. We have developed metsdloporphyrins such as 1 and 2 as excellent initiator for living pol3mierizations of a variety of cyclic and vinyl monomers such as epoxides episulfides, lactones, methacrylates, acrylates, and methacrylonitrile, and also for living cdtemating copolymerization of epoxide ind carbon dioxide " or cyclic acid anhydride . ... 

The most common route to cyclic carbonates is the reaction of epoxides with CO2, which is promoted by a variety of homogeneous, heterogeneous and supported catalysts either cyclic carbonates or polymers are obtained [89]. Main group metal halides [90a] and metal complexes [90b], ammonium salts [91] and supported bases [92], phosphines [93], transition metal systems [88, 94], metal oxides [95], and ionic liquids [96] have been shown to afford monomeric carbonates. A1 porphyrin complexes [97] and Zn salts [89, 94, 98] copolymerize olefins and CO2. 

There are many electrophiles which not only terminate living polymer chains but also produce end-group substitution. For example, macromolecules with hydroxyl, carboxyl, thiol, or chlorine termini can be prepared by reacting living polymers with such compounds as epoxides, aldehydes, ketones, carbon dioxide, anhydrides, cyclic sulfides, disulfides, or chlorine (15-23). However, primary and secondary amino-substituted polymers are not available by terminations with 1° or 2° amines because living polymers react with such functionalities (1.). Yet, tert-amines can be introduced to chain ends by use of -N-N-di-methylamino-benzaldehyde as the terminating agent (24). 

---