Copolymerisation of Oxiranes and Carbon

Catalysts derived from reaction systems such as triethylaluminium-water and triethylaluminium-water-acetylacetone [225], triethylaluminium-triphe-nylphosphine [226], triethylaluminium-pyrogallol [209] and rare-earth metal phosphonate- triisobutylaluminium-glycerol [227] appeared to be effective in the copolymerisation of propylene oxide and carbon dioxide, yielding high molecular weight polypropylene ether-carbonate)s (Table 9.4) but not the respective alternating copolymer which is polypropylene carbonate). 

Although propylene oxide has been the oxirane most widely studied in copolymerisation with carbon dioxide, there are a variety of other oxiranes capable of coordination copolymerisation with carbon dioxide (Table 9.4). 

Another interesting monomer for copolymerisation with carbon dioxide is isomeric 2-butene oxide. In copolymerisation in a ternary comonomer system consisting of 2-butene oxide, 1-butene oxide and carbon dioxide with the diethylzinc-water catalyst, m-2-butene oxide was incorporated in the copolymer, while trans-2-butene oxide hardly underwent an enchainment [230]. Thus, the smaller steric hindrance for the r/.v-isomer than for the irans-isomer throughout the coordination copolymerisation with carbon dioxide is to be taken into account. 

Copolymerisation of propylene oxide as well as other oxiranes with carbon dioxide in the presence of zinc-based coordination catalysts is generally accompanied with the formation of a cyclic five-membered carbonate, propylene carbonate or another alkylene carbonate [147,206,207,210,212,230]. The alky-lene carbonate, however, is not the precursor for poly(alkylene carbonate), since it hardly undergoes a polymerisation under the given conditions [142-146], 

Studies of the regioselectivity of oxirane/carbon dioxide copolymerisation showed the polar effect exerted by the ring substituent, but not the bulkiness, to be the factor determining the direction of ring opening [231,232]. In the case of propylene oxide/carbon dioxide copolymerisation, C g—O bond cleavage 

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Since oxiranes are representative heterocyclic monomers containing an endo-cyclic heteroatom, and the most commonly polymerised of such monomers, they have been subjected to copolymerisations with heterocyclic monomers containing both an endocyclic and an exocyclic heteroatom. Coordination copolymerisations of heterocyclic monomers with different functions are focused on oxirane copolymerisation with cyclic dicarboxylic acid anhydride and cyclic carbonate. However, the statistical copolymerisation of heterocyclic monomers with an endocyclic heteroatom and monomers with both endocyclic and exocyclic heteroatoms have only a limited importance. Also, the block copolymerisation of oxirane with lactone or cyclic dicarboxylic acid anhydride is of interest both from the synthetic and from the mechanistic point of view. Block copolymerisation deserves special interest in terms of the exceptionally wide potential utility of block copolymers obtained from comonomers with various functions. It should be noted, however, that the variety of comonomers that might be subjected to a random, alternating and block polymerisation involving a nucleophilic attack on the coordinating monomer is rather small. 

It seems that the initiation step of the copolymerisation most likely involves the oxirane reaction [according to scheme (3)]. Zinc alcoholate species formed in this reaction can easily propagate the copolymer chain, coordinating and enchaining both the oxirane [scheme (3)] and the cyclic carbonate [scheme (15)] comonomers. However, in the case of the cyclic carbonate, its enchainment may also proceed according to scheme (14), leading to decarboxylation. Thus, the obtained poly(ether-carbonate)s are characterised by a lower content of carbonate units with respect to the ether units [82,146]. 

A concerted mechanism of oxirane/carbon dioxide copolymerisation with catalysts formed in diethylzinc-dihydric phenol systems and related systems has been proposed [76,206,207], According to this mechanism, the oxirane coordination and enchainment occur as shown by the following scheme ... 

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