Copolymerisation of Oxiranes and Cyclic Acid

Although rather scant information concerning the structure of active sites in the above copolymerisation systems is available [183-189], cyclic acid anhydrides can be considered as coordinating to metal species via the carbonyl oxygen atom and reacting by nucleophilic attack of the metal substituent on the carbon atom of the coordinated carbonyl group [190,191], Thus, the oxi-rane/cyclic acid anhydride copolymerisation pathway may be presented schematically as follows [82]  

Concerning metal alkyl catalysts such as diethylzinc for the copolymerisation, it was established that they formed zinc carboxylate species owing to the reaction with maleic anhydride [192], 

The tendency towards alternation in the oxirane/cyclic acid anhydride system with the zinc-based coordination catalyst is connected, according to literature data [193], with the stronger nucleophilic properties of the oxygen atom of the zinc alcoholate species and the weaker electrophilic properties of the zinc atom in such species compared with the respective properties of zinc carboxylate species. Studies of the copolymerisation of propylene oxide and maleic anhydride in the presence of catalysts such as diethylzinc-monoprotic compound (1 1) showed an increasing tendency towards alternation in systems with catalysts of decreasing electrophilicity of the zinc atom [186], which may corroborate the coordination mechanism proposed scheme (24). 

Oxirane/cyclic acid anhydride alternating copolymers of controlled molecular weight with a narrow molecular weight distribution were found by Aida et al. [188,189] to be formed under mild conditions when copolymerising ethylene oxide and phthalic anhydride in the presence of the (tpp)AlCl-quater-nary phosphonium salt catalyst system. The copolymerisation carried out with (tpp)AlCl alone proceeded very slowly, and the product was not polyethylene terephthalate) but contained ether linkages in considerable amount. The development of the living character and the tendency towards alternation of the copolymerisation when using the aluminium porphyrin catalyst, coupled with a quaternary salt, have been postulated [188,189] to be due to the formation of 

Taking the above into account, the oxirane/cyclic acid anhydride/tetrahydrofuran terpolymerisations should rather be considered to involve active sites of a cationic nature, such as those, shown in scheme (26), that are responsible for the formation of oxyalkylene sequences [82]  

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