Anionic ring-opening polymerization of a bicyclic bis(y-lactone) (lb) was carried out under anionic conditions, lb had no homopolymerizability but copolymerized with epoxide 2 to selectively give the corresponding alternating copolymer. IR, NMR, and 13C NMR spectra and products of the alkaline hydrolysis of the obtained polymer strongly suggested an alternating copolymer structure consisting of two successive units derived from lb and 2. The unit from lb was a linear diester, probably formed by a successive double ring-opening polymerization with isomerization. The 1:1 copolymer composition was not changed by varying the monomer feed ratio in the range between 20:80 and 80:20. From the results of the model reaction of an equimolar mixture of lb and 2 with 0.6 equiv of sodium methoxide in THF, the initiation of this copolymerization was suggested to involve a nucleophilic attack of the initiator alkoxide at the carbonyl carbon of the 7-lactone ring of lb to form a carboxylate via a double ring-opening isomerization. The propagation mechanism was studied by the results of the model reaction as well as the formation of the alternating copolymer. It was found that the propagation consisted only of cross-propagation; that is, the resulting carboxylate end derived from lb reacts only with 2, whereas the alkoxide end derived from 2 undergoes selective nucleophilic attack at lb. The carboxylate end attack at 2 was suggested as the rate-determining step from the time-conversion curves. The rate of the copolymerization and the molecular weight and molecular weight distribution of the copolymer were strongly affected by solvent and counterion. The rate and the molecular weight distribution increased in the order of the counterions Na+ < K+ < Cs+ and also increased with increasing solvent polarity.

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Introduction. Bicyclo orthoester (BOE), spiro orthoesters (SOEs), and spiro orthocarbonates (SOCs) are the only monomers which have been reported so far to expand in volume during polymerization, and this unique property is believed to come from their polymerization mode: successive double ring-opening polymerization.1These monomers can polymerize with cationic initiators but not with anionic or radical initiators. Attempts to develop monomers capable of polymerizing with anionic2,3 and radical4,5 initiators have been done, but no volume