ABSTRACT
Ring-opening polymerization usually involves less shrinkage in volume than either simple addition polymerizations or condensation polymerizations. Oxabicyclic and oxaspirocyclic monomers have been developed as expandable monomers and their chemistry has been well studied during the last 15 years. A few anionically polymerizable expandable monomers were developed. However, some problems still remain for their actual use, particularly in the case of bicyclic or spirocyclic monomers. Most spiro ortho esters (SOEs), bicyclo ortho esters, and spiro ortho carbonates (SOCs) are able to polymerize only with ionic catalysts unless they have a group undergoing radical ring-opening polymerization with free radical initiators. Realizing the importance of SOEs with radically polymerizable groups in designing polymers that undergo no shrinkage, several SOEs having vinyl groups have been reported in the literature. The SOCs seem to be the most promising candidates among the three expandable monomers, and further recent studies are gradually clarifying the essential factors required for preparing these expandable monomers.
