Mechanistic Discussion of Cationic Crosslinking Copolymerizations of 1,2-Epoxycyclohexane with Diepoxide Crosslinkers Accompanied by Intramolecular and Intermolecular Chain Transfer Reactions

Our previous mechanistic discussion of the free-radical crosslinking monoallyl/diallyl copolymerizations was extended to the cationic crosslinking monoepoxide/diepoxide copolymerizations, typically including 1,2-epoxycyclohexane (ECH) as a monoepoxide and bis[3,4-epoxycyclohexylmethyl] adipate (BECHMA) as a diepoxide crosslinker. In the cationic polymerization, oligomer is usually obtained because of the occurrence of characteristic chain-forming reactions. Therefore, cationic crosslinking monoepoxide/diepoxide copolymerizations could be in the category of the network formation through free-radical crosslinking monoallyl/diallyl copolymerizations. Thus, the gelation behavior was discussed by comparing the actual gel points with the theoretical ones; the greatly delayed gelation from theory was observed. Then, the resulting network polymer precursors (NPPs) were characterized by SEC-MALLS-viscometry to clarify the cationic crosslinking ECH/BECHMA copolymerization mechanism. Notably, the correlation lines of molecular weight versus elution volume were specific for the NPPs obtained at a high conversion close to the gel point as compared with those obtained by the free-radical crosslinking monoallyl/diallyl copolymerization. This may be ascribed to the occurrence of intramolecular and intermolecular chain transfer reactions characteristic of cationic polymerization; the chain transfer reactions involve the intramolecular and intermolecular nucleophilic attack of ether oxygen or terminal hydroxyl oxygen in the NPPs to a terminal growing cation that leads to the formation of not only the loop-but also the crosslink-structures containing NPPs, providing fragile ultrahigh-molecular-weight NPP in the SEC columns. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4445-4455, 2010