Boron compounds have been used as initiators in the polymerization of dimethylsulfoxonium methylide,a living C1 polymerization discovered by Shea et al and coined the name polyhomologation.1 Encouraged by this work,we discovered that 3-methylallyltriphenylarsonium ylide,a ω-monosubstituted allylarsonium ylide,leads also to C3 polymerization.Moreover,by combining ω-monosubstituted allylic arsonium ylide(C3)with sulfoxonium methylide(C1)we were able to synthesize well-defined methylene/3-methylpropenylene block and random copolymers.2 Recently,we discover benzyltriphenylarsonium ylides could also serve as C1 units.A well-defined poly[(phenylmethylene-co-methylpropenylene)-b-methylene] terpolymers via one-pot borane initiated random copolymerization of ω-methylallyl and benzyltriphenylarsonium ylides,followed by polymerization of sulfoxonium methylide.Other substituted arsonium ylides,such as prenyltriphenyl,propyltriphenyl and(4-fluorobenzyl)triphenyl can also be used instead of benzyltriphenylarsonium.The obtained terpolymers are well-defined,possess a predictable molecular weight and low polydispersity(Mn,NMR = 1.83-9.68×103 g/mol,(D)= 1.09-1.22).More important,an unexpected light emission phenomenon was discovered in these non-conjugated terpolymers as proved by fluorescence and NMR spectroscopy.This phenomenon can be explained by the isomerization of the double bonds of allylic monomeric units along the chain of the terpolymers.3