We report the first synthesis of well-defined poly[(phenylmethylene-co-methylpropenylene)-b-methylene, [(C1-co-C3)-b-C1], terpolymers via one-pot borane initiated random copolymerization of ω-methylallyl (C3 units, chain is growing by three carbon atoms at a time) and benzyltriphenylarsonium (C1 units, chain is growing by one carbon atom at a time) ylides, followed by polymerization of sulfoxonium methylide (C1 units). The synthesized terpolymers possess a predictable molecular weight and monomodal low polydispersity (Mn, NMR=1.83-9.68×103 g /mol, Đ =1.09-1.22). Other substituted arsonium ylides, such as prenyltriphenyl, propyltriphenyl and (4-fluorobenzyl)triphenyl can also be used instead of benzyltriphenylarsonium ylide, affording well-defined terpolymers too (Mn, NMR=4.3-9.1×103 g/mol, Đ=1.16-1.22). An unexpected light emission phenomenon was discovered in these non-conjugated terpolymers, as demonstrated by fluorescence and NMR spectroscopy. This phenomenon can be attributed to the formation of conjugated small blocks along the allylic monomeric units as the result of isomerization of the double bonds (isomerization-induced light emission).