A copolymerization of 10-methyl-2H,8H-benzo-[1,2-b:5,4-b']bipyran-2,8-dione (1) and glycidyl phenyl ether (GPE) was studied. 1 was a bislactone designed as a bifunctional analogue of 3,4-dihydrocoumarin (DHCM), of which anionic 1:1 alternating copolymerization with GPE has been reported by us, previously. This alternating nature was inherited by the present copolymerization of 1 and GPE, leading to an intriguing copolymerization behavior in contrast to the ordinary statistical copolymerizations of monofunctional monomers and bifunctional monomers usually controlled by the proportional dependence of the crosslinking density on the monomer feed ratio: (1) When the feed ratio [GPE](0)/[1](0) was 1, the two monomers underwent the 1:1 alternating copolymerization. In this case, I behaved as a monofunctional monomer, that is, only one of the two lactones in 1 participated in the copolymerization allowing the other lactone moiety to be introduced into the side chain almost quantitatively. (2) Increasing the feed ratio [GPE](0)/[1](0) to larger than 4 allowed almost all of the lactone moieties to participate in the copolymerization system to give the corresponding networked polymers efficiently. The compositions of the copolymers [GPE unit]/[1-derived acyclic ester unit] were always biased to smaller values than the feed ratios [GPE](0)/[lactone moiety in 1](0) by the intrinsic 1:1 alternating nature of the copolymerization. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3662-3668, 2009