Anionic cross-linkings of epoxy group containing polymers such as poly(glycidyl methacrylate) (6) and poly(4-(glycidylmethyl)styrene) (7) with bicyclic acid spirocyclic bis(gamma-lactone)s (1-3) are described. A model study using glycidyl pivalate (8) as the polymer model compound was carried out at 120 degrees C in THF ([C] = 4.0 M) in the presence of potassium tert-butoxide (2 mol %) to obtain the corresponding alternating copolymers (9-11) in 62-69% yields. The anionic cross-linkings of 6 and 7 with 1-3 and homopolymerizations of 6 and 7 under similar conditions afforded dichloromethane-insoluble cross-linked (co)polymers (12-19) quantitatively. The IR spectral change might suggest strongly that the cross-linked copolymers formed had polyester group consisting of an almost completely alternating copolymer unit. Recovery of a small amount of 1-3 would suggest that the polyester side chain contained a small amount of a polyether unit in some cases. The degree of ring-opening of epoxy group of 6 and 7 was 100% in most cases. Homopolymerizations of 6 and 7 were accompanied by 5.4% and 6.3% volume shrinkage, respectively, although the degree of ring-opening of tile epoxy group was very low (27% and 38%). Volume shrinkage (2.5-5.4%) observed in the cross-linkings of 6 and 7 with 1-3 was relatively lower in spite that tile degree Of ring-opening of epoxy group was 100%. It was concluded that volume shrinkage during cross-linking of epoxy group containing polymers can be considerably suppressed by addition of 1-3 as an additive. Thermal properties such as glass transition and 10% weight loss temperatures of the crosslinked polymers 12-19 obtained were evaluated by DSC and TGA.