The first anionic polymerization with small volume shrinkage is described. Volume change in anionic copolymerization of bicyclic bis(gamma-lactone) 1c and glycidyl phenyl ether (2) with potassium tert-butoxide (4 mol %) at 120 degrees C was nearly zero (small expansion, +0.25 +/- 0.15%). The anionic copolymerization of aromatic substituent-containing bicyclic bis(gamma-lactone)s (1e-g) with 2 was carried out at 120 degrees C in THF and in bulk for 72 h in the presence of potassium tert-butoxide (4 mol %). The H-1 MMR, C-13 NMR, and IR spectra of the methanol-insoluble parts clearly suggested the proposed alternating copolymer structures consisting of two successive units derived from 1e-g and 2. The anionic copolymerizations of bicyclic bis(gamma-lactone)s (1a-g) with equimolar amounts of bifunctional epoxides (5a,b) in bulk were carried out at 120-160 degrees C in the presence of potassium tert-butoxide (4 mol %) to afford the corresponding dichloromethane-insoluble copolymers (6aa-gb) in quantitative yields. The copolymer composition was ca. 1:0.5 in any case, as estimated from the H-1 NMR spectra of the dichloromethane-soluble parts. Yield of the copolymers decreased when the monomer feed ratio was deviated from 1:0.5. The IR spectral analysis of 6aa-gb strongly suggested the occurrence of the efficient alternating copolymerization. Small shrinkages or expansions in volume (-2.5 +/- 0.15% to +2.4 +/- 0.15%) were observed during the copolymerizations of 1 and 5, definitely indicating the possibility of 1 as expanding monomers. Thermal properties such as glass transition and 10% weight loss temperatures of the obtained copolymers were evaluated by DSC and TGA.