This article deals with (1) synthesis and anionic polymerization of a six-membered cyclic carbonate having styrene moiety, (2) anionic depolymerization of the obtained polymer, (3) radical cross-linking of the obtained polymer, and (4) anionic de-cross-linking of the cross-linked polymer. The monomer 5-ethyl-5-[(p-vinylphenyl) methoxymethyll-1,3-dioxan-2-one (St6CC) underwent anionic polymerization with potassium tert-butoxide (t-BuOK) as an initiator in THF to afford the corresponding polycarbonate [poly(St6CC)I. It was confirmed that this polymerization was equilibrium polymerization by the relationships between the polymerization temperature and monomer conversion. Poly(St6CC) underwent anionic depolymerization with t-BuOK (5 mol % vs polymer repeating unit) as a catalyst in THF (0.1 M) at 20 degrees C for 24 h to recover St6CC in 60% yield. Treatment of poly(St6CC) with a radical initiator afforded the cross-linked polymer. Employment of styrene as the comonomer satisfactorily afforded the corresponding cross-linked polymer. It underwent anionic de-cross-linking with t-BuOK (10 mol % vs polymer repeating unit) in THF at 50 degrees C for 24 h to afford a THF-soluble polymer. The yield of the THF-soluble part increased as the styrene composition in the cross-linked polymer increased. It was suggested that the de-cross-linking efficiency depended on the cross-linking density.