The isomerization polymerization of three alkyl glycidyl carbonates (4), i.e., glycidyl methyl carbonate (4a), ethyl glycidyl carbonate (4b), and glycidyl propyl carbonate (4c), catalyzed by methylaluminum bis(2,6-di-t-butyl-4-methylphenoxide) (3) to afford novel poly(orthocarbonate)s, poly[(2-alkoxy-1,3-dioxolane-2,4-diyl)ox ene]s (5a-c), is described. The polymerization proceeded best at around room temperature and gave 5 having several thousands of M-n. As the alkoxy chain of 4 was lengthened, the polymer yield decreased, while the polymer molecular weight increased. The yields of 5b and 5c, however, were improved by increasing the feed ratio of 3 to 4 from 0.04 to 0.10. The reactivity of 4 was discussed in relation to that of glycidyl alkanoates (1).