"Immortal" ring-opening polymerization (iROP) of rac-4-methyl-1,3-dioxan-2-one (alpha-methyl trimethylene carbonate, alpha-MeTMC) has been carried out using various catalysts combined with an alcohol acting as a co-initiator and a chain transfer agent. The Lewis acid Al(OTf)(3), the metallo-organic beta-diiminate [(BDI)Zn(N(SiMe(3))(2))] (BDI(iPr) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)-imino)-2-pentene) complex, or the organic bases, 2-tert-butylimino-2-diethylamino-1,3dimethylperhydro-1,3,2diazaphosphorine (BEMP), 1.5.7-triazabicyclo-[4.4.0]dec-5-ene (TBD) and 4-N,N-dimethylaminopyridine (DMAP), successfully afforded the corresponding poly(alpha-MeTMC)s with quite good control and activities. In comparison, at 110 degrees C, these same catalytic systems were more active toward the iROP of native TMC, whereas the iROP of the seven-membered analogue alpha-Me7CC generally featured a poorer control. The dissymmetry of alpha-MeTMC raises the question of catalyst selectivity in the ROP process. Detailed microstructural analyses of the poly(alpha-MeTMC)s using (1)H and (13)C NMR revealed (1) the preferential ring-opening at the most hindered oxygen-acyl O-C(O)O bond, i.e., closest to the alpha-Me substituent of the zinc-based system, in the iROP of alpha-MeTMC, possibly as a result of the favorable steric hindrance; (2) the absence of regioselectivity of the aluminum or guanidine catalyst systems. In addition, MALDI-ToF-MS analyses confirmed the expected alpha-hydroxy,omega-alkoxyester chain-ends in these polymers. DSC analyses revealed a glass transition temperature of the poly(alpha-MeTMC)s (T(g) approximate to -17 degrees C) similar to that of poly(TMC)s.