Metal- and Organo-Catalyzed Ring-Opening Polymerization of α-Methyl-Trimethylene Carbonate: Insights into the Microstructure of the Polycarbonate
Résumé
"Immortal" ring-opening polymerization (iROP) of rac-4-methyl-1,3-dioxan-2-one (α-methyl trimethylene carbonate, α-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 β-diiminate [(BDI)Zn(N(SiMe3)2)] (BDIiPr = 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(α-MeTMC)s with quite good control and activities. In comparison, at 110 °C, these same catalytic systems were more active toward the iROP of native TMC, whereas the iROP of the seven-membered analogue α-Me7CC generally featured a poorer control. The dissymmetry of α-MeTMC raises the question of catalyst selectivity in the ROP process. Detailed microstructural analyses of the poly(α-MeTMC)s using 1H and 13C NMR revealed (1) the preferential ring-opening at the most hindered oxygen-acyl O-C(O)O bond, i.e., closest to the α-Me substituent of the zinc-based system, in the iROP of α-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 α-hydroxy,ω-alkoxyester chain-ends in these polymers. DSC analyses revealed a glass transition temperature of the poly(α-MeTMC)s (Tg ≈ −17 °C) similar to that of poly(TMC)s.