Polycarbonates derived from green acids: Ring-opening polymerization of seven-membered cyclic carbonates

摘要

Methyl substituted seven-membered ring carbonates (7CCs), namely, 4-methyl- and 5-methyl-1,3-dioxepan-2-one (α-Me7CC and β-Me7CC), have been synthesized in high yields (up to 70%) upon cyclization of the corresponding α,ω-diols issued from green renewable acids. ("Immortal") ring-opening polymerization of these monomers 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 organometallic complexes [(BDI~(iPr))Zn(N(SiMe_3)_2)] ((BDI~(iPr)) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)-imino)-2-pentene] and [(ONOO~(tBu))Y(N(SiHMe_2)_2)(THF)] (ONOO ~(tBu) = amino-alkoxy-bis(phenolate)) or the organic 4-N,N-dimethylaminopyridine (DMAP), 1.5.7-triazabicyclo-[4.4.0]dec-5-ene (TBD) or 2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine (BEMP) successfully afforded the corresponding poly(α- and β-Me7CC)s with quite good control and activities. The dissymmetry of the monomers raises the question of catalyst selectivity in the ring-opening of the 7CCs. Detailed microstructural analyses of the poly(7CC)s using ~1H and ~(13)C NMR and MALDI-ToF-MS techniques revealed (1) the higher regioselectivity with preferential ring-opening at the most hindered oxygen-acyl O-C(O)O bond, that is, closest to the α-Me substituent of the zinc-based system followed by the yttrium, as compared to the lack of selectivity of the aluminum one, in the ring-opening polymerization (ROP) of α-Me7CC; (2) the absence of regioselectivity in the ROP of β-Me7CC, whichever the catalyst system used, most likely as a result of the OC(O)O further remote substitution site; (3) the expected α-hydroxy,ω-alkoxyester chain ends. Differential scanning calorimetry (DSC) analyses of these polymers underlined the influence of the position of the methyl substituent on the glass transition temperature. Noteworthy, this study represents the first synthesis of the β-Me7CC monomer and of the resulting polycarbonates.