Functionalized poly(alpha-hydroxy acid)s via ring-opening polymerization: Toward hydrophilic polyesters with pendant hydroxyl groups
Two functionalized dilactones with protected hydroxyl groups, benzyloxymethyl methyl glycolide (4a) and benzyloxymethyl glycolide (4b), were synthesized and converted to the corresponding polyesters by ring-opening polymerization in the melt ( at 110 degrees C using benzyl alcohol and SnOct(2) as initiator and catalyst, respectively, and at 130 degrees C using SnOct(2) as catalyst or in solution at 35 degrees C using ethylzinc phenolate and 2-propanol as catalyst and initiator, respectively). The obtained polymers were amorphous, with a glass transition temperature (T-g) between 15 and 45 degrees C. C-13 NMR analysis showed that poly( 4b) was perfectly alternating, owing to a regioselective ring opening, whereas poly( 4a) had a random distribution of methyl and benzyloxymethyl side groups. Both 4a and 4b could be copolymerized with L-lactide. Copolymers of L-lactide with 4b showed crystallinity at 75% lactide content, whereas copolymers with 4a were amorphous at the same lactide content. Monomer 4b apparently reacts faster than lactide, resulting in composition drift and finally yielding a polymer rich in lactide and consequently in lactide blocks that are large enough to crystallize. Block copolymers were synthesized by sequential polymerization of L-lactide and 4a using ethylzinc phenolate as catalyst. Deprotection of the benzyloxymethyl groups of poly( 4a) and poly( 4b) gave the corresponding hydroxylated polyesters, which were amorphous and semicrystalline, respectively, according to DSC analysis.