Synthesis and characterization of homo- and co-polymers of (R,S)-β-butyrolactone and γ-butyrolactone or β-valerolactone initiated with cyclic tin alkoxide

A series of predominantly syndiotactic poly(R,S)3-hydroxybutyrates (PHB) which were in widely different molecular weights (from 16,000 to 380,000 Da) were prepared by ring-opening polymerization of (R,S)β-butyrolactone(β-BL) initiated with cyclic tin alkoxide, using high vacuum technique in order to create and maintain the impurity-free conditions. Copolyesters poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3HB-co-4HB)) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(3HB-co-3HV)) have also been synthesized from copolymerizations of β-BL with either γ-butyrolactone (γ-BL) or (R,S)β-valerolactone (β-VL) with cyclic tin alkoxide. The resulting polymers were characterized by GPC, DSC, 1H, 13C NMR. Molecular weight of the polymer is largely dependent on an amount of initiator and inversely proportional to that. The glass-transition temperature of the PHB is 5 ± 2 °C, being independent of molecular weight of the samples. 13C NMR spectroscopy resolved stereosequences in synthetic PHB at the diad level for the carbonyl carbon and at the triad level for the methylene carbon. Analysis of diad sequence distributions at various polymerization temperatures for the cyclic tin alkoxide investigated showed an activation energy (ΔE = Es − Ei) of ca. −10.8 kJ/mol for syndiotactic versus isotactic diad formation. These analyses show that the cyclic tin alkoxide is a stereoselective initiator for ring-opening polymerization of β-BL and gives predominantly syndiotactic PHB (syndiotactic diad fraction from 0.59 to 0.79). Triad stereosequence distributions of PHB samples agree well with the Bernoullian statistical model of chain-end control.