A theoretical study of the ring-opening polymerization (ROP) mechanism of 1,5-dioxepan-2-one (DXO) and L-lactide (LLA) with stannous(II) 2-ethylhexanoate (Sn(Oct)2) is presented. The B3LYP density functional method has been used for the quantum chemical calculations. Our results support a coordination-insertion mechanism initiated by a tin-alkoxide species formed prior to the ROP. The ratedetermining step in the ROP was the nucleophilic attack of the alkoxide on the carbonyl carbon of the monomer. The activation energy for the ROP of DXO with Sn(Oct′)2 has been determined to be 19.8 kcal/mol and for L-lactide 20.6 kcal/mol. At normal reaction temperatures, a ligand may dissociate as Oct′H during propagation. An excess of carboxylic acid hinders the coordination of monomer to the initiating/propagating complex.
ABSTRACT:Ring-opening polymerization of 1,5-dioxepan-2-one initiated by 1,1,6,6-tetra-n-butyl-1,6-distanna-2,5,7,10-tetraoxacyclodecane was carried out in chloroform, dichloromethane, or 1,2-dichloroethane. Effects of reaction temperature, solvent, and monomer-to-initiator ratio were investigated. Polymerization kinetics showed a firstorder dependence on the monomer for polymerization in chloroform and dichloromethane at 40°C. The kinetic order with respect to the initiator were a first order when dichloromethane was used as the solvent, the order in initiator changed, depending on the initiator concentration when chloroform was used. A maximum in molecular weight was observed at 40°C when chloroform was used as the solvent. The change of solvent did not markedly alter the polymerization rate or the molecular weight of the polymers prepared, as expected from the coordination insertion mechanism. Depolymerization of the polymers formed was observed when the reaction was allowed to continue after complete monomer conversion in chloroform as reaction medium at 40°C.
Hydroxy telechelic poly(l-lactide)s of different molecular weight have been synthesized by
controlled ring-opening polymerization using cyclic tin alkoxides. NMR analysis showed that the
propagation proceeded through an insertion mechanism. No free hydroxyl or carboxyl end groups were
detected in the polymerization mixture. Complete reaction of the initiator was observed over the entire
range of compositions studied. Both tin−oxygen bonds were reactive and participated in the propagation
reaction. Peak assignments were obtained by 1H, 13C, distortionless enhancement polarization transfer
(DEPT), and heteronuclear multiple quantum coherence−gradient selected (hmqc-gs) nuclear magnetic
resonance spectroscopy. The kinetics of the solution polymerization of l-lactide has been investigated
and showed a first order in monomer. The external order in initiator has been determined to be 3/4 for
initiator concentrations above 5 mmol/L and to 2 below 2 mmol/L. The molecular weight could easily be
adjusted by the monomer-to-initiator ratio, and the molecular weight distribution remained narrow even
at high molecular weight (MWD < 1.15). The polymerization products were characterized by size exclusion
chomatography (SEC) as well as 1H and 13C NMR.
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