Chiral indium salen complexes are highly active, isoselective catalysts for the ring opening polymerization of racemic lactide. The polymerizations are well controlled and polymers with high molecular weights and low molecular weight distributions are obtained. Preliminary kinetic investigations with the enantiopure complex confirm enantiomorphic site control as the dominant contributor to selectivity and formation of block copolymers.
The dinuclear indium catalyst [(NNO)InCl] 2 (μ-OEt)(μ-Cl), previously reported to be highly active for the living ring-opening polymerization of cyclic esters lactide (LA) and β-butyrolactone (BBL), was used to generate a series of triblock copolymers of poly(lactic acid) (PLA) and poly(hydroxybutyrate) (PHB). Copolymers PLLA−PDLLA−PLLA and PLLA−PDLLA−PDLA, synthesized via sequential monomer addition, showed low molecular weight distributions and excellent correlation between the calculated and experiment molecular weights. Significantly, triblock copolymers of the type PLA−PHB−PLA were also synthesized for the first time through a sequential addition technique. Analysis of polymers after each addition of monomer showed that although only 85% conversion was achieved after addition of BBL, the remaining chain ends were active and addition of more lactide yielded a triblock. Rheological studies of PLLA−PHB−PDLA indicated solid like behavior even well above the temperature at which stereocomplex formation was observed. These elastomeric triblocks exhibited elongations at break 5−10 times greater than those of corresponding PLLA−PDLLA−PLDA triblocks.
A methodology for controlling aggregation in highly active and isoselective indium catalysts for the ring opening polymerization of racemic lactide is reported.
We
introduce an air- and moisture-stable hydroxy-bridged indium
salan complex as a highly active and controlled catalyst for the ring-opening
polymerization of cyclic esters in air. The reversible activation
of this complex with linear and branched alcohols leads to immortal
polymerization, allowing the controlled formation of block copolymers
in air. It is the only reported example of a living catalyst that
remains controlled after multiple exposures to ambient air at high
temperatures. Although the prevalent catalyst for ring-opening polymerization,
tin octanoate, is robust, it does not promote controlled polymerization.
Our indium catalyst is exceptional in being both robust and controlled.
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