Heteroleptic Tin(II) Initiators for the Ring-Opening (Co)Polymerization of Lactide and Trimethylene Carbonate: Mechanistic Insights from Experiments and Computations.
Résumé
The tin(II) complexes \LOx\Sn(X) (\LOx\-=aminophenolate ancillary) contg. amido (1-4), chloro (5), or lactyl (6) coligands (X) promote the ring-opening polymn. (ROP) of cyclic esters. Complex 6, which models the first insertion of L-lactide, initiates the living ROP of L-LA on its own, but the amido derivs. 1-4 require the addn. of alc. to do so. Upon addn. of one to ten equiv. of iPrOH, precatalysts 1-4 promote the ROP of trimethylene carbonate (TMC); yet, hardly any activity is obsd. if tert-Bu (R)-lactate is used instead of iPrOH. Strong inhibition of the reactivity of TMC is also detected for the simultaneous copolymn. of L-LA and TMC, or for the block copolymn. of TMC after that of L-LA. Exptl. and computational data for the \LOx\Sn(OR) complexes (OR=lactyl or lactidyl) replicating the active species during the tin(II)-mediated ROP of L-LA demonstrate that the formation of a five-membered chelate is largely favored over that of an eight-membered one, and that it constitutes the resting state of the catalyst during this (co)polymn. Comprehensive DFT calcns. show that, out of the four possible monomer insertion sequences during simultaneous copolymn. of L-LA and TMC: 1) TMC then TMC, 2) TMC then L-LA, 3) L-LA then L-LA, and 4) L-LA then TMC, the first three are possible. By contrast, insertion of L-A followed by that of TMC (i.e., insertion sequence 4) is endothermic by +1.1 kcal mol-1, which compares unfavorably with consecutive insertions of two L-Lunits (i.e., insertion sequence 3) (-10.2 kcal mol-1). The copolymn. of L-LA and TMC thus proceeds under thermodn. control. [on SciFinder(R)]