13C NMR study of the tacticity of poly(propylene oxide)s prepared by polymerization with α,β,γ,δ-tetraphenylporphyrin/AlEt2Cl as initiator system: An example of first-order markovian statistics in ring-opening polymerization

Racemic and enantiomerically enriched poly(propylene oxide)s were prepared using α, β, γ, δ-tetraphenylporphyrin/AlEt₂Cl as initiator system. Triad tacticity of the polymers was analyzed by ¹³C NMR spectroscopy (62,89 MHz). A predominance of isotactic triads was observed in the case of the racemic polymer, corresponding to a non-random distribution of configurational units in the polymer chain. A model correlating tacticity with enantiomeric excess of starting monomer in the case of a first-order Markovian distribution was established. Good agreement between experimental results and calculated triad contents (according to the model previously described) was observed for enantiomerically enriched polymers. This indicates a steric control by the last unit of the growing chain end. The influence of the size of the substituent was examined for the case of poly(1,2-epoxybutane). The origin of the chain-end effect is discussed.