A new superbase, the cyclic trimeric phosphazene base (CTPB), was prepared with high yield and purity. In the presence of alcohol, the CTPB serves as a highly efficient organocatalyst for ring-opening polymerization of the "non-polymerizable" γ-butyrolactone to offer well-defined poly(γ-butyrolactone) with high conversions (up to 98 %) at -60 °C. The produced polymers have high molecular weights (up to 22.9 kg mol ) and low polydispersity distributions (1.27-1.50). NMR analysis of initiation process and the structural analysis of resulting polymers by MALDI-TOF suggest a mechanism involving an activating initiator which leads only to linear polymers with BnO/H chain ends.

Phosphazene base is an important organocatalyst in polymer chemistry owing to its high activity and versatility. In this contribution, we demonstrate that cyclic trimeric phosphazene base (CTPB) can catalyze stereoselective ring-opening polymerization (ROP) of rac-lactide (rac-LA) to produce isotactic stereoblock PLA (P i up to 0.93). The polymerizations are highly controlled, as evidenced by linear relationship between molecular weights (MW) and monomer conversions and the narrow dispersity (Đ = M w/M n) of the resulted polymers with high fidelity of end groups. The investigations on polymerization parameters show that the tacticity of produced PLA depends on the polymerization temperatures and solvents, while the kinetic studies reveal a faster rate for ROP of l-LA as compared to rac-LA under same conditions. Based on these results, the chain end control mechanism is proposed to explain the production of isotactic stereoblock PLA from rac-LA by an achiral catalyst.

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An ew superbase,t he cyclic trimeric phosphazene base (CTPB), was prepared with high yield and purity.Int he presence of alcohol, the CTPB serves as ah ighly efficient organocatalyst for ring-opening polymerization of the "nonpolymerizable" g-butyrolactone to offer well-defined poly(gbutyrolactone) with high conversions (up to 98 %) at À60 8 8C. The produced polymers have high molecular weights (up to 22.9 kg mol À1 )and low polydispersity distributions (1.27-1.50). NMR analysis of initiation process and the structural analysis of resulting polymers by MALDI-TOF suggest am echanism involving an activating initiator which leads only to linear polymers with BnO/H chain ends.Scheme 1. Synthesis of acyclic trimeric phosphazene base.

Linear and star PCLs with controlled and high molecular weights are efficiently prepared by a catalytic system of CTPB/alcohol.