Controlled synthesis of polycarbonate diols and their polylactide block copolymers using amino-bis(phenolate) chromium hydroxide complexes†

A diamine-bis(phenolate) chromium(III) complex, CrOH[L] ([L] = dimethylaminoethylamino-N,N-bis(2-methylene-4,6-tert-butylphenolate)), 2, in the presence of tetrabutylammonium hydroxide effectively copolymerizes CO₂ and cyclohexene oxide (CHO) into a polycarbonate diol. The resultant low molar mass (6.3 kg mol⁻¹) diol is used to initiate ring-opening polymerization of rac-lactide with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) giving ABA-type block copolymers with good molar mass control through varying rac-LA-to-diol loadings and with narrow dispersities. As the degree of rac-LA incorporation increases, the glass transition temperatures (T_g) are found to decrease, whereas decomposition temperatures (T_d) increase. (Diphenylphosphonimido)triphenylphosphorane (Ph₂P(O)NPPh₃) was used as a neutral nucleophilic cocatalyst with 2, giving phosphorus-containing polycarbonates with an M_n value of 28.5 kg mol⁻¹, a dispersity of 1.13, a T_g value of 110 °C and a T_d value of over 300 °C. A related Cr(III) complex (4) having a methoxyethyl pendent group rather than a dimethylaminoethyl group was structurally characterized as a hydroxide-bridged dimer.