Lactide, β-butyrolactone, δ-valerolactone, and ε-caprolactone polymerization with copper diketiminate complexes

Pseudo-first-order rate constants for the polymerization of rac-lactide with dimeric (nacnacBnCu(μ-OiPr))2 (nacnacBn = deprotonated N,N′-dibenzyl-2-amino-4-imino-pent-2-ene), 1, in acetonitrile, THF, dichloromethane, and toluene are kobs = 2.4(1), 5.3(5), 3.6–4.4, and 10(1) min−1, respectively (25 °C, 1.0 mmol L−1 1). A bimodal polymer molecular weight distribution is obtained in toluene, narrow polydispersities of 1.1 for THF and acetonitrile. Under the same conditions (25 °C, 1.0 mmol L−1 1, CH2Cl2), 1 polymerizes β-butyrolactone, ε-caprolactone, and δ-valerolactone with rate constants of kobs = 3.0(1) × 10−2, 1.2–2.7 × 10−2, and 0.11(1) min−1, respectively. Homopolymers showed narrow polydispersities of approximately 1.1. Sequential addition polymerizations showed evidence for transesterification if β-butyrolactone or ε-caprolactone is introduced after a lactide block.