Coupling of Propylene Oxide and Lactide at a Porphyrin Chromium(III) Center

5,10,15,20-Tetraphenylporphyrin chromium chloride (TPPCrCl) with added [Ph₃PNPPh₃]⁺Cl^– (PPN⁺Cl^–) selectively polymerizes lactide (l and <i>rac</i>) dissolved in neat propylene oxide (PO) to yield polylactide (PLA) terminated by the −OCHMeCH₂Cl group. At 0 °C and below, <i>rac</i>-LA yields polymers highly enriched in isotactic tetrads (<i>iii</i>). At 25 °C, some stereoselectivity is lost as transesterification becomes significant, and at 60 °C and above, enchainment of PO leads to the formation of 3,6-dimethyl-1,4-dioxan-2-one by a backbiting mechanism. At 0 °C, after the enchainment of l-(<i>S</i>,<i>S</i>)-LA in neat (<i>R</i>)-(+)-PO, the formation of (3<i>S</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one occurs, while at higher temperatures the ratio of (3<i>S</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one to (3<i>R</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one falls to 3:2

Coupling of Propylene Oxide and Lactide at a Porphyrin Chromium(III) Center

5,10,15,20-Tetraphenylporphyrin chromium chloride (TPPCrCl) with added [Ph₃PNPPh₃]⁺Cl^– (PPN⁺Cl^–) selectively polymerizes lactide (l and <i>rac</i>) dissolved in neat propylene oxide (PO) to yield polylactide (PLA) terminated by the −OCHMeCH₂Cl group. At 0 °C and below, <i>rac</i>-LA yields polymers highly enriched in isotactic tetrads (<i>iii</i>). At 25 °C, some stereoselectivity is lost as transesterification becomes significant, and at 60 °C and above, enchainment of PO leads to the formation of 3,6-dimethyl-1,4-dioxan-2-one by a backbiting mechanism. At 0 °C, after the enchainment of l-(<i>S</i>,<i>S</i>)-LA in neat (<i>R</i>)-(+)-PO, the formation of (3<i>S</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one occurs, while at higher temperatures the ratio of (3<i>S</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one to (3<i>R</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one falls to 3:2