Discrete Cationic Zinc and Magnesium Complexes for Dual Organic/Organometallic-Catalyzed Ring-Opening Polymerization of Trimethylene Carbonate

We describe herein an original approach for the efficient immortal ring-opening polymerization (iROP) of trimethylene carbonate (TMC) under mild conditions using dual-catalyst systems combining a discrete cationic metal complex with a tertiary amine. A series of new zinc and magnesium cationic complexes of the type [{NNO}M](+)[anion](-) ({NNO}(-) = 2,4-di-tert-butyl-6-{[(2'-dimethylaminoethyl)methylamino]methyl}phenolate; M = Zn, [anion](-) = [B(C6F5)(4)](-) (2), [H2N-{B(C6F5)(3)}(2)](-) (3), and [EtB(C6F5)(3)](-) (4); M = Mg, [anion]- = [H2N{B(C6F5)(3)}(2)]- (7)) have been prepared from the corresponding neutral compounds [{NNO}ZnEt] (1) and [{NNO}-Mg(nBu)] (6). Compounds 2-4 and 7 exist as free ion pairs, as revealed by H-1, C-13, F-19, and B-11 NMR spectroscopy in THF solution, and an X-ray crystallographic analysis of the bis(THF) adduct of compound 7, 7 center dot(THF)(2). The neutral complexes 1 and 6, in combination with one equivalent or an excess of benzyl alcohol (BnOH), initiate the rapid iROP of TMC, in bulk or in toluene solution, at 45-60 degrees C (turnover frequency, TOF, up to 25-30 000 mol(TMC)center dot mol(Zn)center dot h(-1) for 1 and 220-240 000 mol(TMC)center dot mol(Mg)center dot h(-1) for 6), to afford H-PTMC-OBn with controlled macromolecular features. ROP reactions mediated by the cationic systems 2/BnOH and 7/BnOH proceeded much more slowly (TOF up to 500 and 3 000 mol(TMC)center dot mol(Zn or Mg)center dot h(-1) at 110 degrees C) than those based on the parent neutral compounds 1/BnOH and 6/BnOH, respectively. Use of original dual organic/organometallic catalyst systems, obtained by adding 0.25 equiv of a tertiary amine such as NEt3 to zinc cationic complexes [{NNO}Zn](+)[anion]- (2-4), promoted high activities (TOF up to 18 300 mol(TMC)center dot mol(Zn)center dot h(-1) at 45 degrees C) giving H-PTMC-OBn with good control over the Mn and M-w/M-n values. Variation of the nature of the anion in 24 did not significantly affect the performance of these catalyst systems. On the other hand, the dual magnesium-based catalyst system 7/NEt3 proved to be poorly effective.