Discrete cationic zinc and magnesium complexes for dual organic/organometallic-catalyzed ring-opening polymerization of trimethylene carbonate

Abstract

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(C(6)F(5))(4)](-) (2), [H(2)N-{B(C(6)F(5))(3)}(2)](-) (3), and [EtB(C(6)F(5))(3)](-) (4); M = Mg, [anion](-) = [H(2)N{B(C(6)F(5))(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 (1)H, (13)C, (19)F, and (11) B NMR spectroscopy in THF solution, and an X-ray crystallographic analysis of the bis(THF) adduct of compound 7, 7⋅(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 °C (turnover frequency, TOF, up to 25-30,000 mol(TMC)⋅mol(Zn)⋅h(-1) for 1 and 220-240,000 mol(TMC)⋅mol(Mg)⋅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 3000 mol(TMC)⋅mol(Zn or Mg)⋅h(-1) at 110 °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 0.2-5 equiv of a tertiary amine such as NEt(3) to zinc cationic complexes [{NNO}Zn](+) [anion](-) (2-4), promoted high activities (TOF up to 18,300 mol(TMC)⋅mol(Zn)⋅h(-1) at 45 °C) giving H-PTMC-OBn with good control over the M(n) and M(w)/M(n) values. Variation of the nature of the anion in 2-4 did not significantly affect the performance of these catalyst systems. On the other hand, the dual magnesium-based catalyst system 7/NEt(3) proved to be poorly effective.