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(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( n Bu)] ( 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 3 000 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.