Living Coordinative Chain Transfer Polymerization of 1-Alkenes

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A novel polymerization method, living coordinative chain transfer polymerization (CCTP), was recently developed with monocyclopentadienyl monoamidinate (CpAm) Group 4 metal complexes, which were previously applied for the traditional living coordination polymerization (TLCP) and stereomodulated degenerative transfer living (SDTL) coordination polymerization. In addition to a CpAm precatalyst and a cocatalyst, a chain transfer agent (CTA) was also added to the CCTP system. The CTA undergoes a rapid and reversible chain transfer with the Group 4 metal catalyst, which results in chain growth on an inexpensive main group metal alkyl. This new CCTP technique provides a practical solution towards the intrinsic problem, one chain per catalytic center, for a TLCP polymerization process.

The first example of living CCTP was provided with ZnEt2 via Cp*HfMe2[N(Et)C(Me)N(Et)] (35) activated by [PhNHMe2][B(C6F5)4]. It was very efficient for the polymerization of ethene, propene, higher α-olefins and α,ω-nonconjugated dienes, and copolymerization of these monomers. The (co)polymers obtained possess very narrow polydispersity (PDI 1.03-1.10) and tunable molecular weights by several factors including a wide range of equivalents of ZnEt2. The living property of this CCTP system was further confirmed by kinetic studies and end group functionalization. The quantitative chain extension on zinc was clearly shown by in situ NMR spectroscopy. The coordinative chain shuttling polymerization (CCSP) was also studied while binary precatalysts, cocatalysts, or chain transfer agents were applied.

The TLCP, SDTL and CCTP of propene via some new CpAm complexes other than 35 were also studied, including the zirconium analogue of 35, CpZrMe2[N(Et)C(Me)N(Et)], and a series of binuclear complexes which have the common structure of [CpZrMe2]2[N(tBu)C(Me)N(CH2)xNC(Me)N(tBu)] (26, x = 8; 27, x = 6; 28, x = 4). The formamidinate precatalyst Cp*ZrMe2[N(tBu)C(H)N(Et)] (12) was also covered in this study. Under both SDTL and CCTP conditions, the binuclear catalysts showed a tether-length dependent chain transfer process as observed by the polymerization results especially by the tacticity of resulting polypropene. Using CCSP process, multi-stereoblock polypropene was successfully prepared via 12 and 27. The structures and properties of these new complexes and (co)polymers were fully characterized by X-ray crystallography, elemental analysis, GPC, DSC, GC and high field NMR spectroscopy.