Center for Polymer Chemistry, Department of Chemistry, University of Houston , Houston, Texas 77204-5003, United States.
Department of Chemistry, University of North Carolina at Chapel Hill , Chapel Hill, North Carolina 27599-3290, United States.
J Am Chem Soc. 2017 Nov 8;139(44):16013-16022. doi: 10.1021/jacs.7b10281. Epub 2017 Oct 30.
Copolymerizations of ethylene with vinyltrialkoxysilanes using cationic (α-diimine)Ni(Me)(CHCN) complexes 4a,b/B(CF) yield high molecular weight copolymers exhibiting highly branched to nearly linear backbones depending on reaction conditions and catalyst choice. Polymerizations are first-order in ethylene pressure and inverse-order in silane concentration. Microstructural analysis of the copolymers reveals both in-chain and chain-end incorporation of -Si(OR) groups whose ratios depend on temperature and ethylene pressure. Detailed low-temperature NMR spectroscopic investigations show that well-defined complex 3b (α-diimine)Ni(Me)(OEt) reacts rapidly at -60 °C with vinyltrialkoxysilanes via both 2,1 and 1,2 insertion pathways to yield 4- and 5-membered chelates, respectively. Such chelates are the major catalyst resting states but are in rapid equilibrium with ethylene-opened chelates, (α-diimine)Ni(R)(CH) complexes, the species responsible for chain growth. Chelate rearrangement via β-silyl elimination accounts for formation of chain-end -Si(OR) groups and constitutes a chain-transfer mechanism. Chelate formation and coordination of the Ni center to the ether moiety, R-O-Si, of the vinylsilane somewhat decreases the turnover frequency (TOF) relative to ethylene homopolymerization, but still remarkably high TOFs of up to 4.5 × 10 h and overall productivities can be achieved. Activation of readily available (α-diimine)NiBr complexes 2 with a combination of AlMe/B(CF)/[PhC][B(CF)] yields a highly active and productive catalyst system for the convenient synthesis of the copolymer, a cross-linkable PE. For example, copolymers containing 0.23 mol % silane can be generated at 60 °C, 600 psig ethylene over 4 h with a productivity of 560 kg copolymer/g Ni. This method offers an alternative route to these materials, normally prepared via radical routes, which are precursors to the commercial cross-linked polyethylene, PEX-b.
使用阳离子（α-二亚胺）Ni(Me)(CHCN)配合物 4a,b/B(CF) 将乙烯与乙烯基三烷氧基硅烷共聚，得到的高分子量共聚物具有高度支化至近乎线性的骨架，这取决于反应条件和催化剂的选择。聚合反应对乙烯压力呈一级反应，对硅烷浓度呈逆一级反应。对共聚物的微观结构分析表明，-Si(OR)基团同时存在于链内和链端，其比例取决于温度和乙烯压力。详细的低温 NMR 光谱研究表明，在-60°C 下，具有确定结构的配合物 3b（α-二亚胺）Ni(Me)(OEt)与乙烯基三烷氧基硅烷快速反应，通过 2,1 和 1,2 插入途径分别生成 4-和 5 元螯合物。这些螯合物是主要的催化剂休眠状态，但与乙烯开环的螯合物（α-二亚胺）Ni(R)(CH)配合物处于快速平衡状态，后者是链增长的物种。通过β-硅基消除进行的螯合物重排导致链端-Si(OR)基团的形成，并构成链转移机制。螯合物的形成以及镍中心与乙烯基硅烷的醚部分 R-O-Si 的配位，相对乙烯均聚而言，略微降低了周转频率（TOF），但仍可达到高达 4.5×10 h 的非常高的 TOF 和整体产率。用 AlMe/B(CF)/[PhC][B(CF)] 组合激活易得的（α-二亚胺）NiBr 配合物 2，可得到用于方便合成共聚物（可交联的 PE）的高活性和高产率的催化剂体系。例如，在 60°C、600 psig 乙烯下，4 小时内可生成含有 0.23 mol%硅烷的共聚物，产率为 560 kg 共聚物/g Ni。该方法为这些材料提供了一种替代途径，这些材料通常通过自由基途径制备，是商业交联聚乙烯 PEX-b 的前体。
