Mechanistic Studies of Hafnium-Pyridyl Amido-Catalyzed 1‑Octene Polymerization and Chain Transfer Using Quench-Labeling Methods

Chromophore quench-labeling applied to 1-octene polymerization as catalyzed by hafnium-pyridyl amido precursors enables quantification of the amount of active catalyst and observation of the molecular weight distribution (MWD) of Hf-bound polymers via UV-GPC analysis. Comparison of the UV-detected MWD with the MWD of the “bulk” (all polymers, from RI-GPC analysis) provides important mechanistic information. The time evolution of the dual-detection GPC data, concentration of active catalyst, and monomer consumption suggests optimal activation conditions for the Hf pre-catalyst in the presence of the activator [Ph3C]­[B­(C6F5)4]. The chromophore quench-labeling agents do not react with the chain-transfer agent ZnEt2 under the reaction conditions. Thus, Hf-bound polymeryls are selectively labeled in the presence of zinc-polymeryls. Quench-labeling studies in the presence of ZnEt2 reveal that ZnEt2 does not influence the rate of propagation at the Hf center, and chain transfer of Hf-bound polymers to ZnEt2 is fast and quasi-irreversible. The quench-label techniques represent a means to study commercial polymerization catalysts that operate with high efficiency at low catalyst concentrations without the need for specialized equipment.发色团淬灭标记法（chromophore quench-labeling）应用于铪-吡啶酰胺类前驱体（hafnium-pyridyl amido precursors）催化的1-辛烯聚合反应，可实现活性催化剂用量的定量分析，并可通过紫外凝胶渗透色谱（UV-GPC）分析观测铪键合聚合物的分子量分布（MWD）。将紫外检测得到的分子量分布与示差折光凝胶渗透色谱（RI-GPC）分析所得“整体”聚合物（即所有聚合产物）的分子量分布进行对比，可获取关键反应机理信息。双检测凝胶渗透色谱数据、活性催化剂浓度与单体消耗的时间演化规律，揭示了活化剂[Ph3C][B(C6F5)4]存在下该铪预催化剂的最优活化条件。该发色团淬灭标记试剂在反应条件下不会与链转移剂二乙基锌（ZnEt2）发生反应，因此可在锌键合聚合物存在下实现铪键合聚合物的选择性标记。在二乙基锌存在下开展的淬灭标记研究显示，二乙基锌不会影响铪中心的链增长速率，且铪键合聚合物向二乙基锌的链转移过程快速且准不可逆。该淬灭标记技术为研究在低催化剂浓度下高效运行的商用聚合催化剂提供了一种无需专用设备的研究路径。