Quantitative Validation of the Living Coordinative Chain-Transfer Polymerization of 1-Hexene Using Chromophore Quench Labeling

In this report, we apply the chromophore quench-labeling (CQL) technique to living, homogeneous polymerization catalysts and living coordinative chain-transfer polymerization (LCCTP) conditions. We observe 100% active site counts via CQL for 1-hexene polymerization as catalyzed by two different cyclopentadienyl amidinate (CPAM) group 4 metal complexes, both previously reported to be living polymerization catalysts. We obtain a rate constant for the propagation of 1-hexene catalyzed by the CPAM hafnium complex. Selective iodinolysis and H-1 NMR analysis of polymer chain ends enable the quantification of Zn-polymeryls; at low conversions (similar to 10% conversion), nearly 2 polymer chains per Zn are observed. Through Mayo analysis and kinetic simulations, we ks) estimate the rate of Hf-polymeryl for Zn-ethyl exchange in polymerization. Beyond active site counting, the chromophore quench-labeling method enables the observation of catalyst-bound polymeryl molecular mass distributions (MMDs) using gel permeation chromatography (GPC) with an inline UV detector (UV-GPC). Comparison of the MMDs of catalyst-bound polymeryls and all polymer chains (measured using refractive index detection, RI-GPC) reveals important mechanistic details. With the exception of early time points in the reaction, we observe excellent overlap between the UV-GPC and the RI-GPC traces. These results reveal that equilibration of polymer chains on Hf and Zn occurs rapidly in the LCCTP of 1-hexene using the CPAM hafnium complex as the catalyst and diethyl zinc as the chain-transfer agent. Kinetic simulations indicate that the Hf-polymeryl for Zn-polymeryl exchange leads to the good overlap between UV-GPC and RI-GPC traces and a crude estimate for the rate constant of this process.