Claudino Mauro, Zhang Xinpeng, Alim Marvin D, Podgórski Maciej, Bowman Christopher N
Department of Chemical and Biological Engineering, University of Colorado, UCB 596, Boulder, Colorado 80309, United States.
Faculty of Chemistry, Department of Polymer Chemistry, MCS University, pl. Marii Curie-Skłodowskiej 5, 20-031 Lublin, Poland.
Macromolecules. 2016 Nov 8;49(21):8061-8074. doi: 10.1021/acs.macromol.6b01605. Epub 2016 Oct 18.
A kinetic mechanism and the accompanying mathematical framework are presented for base-mediated thiol-Michael photopolymerization kinetics involving a photobase generator. Here, model kinetic predictions demonstrate excellent agreement with a representative experimental system composed of 2-(2-nitrophenyl)propyloxycarbonyl-1,1,3,3-tetramethylguanidine (NPPOC-TMG) as a photobase generator that is used to initiate thiol-vinyl sulfone Michael addition reactions and polymerizations. Modeling equations derived from a basic mechanistic scheme indicate overall polymerization rates that follow a pseudo-first-order kinetic process in the base and coreactant concentrations, controlled by the ratio of the propagation to chain-transfer kinetic parameters (/) which is dictated by the rate-limiting step and controls the time necessary to reach gelation. Gelation occurs earlier as the / ratio reaches a critical value, wherefrom gel times become nearly independent of /. The theoretical approach allowed determining the effect of induction time on the reaction kinetics due to initial acid-base neutralization for the photogenerated base caused by the presence of protic contaminants. Such inhibition kinetics may be challenging for reaction systems that require high curing rates but are relevant for chemical systems that need to remain kinetically dormant until activated although at the ultimate cost of lower polymerization rates. The pure step-growth character of this living polymerization and the exhibited kinetics provide unique potential for extended dark-cure reactions and uniform material properties. The general kinetic model is applicable to photobase initiators where photolysis follows a unimolecular cleavage process releasing a strong base catalyst without cogeneration of intermediate radical species.
本文提出了一种涉及光碱发生器的碱介导硫醇-迈克尔光聚合动力学的动力学机制及相应的数学框架。在此，模型动力学预测结果与一个典型实验体系表现出极佳的一致性，该实验体系由2-(2-硝基苯基)丙氧基羰基-1,1,3,3-四甲基胍（NPPOC-TMG）作为光碱发生器，用于引发硫醇-乙烯基砜迈克尔加成反应和聚合反应。从基本机理方案推导得出的建模方程表明，总聚合速率在碱和共反应物浓度方面遵循准一级动力学过程，受链增长与链转移动力学参数之比（/）控制，该比值由限速步骤决定，并控制达到凝胶化所需的时间。当/比值达到临界值时，凝胶化更早发生，此后凝胶时间几乎与/无关。该理论方法能够确定由于质子性污染物的存在导致光生碱的初始酸碱中和而对反应动力学产生的诱导时间效应。这种抑制动力学对于需要高固化速率的反应体系可能具有挑战性，但对于那些需要在动力学上保持休眠直至被激活的化学体系却很重要，尽管最终代价是聚合速率较低。这种活性聚合的纯逐步增长特性及其表现出的动力学为延长暗固化反应和获得均匀的材料性能提供了独特的潜力。该通用动力学模型适用于光解遵循单分子裂解过程释放强碱催化剂且不联产中间自由基物种的光碱引发剂。
