The Dynamics of Propagating Polymerization Fronts

在未搅拌的介质中，放热反应有时可以支持反应前沿的传播。聚合反应是非常放热的，可以用不稳定化合物(如过氧化氢)热分解产生的自由基引发加成聚合。因此，聚合产生的热可以扩散并导致在邻近区域引发聚合，从而产生传播前沿。例如，在前沿区域进行聚合具有材料合成的潜力，原因有两个：(1)如果聚合是在流动反应器中进行的，并且输入流速等于前沿传播速度，则可以保持高温、强氧化反应区远离壁面。如果氧化区与金属壁接触，就像在标准的连续搅拌釜式反应器(CSTR)中一样，那么氧化的杂质必然会污染产品。然而，在锋面区域，反应区不与金属接触，因此不会发生污染。用这种方法在商业装置上生产超纯聚苯乙烯和聚甲基丙烯酸甲酯是可能的。(2)如果前锋是在静态介质中进行的，则由于前锋的自组织特性，可能形成新的材料。但是，前线的不稳定可能会干扰这些方法。因此，PI计划观察聚合前锋的动态行为，原因有四：(1)聚合前锋提供了易于访问的实验系统来测试反应扩散系统中的热不稳定性理论，包括振荡、自旋模式、倍周期和混沌；(2)聚合前锋表现出有趣的对流不稳定性，但人们对此知之甚少；(3)聚合前锋作为一种材料生产手段很有希望，但不稳定性可能会受到干扰(了解导致不稳定性的条件可以允许设计消除不稳定性或利用它们的实验条件)；(4)建立完整的前向聚合模型，可以对非稳态聚合动力学有一个基本的认识。

Abstract - Pojman - 9319175 An exothermic reaction can sometimes support a propagating front of reaction in an unstirred medium. Polymerization reactions are very exothermic, and addition polymerizations can be initiated with free radicals produced from the thermal decomposition of an unstable compound such as a peroxide. Thus, the heat produced by the polymerization can diffuse and cause the initiation of polymerization in a neighboring region, resulting in a propagating front. Performing polymerization in the frontal regime has, for example, potential for materials synthesis for two reasons: (1) If the polymerization were performed in a flow reactor such that the input flow velocity would equal the front propagation velocity, then it would be possible to maintain the high temperature, strongly oxidizing reaction zone away from the walls. If the oxidizing region is in contact with the metal walls, as in a standard Continuous Stirred Tank Reactor (CSTR), then oxidized impurities will necessarily contaminate the product. However, in the frontal regime, the reaction zone is not in contact with metal and thus, no contamination occurs. It has been possible to produce ultra-pure polystyrene and poly(methyl methacrylate) in a commercial plant using this approach. (2) If the front is carried out in a static medium, then novel materials may be formed because of the self-organizing character of the front. But, instabilities in the fronts can interfere with these approaches. The PI is therefore planning to look at the dynamic behavior of propagating polymerization fronts for four reasons: (1) polymerization fronts offer readily accessible experimental systems to test theories of thermal instabilities in reaction-diffusion systems, including oscillations, spinning modes, period-doubling and chaos; (2) polymerization fronts exhibit interesting convective instabilities, that are poorly understood; (3) polymerization fronts hold promise as a means of materials production but instabilities c an interfere (understanding the conditions that lead to instabilities could allow the design of experimental conditions that eliminate the instabilities or exploit them); and (4) developing complete models of frontal polymerization could provide fundamental understanding of non-steady state polymerization kinetics.