Low molecular reactions are frequently accompanied by side reactions producing a whole spectrum of by-products whose removal in purification operations generally causes little difficulty because the major and side products usually differ sufficiently in their properties. Hence detailed kinetic studies in low molecular chemistry can claim industrial interest only insofar as they aim to reduce the rate of formation of side products. The situation is completely different in polymer chemistry, where the terms major and side product no longer apply. Instead, one obtains mixtures of similar and/or isomeric molecules which differ so little with regard to molecular weight and structure that resolution into pure substances or isolation of pure substances becomes practically impossible. However, since all partial reactions exert a direct influence on the product spread, i.e. the composition and properties of a product mixture are modified by any change in operating conditions, detailed kinetic analyses embracing all relevant partial reactions are an essential prerequisite for the production of defined polymeric products on any scale. This progress report illustrates the importance of kinetics for radical polymerization with the aid of selected examples. 相似文献
Batch radical polymerization of non‐ionized methacrylic acid, 30 wt.‐% in aqueous solution, has been studied at 50 °C and ambient pressure with 2‐mercaptoethanol (ME) as the chain‐transfer agent (CTA). Initial polymerization rate decreases with CTA concentration, which has been varied up to 20 mol‐%. A kinetic model is presented which includes chain‐length‐dependent termination and uses an empirical function to account for the dependence of termination rate on both monomer conversion and molar mass of the polymeric product. In conjunction with PREDICI simulation, this model affords for an adequate representation of the measured monomer conversion vs. time profiles.
There is a fascinating interplay between termination and transfer that shapes the kinetics of radical polymerization (RP). In one limit all dead-chain formation is by termination, in the other by transfer. Because of chain-length-dependent termination (CLDT), the rate law for RP takes a different form in each limit. However, common behavior is observed if one instead considers how the average termination rate coefficient varies with average degree of polymerization. Examples are given of using these principles to understand trends in actual RP data, and it is also demonstrated how to extract quantitative information on CLDT from simple steady-state experiments. 相似文献
