Bimetallic chromium catalysts are investigated for the enantioselective polymerization of propylene oxide. The catalyst is composed of two salen chromium species linked by an alkyl chain, the length of which significantly impacts the rate of polymerization. While the use of a chloride initiator on the catalyst resulted in bimodal molecular weight distributions, switching to a trifluoroacetate initiating group and adding a diol chain transfer agent afforded polymers of controllable molecular weight with low, unimodal dispersities. 相似文献
Epoxy resins are currently used for many important applications such as adhesives, encapsulates and ad-vanced composite matrixes. However, the further use of epoxies is limited because of their inherent brittle-ness. Thus, the modifications of epoxy resin… 相似文献
Chain transfer reactions widely exist in the free radical polymerization and controlled radical polymerization, which can significantly influence polymer molecular weight and molecular weight distribution. In this work, the chain transfer reactions in modeling the reversible addition–fragmentation transfer (RAFT) solution copolymerization are included and the effects of chain transfer rate constant, monomer concentration, and comonomer ratio on the polymerization kinetics and polymer molecular weight development are investigated. The model is verified with the experimental RAFT solution copolymerization of styrene and butyl acrylate, with good agreements achieved. This work has demonstrated that the chain transfer reactions to monomer and solvent can have significant impacts on the number‐average molecular weight (Mn) and dispersity (Ð). 相似文献
Summary: A “series” hybrid model based on material balances and artificial neural networks to predict the evolution of weight average molecular weight, , in semicontinuous emulsion polymerization with long chain branching kinetics is presented. The core of the model is composed by two artificial neural networks (ANNs) that calculate polymerization rate, Rp, and instantaneous weight‐average molecular weight, from reactor process variables. The subsequent integration of the material balances allowed to obtain the time evolution of conversion and , along the polymerization process. The accuracy of the proposed model under a wide range of conditions was assessed. The low computer‐time load makes the hybrid model suitable for optimization strategies.
In this work, the functional polylactic acid (PLA) was synthesized using epoxy chain extender (ADR) as a chain extender agent through melt blending method. The effects of ADR content on the molecular structure, thermal properties, and tensile properties of the functional PLA were investigated. Meanwhile, the hydrolytic behavior of the PLA/ADR materials at different hydrolysis temperatures was explored. It was found that ADR effectively regulated the molecular structure of PLA in the molten state and significantly increased the relative molecular weight, storage modulus, and complex viscosity of PLA. In addition, the Cole-Cole diagram results suggested the branched structure of PLA chain expansion systems. Based on mechanical property tests, it was noted that the addition of ADR made the molecular chain form a micro-crosslinked structure. Additionally, the mass loss rate of PLA/1.6ADR (the dosage of ADR was 1.6 wt%) was 14.75% after 14 weeks of hydrolysis under hydrolysis conditions at 58°C, while that of pure PLA was 25.89%. Moreover, the functional PLA/ADR materials exhibited significantly slower decrease rates in molecular weight, melting temperature, and tensile strength, and still maintained intact morphology after 14 weeks of hydrolysis compared to pure PLA. Therefore, the molecular structure of PLA is effectively regulated by ADR, which greatly enhances the hydrolysis resistance and further promotes the range of application of PLA. 相似文献
The effect of polymerization conditions on the molecular weight of polystyrene grafted onto silica obtained from the radical graft polymerization initiated by azo and peroxyester groups introduced onto the surface was investigated. The molecular weight of polystyrene grafted onto silica obtained from the radical graft polymerization initiated by surface azo and peroxyester groups decreased with decreasing monomer concentration and polymerization temperature. The molecular weight of polystyrene was found to be controlled to some extent by the addition of a chain transfer agent. The molecular weight of grafted chain on silica surface obtained from the graft polymerization initiated by surface radicals formed by photodecomposition of azo groups was considerably smaller than that by thermal decomposition. The number of grafted polystyrene in photopolymeriztion, however, was much larger than that in thermal polymerization. These results are explained by the blocking of surface radicals formed on the silica surface by previously grafted polymer chain: when the decomposition of surface azo and peroxyester groups proceed instantaneously at the initial stage of the polymerization, the number of grafted polymer chains increased. 相似文献