Model catalysts having specially designed active species on support surfaces were employed to understand relationships between the structure/state of active sites and their catalytic performances for two classes of industrial olefin polymerization catalysts: Ziegler‐Natta and Phillips catalysts. Propylene polymerization using a TiCl3/MgCl2 model Ziegler‐Natta catalyst clarified an important role of the Ti dispersion state for the polymer stereo & chemical structures. Model Phillips catalysts having monochromate and dichromate structures on silica support imparted knowledge that dinuclear species is more responsible for the production of α‐olefin comonomer in ethylene polymerization, thus giving more branched polyethylene.
An improved stopped‐flow (SF) technique is employed to clarify the origin of kinetics in propylene polymerization with a Mg(OEt)2‐based Ziegler–Natta catalyst. Polymerization in the range of 0.1–5 s exhibits a kinetic transition from a linear development to a build‐up‐type development of the yield. It is found that a lower alkylaluminum concentration leads to a lower activity in the linear regime, whereas the extent of the activation becomes greater in the build‐up regime. The origin of these kinetic behaviors is studied using scanning electron microscopy (SEM) for catalyst/polymer particles and cross‐fractionation analyses for polymer structures. It is found that the kinetic transition mainly arises from the fragmentation of the catalyst particles and resultant increase in the active site concentration. The fragmentation manner strongly depends on the alkylaluminum concentration, which affects not only the amount, but also the placement of initial polymer formation. The nature of the active sites varies as a result of an aging effect with alkylaluminum: their stereospecificity, propagation rate constant, and tolerance for chain transfer reaction increase as the polymerization progresses.
High-performance water-soluble polymers have a wide range of applications from engineering materials to biomedical plastics. This article discusses the synthesis of water-soluble polyimide from bio-based monomers.
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