Polymerization physically controlled

Many transition metals and their compounds with organic ligands initiate the polymerization of alkenes and/or dienes. Some of them do not need any special treatment to this end while others require the presence of some organic or mineral compound or a special physical modification. In contrast to ZN catalysts, they are active without an organometal of Groups I—III. They are commonly known as metal alkyl free (MAF) catalysts. Many of their features are, of course, in common with ZN catalysts. MAF catalysts initiate stereoselectively controlled polymerization. Even less is known of their operating mechanism than that of ZN catalysts. It is assumed that propagation also occurs on the transition metal-carbon bond. 

TABLE 1. Physical properties of polymers in the controlled polymerization of styrene using the Step 3 product. 

A similar, physically bound CuBr catalyst on a silica gel support (L-29) was also employed for MMA polymerization.149 The polymers had narrow MWDs (MJMn 1.3), but the molecular weights were higher than the calculated values. The recycled catalysts have a lower activity but lead to better control of molecular weights i.e., the Mn agreed well with the calculated values, and the MWDs were narrower MJ Mn 1.2). The physically supported catalysts were further employed for the synthesis of end-functionalized polymers.150 When physically supported silica gel catalysts are packed into a continuous column reactor, a controlled polymerization is possible.151... 

Nitroxide mediated SFRP, DPE mediated polymerization, ATRP, RAFT polymerization, etc. achieve polymerization control through the use of kinetic mediators or transfer agents, which protect a propagating free radical from imdesirable transfer and termination reactions. The emulsion block copolymer method is unique in that it does not require the use of any chemical mediators to achieve this control. Polymerization control is achieved by physically trapping radicals by... 

Natta also serendipitously isolated polypropylene fractions that exhibited novel elastomeric behavior that he proposed were a manifestation of properties linked to an unique isotactic-atactic stereoblock polypropylene (sbPP) microstructure (Vll in Figure 3.1) [13]. In this model, the elastic properties of sbPP were hypothesized to originate with interchain associations of hard, crystalline isotactic) domains that function as nonbonded physical crosslinks within an amorphous atactic) matrix, with the former serving to dimensionally restore the material upon the removal of a deforming strain. Unfortunately, this sbPP material was not the principal product of a controlled polymerization for which a sound mechanism could be established to account for chain growth that, in this case, must proceed in alternating stereoselective and nonselective fashion. Indeed, both sPP and sbPP... 

Nature is replete with examples of functional nanostructures capable of completing complex tasks. These structures are composed of well-defined linear polymer chains folded into precise, three-dimensional shapes. An obvious yet unmet goal of chemists is to reproduce the functionality of natural systems using synthetic polymers. This warrants a tour de force involving modern controlled polymerization techniques and postpolymerization modification strategies in combination with current theories of polymer physics. Areas such as catalysis,sensing, nanoreactors, and nanomedicine ° stand to benefit from advances in this research. 

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