Radical polymerization early polymer research

This paper has provided, we believe, a comprehensive, up-to-date, critical, and objective review on the discovery and the subsequent fast development of living radical polymerizations catalyzed by transition-metal complexes in the period from 1994 to early 2001. These metal-catalyzed living radical polymerizations have rapidly been developing since their discovery in 1994, and the scope of applicable monomers, metal catalysts, and initiators has been expanding. Their advantages include versatility toward a variety of monomers, feasibility in a wide range of reaction conditions, and relatively easy access to the materials. This permits many researchers to use the systems for the precision synthesis of various polymers with controlled architectures. 

In the early stages of ESR application to polymer research, many studies on the identification of free radicals produced by irradiation with ionizing radiation, x-ray, and ultraviolet light were made. Some of the irradiation effects in polymeric materials were considered to originate from the radical processes and, therefore, clear identification of the radicals trapped in irradiated polymers was one of the most important problems at that stage. In this meaning, ESR application was considered to be a very convenient technique for this purpose, because detection and identification of the free radicals bearing unpaired electrons in principle can be done easily by the ESR method without any chemical modification of the materials. 

Anionic polymerization offers very fast polymerization rates because of the long lifetime of polystyryl carbanions. Early research focused on this attribute, most studies being conducted at short reactor residence times (<1 h), at relatively low temperatures (10-50°C), and in low chain-transfer solvents (typically benzene) to ensure that premature termination did not take place. Also, relatively low DPs were typically studied. Continuous commercial free-radical solution polymerization processes to make PS, on the other hand, operate at relatively high temperatures (>100°C), at long residence times (>1.5 h), utilize a chain-transfer solvent (ethylbenzene), and produce polymer in the range of 1000-1500 DP. 

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