Radical polymerization controlled chain lengths

In copolymerizing acrylonitrile with another monomer, conditions must be controlled in such a way that the reaction produces a polymer having the desired chain structure and length. The reaction takes place in the presence of substances capable of producing free radicals. In addition, certain trace metals that have been found to increase reaction rates offer a means of controlling chain length. When polymerization is carried... 

One of the problems of radical polymerization is high-termination-rate constants by combination ( ) or by disproportionation ( ). In view of this, polymer chains of controlled chain length cannot be formed and this technique is ill-suited for precise control of molecular structure (e.g., in star, comb, dendri-mers, etc.) required for newer apphcations like microelectronics. The major breakthrough occurred when nonterminatmg initiators (which are also stable radicals) were used. Because of its nonterminating nature, this is sometimes called living radical polymerization and the first initiator that was utilized for this purpose was TEMPO (2,2,6,6-tetramethylpiperidinyl-l-oxo) [36,37]. A variation of this is atom-transfer radical polymerization (ATRP) in which, say for styrene, a mixture of 1 mol% of 1-phenyl ether chloride (R—X) and 1 mol% CuCl with two equivalents of bipyridine (bpy) is used for initiation of polymerization. Upon heating at 130°C in a sealed tube, bpy forms a complex with CuCl (bpy/CuCl),... 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

Before any chemistry can take place the radical centers of the propagating species must conic into appropriate proximity and it is now generally accepted that the self-reaction of propagating radicals- is a diffusion-controlled process. For this reason there is no single rate constant for termination in radical polymerization. The average rate constant usually quoted is a composite term that depends on the nature of the medium and the chain lengths of the two propagating species. Diffusion mechanisms and other factors that affect the absolute rate constants for termination are discussed in Section 5.2.1.4. 

The functional semitelechelic HPMA polymers can be readily prepared by free radical polymerization in the presence of functional mercaptans. The functional groups and chain length of the ST polymers can be controlled by the choice of a particular mercaptan and the reaction conditions. ST HPMA polymers can be used for the modification of proteins... 

Anionic polymerization represents a powerful technique for synthesizing polymers with low PDI values, thus providing good control over the chain length. This method leads to less side reactions than radical polymerizations. For instance, unlike in radical polymerization, there is no termination by the combination of two active chains. However, the mechanism is more sensitive to impurities and functional groups, and therefore applicable for only a limited class of monomers. 

The inclusion of stable free radicals such as TEMPO (18, TO ) in free radical polymerizations leads to precise control of chain length by restricting the number of polymerizing chains (equation 96). This process is known as nitroxyl radical mediated polymerization (NRMP). 

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