Free radical initiators homopolymer formation

Consider the free radical reactions during formation of homopolymer P from monomer M. Let the initiator be denoted by I ... 

While reverse initiation simplified setting up a miniemulsion ATRP, it limited the range of materials that could be prepared and the catalysts that could be employed in the reaction. In a reverse initiation procedure one cannot independently adjust catalyst level and DPo,ge since formation of both the lower oxidation state catalyst complex and the ATRP initiator depends on the amount of standard free radical initiator added to the reaction. Therefore, a new initiation system was developed, SR NI, that involved addition of both an ATRP initiator and a free radical initiator. SR NI allowed the preparation of block and star copolymers in a miniemulsion with low levels of an active catalyst and low levels of surfactant.The use of SR Nl provided stable latexes with high soUds content. As shown in the 2D chromatography plot in Figure 13(a), only 4.5% of linear homopolystyrene was present in the final polymer when a tri-arm poly(methyl acrylate) macroinitiator was chain extended by styrene in an SR NI it miniemulsion process. The homopolymer results from the... 

This technique is based in the fact that when cellulose is oxidized by ceric salts such as ceric ammonium nitrate Ce(NH4)2(N03)6 free radicals capable of initiating vinyl polymerization are formed on the cellulose. However, the possibility remains that the radical formed is an oxygen radical or that the radical is formed on the C-2 or C-3 instead of the C-6 carbon atom. Another mechanism, proposed by Livshits and coworkers [13], involves the oxidation of the glycolic portion of the an-hydroglucose unit. Several workers [14,15], however, have found evidence for the formation of some homopolymer. In the ceric ion method free radicals are first generated and are then capable of initiating the grafting process [16-18]. 

Collagen contains alcoholic groups in the hydroxyproline, serine, threonine, and hydroxy lysine moieties. Free radicals are probably formed at such sites which, in the presence of a vinyl monomer, serve to initiate grafted side chains. Since the free radicals are formed on the side chains of the substrate, a high grafting efficiency and a minimum amount of homopolymer formation compared with other redox systems can be expected. 

Polymers were prepared by AIBN-initiated free-radical polymerization in degassed toluene at 6O C. With the exception of poly(4-aceto-l-naphthyl methacrylate), the polymers precipitated on formation. Polymers were purified by reprecipitation from CHCl 3 into methanol. Copolymer compositions were determined by UV spectroscopy using the extinction coefficients of the corresponding homopolymers and of 2,4-di aceto-l-naphthyl methacrylate monomer as references. Tables I and II list the properties of the homopolymers and copolymers used in the present study. 

Gak et al. (100, 102) examined the nature of free radicals in polyoxymethylene. When formaldehyde homopolymer, PFA-OH was irradiated at —196° C, the initial rate of formation of the free radical — CH20- was found to be proportional to the square of the incident light intensity, which indicates the two-photon process of —CH20-radical formation. When irradiated PFA-OH was kept in the dark, some increase occurred in the concentration of -CH20- and there was a simultaneous fall in the concentration of -CH2 at temperatures above —196° C. The maximum rate was attained in the temperature range —130° C----123° C. This is caused by reaction (41). 

Better control of grafting and less homopolymer formation is achievable in ionic reactions than can be obtained in free-radical reactions. Anionic grafting via backbone initiation has been demonstrated (22) with caprolacteim on macromolecular ester sites of styrene/methyl methacrylate copolymers. Cationic grafting of isobutylene onto poly(vinyl chloride) with the aid of aluminum alkyl has been carried out by J. P. Kennedy (23). 

The grafting from procednre requires active sites to be created on the polymer chain capable of initiating the growth of otha- chain branches comprising a second monomer. Free-radical sites can be formed by direct or mntual radiation with y-rays of a polymer in the presence of the second monomer. This is a simple method but can also lead to homopolymer formation. 

Thermal cleavage of the above peroxides results in macromolecules with free-radicals sites. Hydroxy radicals also form and initiate formations of homopolymers. Decompositions of the peroxides by redox mechanisms, however, increase the yields of graft copolymers, but do not stop all formations of hydroxy radicals ... 

These techniques rely upon high shear to cause bond scissions. Ruptured bonds result in formations of free-radical and ionic species. When this application of shear is carried out in the presence of monomers, block copolymers can form. This approach is exploited fairly extensively. Such cleavages of macromolecules can take place during cold mastication, milling, and extrusion of the polymers in the viscoelastic state. Both homolytic and heterolytic scissions are possible. The first yields free-radical and the second ionic species. Heterolytic scissions require more energy but should not be written off as completely unlikely." Early work was done with natural rubber. It swells when exposed to many monomers and forms a viscoelastic mass. When this swollen mass is subjected to shear and mechanical scission, the resultant radicals initiate polymerizations. The mastication reaction was shown to be accompanied by formation of homopolymers. Later, the technique was applied to many different polymers with many different monomers. ... 

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