Initiators, radical polymerization

Radical initiators Radical polymerization Radical scavengers... 

Dimethyl sulfonium-2-pyridyl carbonyl methylide (Ypy-5) [65] initiated radical polymerization of styrene in dimethyl sulfoxide at 85 0.1°C for 6 h under a nitrogen blanket. (See Table 4.)... 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

Hyperbranched poly(ethyl methacrylate)s prepared by the photo-initiated radical polymerization of the inimer 13 were characterized by GPC with a lightscattering detector [51]. The hydrodynamic volume and radius of gyration (i g) of the resulting hyperbranched polymers were determined by DLS and SAXS, respectively. The ratios of Rg/R are in the range of 0.75-0.84, which are comparable to the value of hard spheres (0.775) and significantly lower than that of the linear unperturbed polymer coils (1.25-1.37). The compact nature of the hyperbranched poly(ethyl methacrylate)s is demonstrated by solution properties which are different from those of the linear analogs. 

Synthesis. Graft copolymer was formed in aqueous solution by ceric-ion-initiated, radical polymerization of monomer on starch. Polymerization was conducted in an inert, atmosphere. Details of the synthesis procedure may be found in references 41 to 43 In recovering the polymer product, freeze drying was used with care since freeze drying produces a more dissolvable and useful product but can degrade polymers with molecular weights of 1 million or more. Poly(starch-g-(1-amidoethylene)) Poly(starch-g-(1-amidoethylene))... 

The tetraphenylethanes described above are symmetrical compounds used to generate the same two radicals by dissociation, while pentaphenylethane (28) is an unsymmetrical derivative, giving two different radicals, triphenylmethyl and diphenylmethyl radicals [138]. The former cannot initiate radical polymerization, but the latter is available as an initiating radical to produce the polymer 28, which can function as the polymeric iniferter [106]. 

Poly(styrene) and PMMA were synthesized from their respective monomers using azobisisobutyronitrile-initiated radical polymerization in benzene. Four freeze-pump-thaw cycles were used to degas the monomer solutions and polymerization was carried out for 48 hours at 60°C. The polymers were purified by multiple reprecipitations from dichloromethane into methanol. Films of these polymers were prepared and found to be free of any fluorescent impurity. 

The combination of a variety of inorganic reductants and inorganic oxidants initiates radical polymerization, for example... 

Radicals produced in Eq. 8-31 are capable of initiating radical polymerizations with some monomers, for example, vinyl chloride. 

Table 3.5. Some common oxidizing and reducing agents that are suitable for initiating radical polymerization by redox reactions...

Radical Polymerization. In this modification, a functional group like an azo or peroxy group, being able to initiate radical polymerization, must be introduced into the particle surface (Scheme 12.1.2). 

Fig. 13 Structures of compounds discussed in the context of 2P-initiated radical polymerization...

Fig. 15 Three-dimensional microstructures produced by 2P-initiated radical polymerization of triacrylates using q.ll or r.l as initiators a photonic bandgap structure b close-up view of the structure in (a) c tapered waveguide structure and d array of cantilevers. Reproduced with permission from [21]...

Polymer-bound dinitrogen complexes containing Mn have been prepared [8] directly from the polymer bound (n5-vinylmethyl-cyclopentadienyl) tricar-bonylmanganese (VCM) and molecular nitrogen in THF/benzene. The metal-containing base polymer may be readily prepared by AIBN initiated radical polymerization of VCM with styrene or /V-vinylpyrrolidone (Fig. 4). 

Fristrup CJ, Jankova K, Hvilsted S (2009) Surface-initiated radical polymerization -a technique to develop biofunctional coatings. Soft Matter 23 4623-4634... 

The Ru(II) catalysts currently used for olefin hydroarylation reactions are limited in terms of incorporation of substituents into the olefin substrate. For example, attempted hydrophenylation of isobutylene with TpRu(CO)(NCMe)(Ph) as catalyst does not yield new organic products. In addition, extension of catalysis to hetero-functionalized olefins using the TpRu(CO)(NCMe)(aryl) systems has not been successful. For electron-deficient olefins (e.g. styrene, methyl methacrylate, acrylonitrile) the TpRu(II) complexes initiate radical polymerization of the olefin in transformations that probably involve a Ru(III/II) redox change [4]. The... 

Trialkyl boron was first claimed as a new anionic initiator for the polymerization of vinyl compounds (264), although it was rather improbable in view of the low ionic character of the boron-carbon bond. The error was quickly corrected when it was shown that free radicals were involved (265, 266) and that oxygen, peroxides, silver salts and copper salts were co-catalysts (262, 267). Aluminum alkyls can also initiate radical polymerizations in the presence of oxygen (267,262) but, as in the case of zinc, cadmium or boron alkyls, the products were not stereoregular. Thus, complexing between catalyst and monomer probably does not occur. 

It is evident that the tendency for free radical polymerization is a function of the monomer. Natta (287) stated that these radicals can initiate polymerization of styrene and diolefins to high molecular weight products but not that of aliphatic alpha olefins. North (339) showed that the decomposition of phenyl tri-isopropoxy titanium initiates radical polymerization of styrene but not of ethylene. 

Cage effects also account for the fact that not all the radicals produced from the decomposition of initiators such as azobisisobutyronitrile (AIBN) are effective in initiating radical polymerizations. In the somewhat simplified reaction Scheme (5-168) depicting the thermolysis of AIBN, two types of cyanopropyl radicals are shown, one still within the solvent cage, whereas the others have reached their statistical separation... 

Kinetics of initiated radical polymerization of styrene is well studied at the initial stages. At the high-conversion of polymerization the growth of viscosity of media- influences the mechanism and kinetics of process greatly (12),In our research we have attempted to obtain tne empirical dependence of some rate constants of initiated polymerization of styrene on conversion with the aim of using them at mathematical simulation of the process. 

Fe(III) species can also be employed in a so-called reverse or alternative atom-transfer radical polymerization (section II.B.9). A mixture of FeCl3 and PPh3 can mediate a controlled polymerization of MMA in the presence of AIBN to give similarly narrow MWDs MJMn = 1.1—1.3).80 An ammonium halide such as />Bu4NBr can be employed in place of PPh3 as a ligand for FeBr3 in the AIBN-initiated radical polymerization.74... 

Side reactions such as termination and transfer were investigated in the polymerizations of styrene,291 acrylates,292 and methacrylates.293 The occurrence of thermally initiated radical polymerizations was observed in the copper-catalyzed styrene polymerization, while the resulting polymer chain can be converted into the dormant polymer terminal via abstraction of halogens from the persistent metal radical in higher oxidation states.294... 

Polymer brushes can be grown from gold surface in a similarly controlled manner by the metal-catalyzed surface-initiated radical polymerization. The attachment of the initiator moiety can be attained via the thiol group as in S-6455 and S-7.456... 

Another example is the copper-catalyzed surface-initiated radical polymerization of MMA from S-7 at room temperature without addition of free initiator. The molecular weights and MWDs of the polymers were directly measured after removing the brushes from the surface. For example, the surface with 40 nm thickness had Mn of 68900 and MWDs of 1.45. A high graft density (180 A2/chain) and decreased surface roughens (0.54 nm) were observed. This method is free from solution and thermal polymerizations due to the absence of free initiators and a low polymerization temperature, which permits a simple washing step without Soxhlet extraction. 

Other variables in the imprinting process are stoichiometry and temperature, which are particularly important in the cases of noncovalent imprinted polymers. Lower temperatures and higher functional monomer to template ratios stabilize noncovalent prepolymerization complexes, resulting in noncovalent MIPs with higher capacities and selectivities. The imprinting process can be carried out at lower temperatures by either UV initiated radical polymerizations or by using azo-based radical initiators. 

Liu and O Brien [12] prepared extremely stable unilamellar vesicles from two different cross-linkable lipids, l-palmitoyl-2-[14-acryloxy-2,4-tetradecadienoic]-s/i-glycerol-3-phosphocholine (Fig. 16) and DenSorbPC (Fig. 6). Light scattering measurements showed that after redox-initiated radical polymerization, both types of vesicles were stable to the addition of 12 molar equivalents of Triton X-100... 

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