Transfer grafting

In the presence of monomers, of course, graft and block copolymers are formed. The polymerization is initiated by the macro free radicals generated by mechanical stresses (block copolymers) or by free radicals obtained by intermolecular transfer (graft polymers), such as... 

The crosslinking of the rubber is important to maintain the structural integrity during the mixing and curing processes. In order to favor internal cavitation of the rubbery core and a good stress transfer, grafting of the inner shell with the rubbery core is necessary. 

NR and synthetic diene based elastomers are a very versatile material for grafting. Chain transfer grafting may be carried out by means of a functionalized monomer-initiator system in a rubber solution, in the latex phase, in swollen rubber or during vulcanization. The chain transfer can be accompanied in diene-based polymers by addition reactions involving both the starting radicals and growing polymer radicals [217]. 

Barner, L. Li, C.E. Hao, X. Stenzel, M.H. Barner-Kowollik, C. Davis, T.P. Synthesis of core-shell poly(divinylbenzene) microspheres via reversible addition fragmentation chain transfer graft polymerization of styrene. J. Polym. Sci. A 2004,42 (20), 5067-5076. 

Pendant nitro groups are also effective in chain-transfer grafting reactions. Thus, graft copolymers of polystyrene with cellulose acetate p-nitrobenzoate and with poly(vinyl p-nitrobenzoate) form readily. Nitro groups appear to be more effective in formations of graft copolymers by a radical mechanism than are double bonds located as pendant groups. " ... 

A study on chain transfer grafting of butyl acrylate onto PVC in emulsion following the synthesis of graft copolymers showed that for the chain transfer method, the grafting efficiency does not increase as Flory s chain transfer theory predicted. 

Graft copolymers may be prepared in three general ways, namely transfer grafting, irradiation grafting and chemical grafting. Transfer grafting is most commonly free radical initiated. Typically, a vinyl or diene polymer is treated with a peroxide in the presence of a vinyl monomer. Transfer occurs between the polymer chain and radicals derived from the initiator the resultant polymer chain radical then initiates polymerization of the monomer, e.g. ... 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

The anionic polymerization of methacrylates using a silyl ketene acetal initiator has been termed group-transfer polymerization (GTP). First reported by Du Pont researchers in 1983 (100), group-transfer polymerization allows the control of methacrylate molecular stmcture typical of living polymers, but can be conveniendy mn at room temperature and above. The use of GTP to prepare block polymers, comb-graft polymers, loop polymers, star polymers, and functional polymers has been reported (100,101). 

Emulsion polymerizations of vinyl acetate in the presence of ethylene oxide- or propylene oxide-based surfactants and protective coUoids also are characterized by the formation of graft copolymers of vinyl acetate on these materials. This was also observed in mixed systems of hydroxyethyl cellulose and nonylphenol ethoxylates. The oxyethylene chain groups supply the specific site of transfer (111). The concentration of insoluble (grafted) polymer decreases with increase in surfactant ratio, and (max) is observed at an ethoxylation degree of 8 (112). 

Group-Transfer Polymerization. Living polymerization of acrylic monomers has been carried out using ketene silyl acetals as initiators. This chemistry can be used to make random, block, or graft copolymers of polar monomers. The following scheme demonstrates the synthesis of a methyl methacrylate—lauryl methacrylate (MMA—LMA) AB block copolymer (38). LMA is CH2=C(CH2)COO(CH2) CH2. 

Eor the selective pre-concentration of deactivated phenols a new silica-based material with the grafted 2,3,5-triphenyltetrazole was proposed. This method is based on the formation of molecular chai ge-transfer comlexes of 2,3,5-triphenyltetrazole (7t-acceptor) with picric acid (7t-donor) in the phase of the sorbent. Proposed SPE is suitable for HPEC analysis of nitrophenols after their desorption by acetonitrile. Test-system for visual monitoring of polynitrophenols under their maximum concentration limits was developed using the proposed adsorbent. 

Mention may finally be made of graft polymers derived from natural rubber which have been the subject of intensive investigation but which have not achieved commercial significance. It has been found that natural rubber is an efficient chain transfer agent for free-radical polymerisation and that grafting appears to occur by the mechanism shown in Figure 30.8. 

Polyalkylene oxides (PAO) are prone to chain transfer. This tendency was used by Union Carbide to graft acrylate side chains via UV exposure of a PAO/acrylic monomer(carbamyloxy alkyl acrylate)/photoinitiator blend [58]. The final product was a water-sensitive PSA. 

On the basis of the principle of grafted TLC, reversed-phase (RP) and normal-phase (NP) stationary phases can also be coupled. The sample to be separated must be applied to the first (2.5 cm X 20 cm) reversed-phase plate (Figure 8.16(a)). After development with the appropriate (5ti 5yi) mobile phase (Figure 8.16(b)), the first plate must be dried. The second (20 cm X 20 cm) (silica gel) plate (Figure 8.16(c)) must be clamped to the first (reversed-phase) plate in such a way that by use of a strong solvent system (Sj/, SyJ the separated compounds can be transferred to the second plate (Figure 8.16(d)). Figure 8.16(e) illustrates the applied, re-concentrated... 

If (P ) is terminated by a chain transfer to a solvent or a monomer, a graft copolymer is formed, or, if the termination is from a combination, a crosslinked network polymer is formed. If the pre-existing polymer (B) contains an end group that itself is photosensitive (or can produce a radical by interacting with photoinitiator) and in the presence of a vinyl monomer (A), block copolymer of type AB can be produced if the photosensitive group is on one end of the polymeric chain. Type ABA block copolymer can be produced if the polymer chain (B) contains a photosensitive group on both ends. 

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