Termination steps styrene polymerization

Anionic polymerization is better for vinyl monomers with electron withdrawing groups that stabilize the intermediates. Typical monomers best polymerized by anionic initiators include acrylonitrile, styrene, and butadiene. As with cationic polymerization, a counter ion is present with the propagating chain. The propagation and the termination steps are similar to cationic polymerization. 

Based on the literature data available for styrene polymerized with benzoyl peroxide, (10,12,14) transfer to monomer and termination by disproportionation will be neglected. For the Initiation step, only primary and Induced decomposition reactions will be considered. 

The latter mechanism is supported by evidence obtained from the initiation and termination steps in the syndiospecific polymerization of styrene [190]. The 13C-enriched titanium catalyst afforded polystyrene with a CH(Ph)CH213CH3 end group, which indicates that the initiation step proceeded by secondary insertion (2,1-insertion) of styrene into the Ti-13C bond of the active species (Eq. 10). In contrast to this mechanism, termination by the addition of 13C-enriched methanol or tert-butyl alcohol afforded polymers without 13CH30 or tertbutoxy end groups. 

On the basis of the polarographic studies, the direct electron transfer polymerizations of the monomers, of which half-wave potentials could be measured, were conducted keeping the potential at a level where the monomer alone was reduced and the electrolyte was not affected. During the electrolysis of a-methyl-styrene, for example, the red color of the carbanion was observed around the cathode, but dissipacted and vanished quickly. Only low polymers were found in the cathodic compartment, but no polymer in the anolyte. In the polymerization of other monomers almost identical results were obtained. From the fact that tri-n-butyl-amine was detected in the catholyte and analysis of the end group of polymers obtained, two possible termination steps were postulated ... 

With both styrene and vinylpyridine, the autoacceleration index decreases as the reaction temperature rises. This effect can be considered normal behavior of polymerizing systems in which the gel effect is operative. As the temperature rises, the termination step, which involves the interaction of two polymeric chains in a highly viscous medium, increases in rate, and the over-all reaction tends to become normal. Ultimately, the stationary-state conditions may eventually apply. 

The requirements for a polymerization to be truly living are that the propagating chain ends must not terrninate during polymerization. If the initiation, propagation, and termination steps are sequential, ie, all of the chains are initiated and then propagate at the same time without any termination, then monodisperse (ie, = 1.0) polymer is produced. In general, anionic polymerization is the only mechanism that yields truly living styrene... 

Polymerization reactions proceed via initiation, propagation, and termination steps as illustrated in Section 4.1. A simplified network to describe the styrene polymerization is ... 

Although the unique features of the above polymerizations were recognized, the same was difficult to do for vinyl and diene monomers in which the much more reactive anion, that is, a car-banion, was involved. Thus, although some of the earlier studies, such as those of Robertson and Marion (10) on sodium polymerization of butadiene in toluene, and Higginson and Wooding (11) on styrene polymerization by potassium amide in liquid ammonia, demonstrated the presence of an anionic mechanism, the absence of any true termination step in these investigations was not recognized because of the presence of many transfer reactions. 

Because no termination step exists, styrene/butadiene/styrene (SBS) triblock polymers can easily be made by charging styrene, butadiene, and again styrene, in succession, to the catalyst. Because the polystyrene blocks behave like styrene homopolymer, these triblock polymers are not suitable for use in automobile tires, but they lend themselves well in polyblends and as the backbone in the graft polymerization of Impact polystyrene and ABS... 

It changes the rate at which reacting molecules diffuse together, and newly produced molecules diffuse apart. This effect was first demonstrated by Nicholson and Norrish (1956) for the termination step of the ffee-radical-initiated polymerization of styrene, which is the combination of two larj free radicals, tmd which appears to be diffusion-controlled under ordinary conditions. Hamann (1958) has demonstrated difforion control for several alkaline etherifications of ethyl bromide... 

One of the first anionic reactions studied in detail was the polymerization of styrene in liquid ammonia, with potassium amide as initiator, reported by Higginson and Wooding (1952). The reaction scheme proposed for this contains no formal termination step, and if all the impurities that are liable to react with the carbanions are excluded from the system, propagation should continue until all monomer has bear consumed, leaving the carbanion intact and still active. This means that if more monomer could be introduced, the active end would continue growing unless inadvertently terminated. These active polycarbanions were first referred to as living polymers by Szwarc (1968). 

The termination steps in anionic polymerizations can result from deliberate introductions of carbanion quenchers, such as water or acids, or from impurities. Terminations, however, can take place in some instances through chain transferring a proton from another molecule like a solvent or a monomer, or even from a molecule of another polymer. In some solvents, like liquid ammonia, transfer to solvent is extensive, as in styrene polymerization by amide ions. ... 

ABC triblock copolymer is formed typically through sequential monomer addition in three ATRP steps. Step One polymerization of styrene (St) by mixing the components together in the following order - CuBr, St, PMDETA, and 1-phenyl ethyl bromide, typically at 100°C. The product is a monofunctional macroinitiator, namely, bromo-terminated polystyrene, P(St)-Br, which is isolated from the reaction products mixture and used in Step Two for the ATRP of tert-butyl acrylate (IBA) at 80°C, the order of addition of the components to the reaction mixture being P(St)-Br, CuBr, tBA, and PMDETA. The product of step 2 is a monofunctional, bromo-terminated macroinitiator, P(St)-l>-P(tBA)-Br, which is isolated from the reaction products mixture and used in Step Three for the synthesis of ABC triblock copolymer by ATRP of methyl acrylate (MA) at 70°C, the order of addition of the components to the reaction mixture being CuBr, P(St)-l>-P(tBA)-Br, MA, and PMDETA. The product of step 3 is the triblock copolymer P(St)-l>-P(tBA)-l>-P(MA)-Br. 

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