Styrene polymerization combination disproportionation

The paper industry uses large amounts of the sodium salt of rosin as paper size, which accounts for the greatest single use of rosin. The synthetic rubber industry is the second most important user of rosin. In making styrene-butadiene rubber, disproportionated rosin soaps are used alone or in combination with fatty acid soaps as emulsifiers in the polymerization process. Disproportionation decreases the number of double bonds in the abietic acid of the rosin, making a more stable material. 

Since the nitroxide and the carbon-centered radical diffuse away from each other, termination by combination or disproportionation of two carbon-centered radicals cannot be excluded. This will lead to the formation of dead polymer chains and an excess of free nitroxide. The build-up of free nitroxide is referred to as the Persistent Radical Effect [207] and slows down the polymerization, since it will favor trapping (radical-radical coupling) over propagation. Besides termination, other side reactions play an important role in nitroxide-mediated CRP. One of the important side reactions is the decomposition of dormant chains [208], yielding polymer chains with an unsaturated end-group and a hydroxyamine, TH (Scheme 3, reaction 6). Another side reaction is thermal self-initiation [209], which is observed in styrene polymerizations at high temperatures. Here two styrene monomers can form a dimer, which, after reaction with another styrene monomer, results in the formation of two radicals (Scheme 3, reaction 7). This additional radical flux can compensate for the loss of radicals due to irreversible termination and allows the poly-... 

Polystyrene. The polymerization of styrene is most commonly done under free radical conditions. Peroxides are used to initiate the reaction at low temperatures. At 100°C styrene acts as its own initiator. Below 80°C the termination mechanism primarily involves combination of radicals. Above 80°C both disproportionation and chain transfer with the Diels-Alder dimer are important. 

An inverted sequence of the same procedure has also been used [139] to prepare the same three-block copolymers. Indeed, thermal polymerization of MMA by ABME gives rise to a polymer mainly containing only one benzoin methyl ether moiety per macromolecule, since growing MMA radicals terminate mostly by disproportionation. Thus, terminally photoactive poly(MMA) is used to obtain the photoinitiated block copolymerization of styrene. In this case, a 90% yield of block copolymers is obtained, appreciably higher than in the preceding method, fully consistent with the usual assumption that the termination in styrene polymers occurs by combination. In fact, coupling of the growing styryl radicals with the less reactive poly(MMA)-bound methoxy benzyl radicals also contributes to the formation of block copolymers. 

Tip 16 Polymerization of methyl methacrylate, styrene, and vinyl acetate. MMA, when polymerized, exhibits termination by both combination and disproportionation (in fact, disproportionation is promoted at higher temperatures). Termination by disproportionation leads to the formation of radicals and, eventually, polymer molecules with a TDB. We also know that TDBs will become competitive with the monomer vinyl bonds for radicals as conversion increases. TDB polymerization (characterized by rate constants close (in value) to propagation rate constants) leads to trifunctional LCB. Yet, upon analysis, poly(MMA) chains are linear. How come What is the explanation/reasoning for this observation We also know that styrene terminates predominantly via combination. Styrene also exhibits transfer to monomer, which is enhanced at higher temperature levels. Transfer to monomer generates chains with TDBs. Yet, polystyrene is linear. What is the explanation ... 

The ratio between the rates of termination by disproportionation and combination, k lk has been evaluated by Berger (1975) for the polymerization of styrene ... 

In general, both of these termination reactions contribute, but to different extends depending upon the monomer and the polymerization conditions. For example, in polymerization of styrene (CH2=CHPh) the chain radicals terminate principally by combination, whereas for methyl methacrylate (CH2=CMeC02Me), the chain radicals terminate predominantly by disproportionation at temperatures above 60 °C. 

Chain growth addition polymerization based on active carbene (e.g., styrene, vinyl carbazole, isobutylene) is a fast reaction that is most difficult to control since termination by combination or disproportionation is not possible. The initiator for cationic polymerization is a proton from a donor such as water, in the presence of a Lewis acid or sulfuric acid. Polymerization tends to be uncontrollable when undiluted monomers are used, and so these reactions are carried out at low monomer levels, usually in chlorinated... 

The Mw/Mn values observed for the copolymers tend to increase with styrene content. This may reflect the fact that termination by disproportionation of propagating methacrylate radicals may be significant when the MMA content of the copolymerization mixture is high but that termination by combination of propagating styrene radicals with other propagating styrene radicals or with propagating MMA radicals will become of increasing importance as the styrene content of the polymerization mixtures increases. 

It is not likely that all A-Sty trimer arises from an ene reaction. (I.e., from reaction in Fig. lA.) As I have pointed out (J), phenyltetralin probably arises from the disproportionation reaction of caged radicals, shown as eq in Fig. lA. (Any A that diffuses into free solution would be expected to add to styrene and not to abstract hydrogen to give phenyltetralin.) If cage disproportionation occurs, then cage combination also must occur. (See eq Ji in Fig. lA.) However, the conclusion reached here is not dependent on whether trimer arises from the combination of radicals within a cage or an ene process, since neither reaction produces free radicals that can initiate the polymerization of styrene. 

Write down a reaction scheme for polymerization of styrene (CH2=CHPh) initiated by thermolysis of azobisisobutyronitrile (AIBN), including both combination and disproportionation as possible modes of termination. 

Using radioactive AZBN as an initiator, a sample of styrene is polymerized to a number average degree of polymerization of 10 000. The AZBN has a radioactivity of 6 X 10 counts s mol in a liquid-scintillation counter. If 0.001 kg of the polystyrene has a radioactivity of 6 x 10 counts s determine whether the termination mechanism is combination or disproportionation. 

The radicals thus generated initiate the polymerization, provided they do not deactivate by mutual combination or disproportionation. Due to their low ceiling temperature, a-substituted styrenes hardly undergo thermal polymerization in the absence of initiator. 

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),... 

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