Styrenes radical attack

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

This mechanism is based on initiation by electron-transfer which leads to a styrene radical anion, which couples rapidly due to its high concentration, and forms a dimeric styrene dianion that is capable of further propagation by anionic attack on styrene monomer. 

Taylor in 1925 demonstrated that hydrogen atoms generated by the mercury sensitized photodecomposition of hydrogen gas add to ethylene to form ethyl radicals, which were proposed to react with H2 to give the observed ethane and another hydrogen atom. Evidence that polymerization could occur by free radical reactions was found by Taylor and Jones in 1930, by the observation that ethyl radicals formed by the gas phase pyrolysis of diethylmercury or tetraethyllead initiated the polymerization of ethylene, and this process was extended to the solution phase by Cramer. The mechanism of equation (37) (with participation by a third body) was presented for the reaction, - which is in accord with current views, and the mechanism of equation (38) was shown for disproportionation. Staudinger in 1932 wrote a mechanism for free radical polymerization of styrene,but just as did Rice and Rice (equation 32), showed the radical attack on the most substituted carbon (anti-Markovnikov attack). The correct orientation was shown by Flory in 1937. In 1935, O.K. Rice and Sickman reported that ethylene polymerization was also induced by methyl radicals generated from thermolysis of azomethane. 

TABLE 12. Rate constants for radical attack on [l.l.ljpropellane and styrene"... 

It is evident that the values of the transfer constants are dependent on the nature both of the attacking radicals and of the transfer agent itself, and that similar effects should be expected during the synthesis of graft copolymers by chain transfer methods. For example, with respect to toluene the chain transfer constant is a little greater for methyl methacrylate radicals than for styrene radicals on the contrary, with respect to halogenated solvents (CC14) the polystyrene radical is much more effective in the removal of a chlorine atom. Vinyl acetate chains are far more effective than either of the other two polymer radicals. 

Chain propagation occurs by the growing chain free radical attacking either the butadiene or styrene monomer. The active radical chain can react with mercaptan to form a new mercaptyl radical and a terminated chain. The mercaptyl radical then can initiate an additional chain. The molecular weight of the chain P can be controlled by the concentration of mercaptan via this chain transfer mechanism. 

Radical attack on the central bond in [l.l.l]propellane 1 occurs 2-3 times faster than attack on styrene and yields bridgehead bicyclo[l.l.l]pent-l-yl radicals. Laser flash photolysis techniques were used to measure the rate constants for the reactions of la with five different radicals (Table 12). The addition of the phenylthiyl radical to la is... 

Products of addition to styrene double bonds can arise as a result of light induced electron transfer reactions. Lewis has studied the intramolecular reaction of l-phenyl-w-amino alkenes (422) 289,290 products arise from electron transfer from the amine nitrogen to the excited state of the styryl group followed by intramolecular proton transfer in the radical ion pair produced. The resultant biradical then couples to yield the isolated products (423) and (424). Sensitisation of the intermolecular analogue of this reaction by 1,4-dicyanobenzene has been reported and is proposed to occur by electron transfer from the styrene to the excited state of the sensitiser followed by attack of an amine on the styrene radical cation. This ultimately leads to the product of anti-Markovnikov addition of the amine across the double bond of the styrene. This is similar to the sequence long since established by... 

The failure to incorporate moieties arising from radical attack on the solvent into the alternating copolymers, coupled with the virtual absence of catalyst residues in the copolymer when the copolymerization of styrene and maleic anhydride is initiated by AIBN (3, 4), indicates that radical species may initiate the polymerization of comonomer charge-transfer complexes, but they are not incorporated into the polymer chain. 

Electron-transfer photosensitization (2,4,6-triphenylpyrylium tetrafluorobor-ate) is reported to induce a photo-Diels Alder reaction between A-arylimines (55) and styrene or a-methylstyrene. The reaction is considered to proceed by attack of the styrene radical cation onto the arylimine and affords both dia-stereoisomers (56) and (57) in reasonable yields, although amounts of the quinoline (58) and the amine (59) are formed in some cases. 

The thermal volatilization analysis of a mixture of polyvinylchloride and polystyrene is given in Fig. 81. The first peak corresponds to the elimination of HC1 and the second to that of styrene. Dehydrochlorination is retarded in the mixture. The production of styrene is also retarded styrene evolution, in fact, does not occur below 350°C. This contrasts with the behaviour of polyvinylchloride-polymethylmethacrylate mixtures for which methacrylate formation accompanies dehydrochlorination. The observed behaviour implies that, if chlorine radical attack on polystyrene occurs, the polystyrene radicals produced are unable to undergo depolymerization at 300° C. According to McNeill et al. [323], structural changes leading to increased stability in the polystyrene must take place. This could also occur by addition of Cl to the aromatic ring, yielding a cyclohexadienyl-type radical which is unable to induce depolymerization of the styrene chain. 

We have already alluded to the fact that the addition of a second substituent to the 1- or a-position increases monomer reactivity. However, when the same substituent is in the 2- or (3-position (i.e., 1,2-disubstitution), the reactivity of the monomer decreases 2- to 20-fold. This has been attributed to the resulting steric hindrance between the substituent and the attacking radical. The role of steric hindrance in the reduction of the reactivity of 1,2-disubstituted vinyl monomers can be further illustrated by the fact that while these monomers undergo oopolymerization with other monomers, say, styrene, they exhibit extreme reluctance to homopolymerize. Homopolymerization is prevented because of the steric hindrance between a 2-substituent on the attacking radical and the monomer. On the other hand, there is no 2- or 3-substituent on the attacking styrene radical consequently, copolymerization is possible. 

Examples of stable complexes are reactions of sulfur dioxide with styrene, or vinyl ethers with maleic anhydride,also a-olefins with maleic anhydride.Also, a reaction of /ran -stilbene with maleic anhydride. In these reactions chaige-transfer complexes form. They are stable and their existence can be detected by spectroscopic means. Additional energy, such as heat or a free-radical attack, converts them to diradicals and polymerizes them into alternating copolymers.i 1 1 ... 

Addition of rubbery materials, however, does improve the impact resistance of polystyrene. This is therefore done extensively. The most common rubbers used for this purpose are butadiene-styrene copolymers. Some butadiene homopolymers are also used, but to a lesser extent. The high-impact polystyrene is presently prepared by dissolving the rubber in a styrene monomer and then polymerizing the styrene. This polymerization is either done in bulk or in suspension. The product contains styrene-butadiene rubber, styrene homopolymer, and a considerable portion of styrene-graft copolymer that forms when polystyrene radicals attack the rubber molecules. The product has very enhanced impact resistance. 

Whichever method of activation is used, decomposition of peroxide leads to free radical attack, probably on the styrene ... 

In the radical polymerization of styrene, the attack on the fi-C atom is unhindered, and the poly(styrene) radical—CH(C6H5) is, in addition, resonance-stabilized. In the polymerization of vinyl acetate, CH2==CH(0C0CH3), there are weak dipole-dipole interactions between the —COO— groups in the transition state, which facilitates an occasional attack on the a-C atom despite steric hindrance by these groups. Poly(vinyl acetate) therefore contains 1-2% head-to-head structures. The attack of the a position is furthermore the easier, the smaller the substituents. Poly-(vinyl fluoride), therefore, has up to 30% head-to-head sequences. 

In a further smdy, it was investigated if sulfonated low-cost ionomers can also be used as acidic cross-linkers for PBI polymers in order to reduce membrane costs. For this purpose, sulfonated polystyrene (S4a, Fig. 4.5) and poly (a-methylstyrene sulfonic acid), (S4b, Fig. 4.5) have been blended with commercial PBIOO (B4, Fuma-Tech) to 70 wt% PBIOO/30 wt% sulfonated polystyrene blend membranes. The membranes were characterized in terms of chemical stability by the immersion in Fenton s Reagent, thermal stability in terms of TGA-FTIR coupling and in terms of proton conductivity after PA doping [58]. Poly(-a-methylstyrene sulfonic acid) was chosen for comparison with sulfonated polystyrene since it is known that the main radical attack target of polystyrene is the tertiary C-H bond [59] which is not present in poly(a-methylstyrene), leading to verified better radical stabilities of poly(-a-methylstyrene), compared to poly(styrene) [60]. In Table 4.4, the results of thermal and FT stability of the blend membranes B3/S4a and B3/ S4b are listed. 

Chain propagation occurs by the growing-chain free radical attacking either butadiene or styrene monomer. The active radical chain can react with mercaptan to form a new... 

FIGURE 1111 Cham propagation in polymerization of styrene The growing polymer chain has a free radical site at the benzylic carbon It adds to a molecule of styrene to extend the chain by one styrene unit The new polymer chain is also a benzylic radical it attacks another molecule of styrene and the process repeats over and over again... 

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