Polystyrene radical chain polymerization

Another polymer that can be produced by radical chain polymerization is polystyrene ... 

So far we have discovered very few polymerization techniques for making macromolecules with narrow molar mass distributions and for preparing di-and triblock copolymers. These types of polymers are usually made by anionic or cationic techniques, which require special equipment, ultrapure reagents, and low temperatures. In contrast, most of the commodity polymers in the world such as LDPE, poly(methyl methacrylate), polystyrene, poly(vinyl chloride), vinyl latexes, and so on are prepared by free radical chain polymerization. Free radical polymerizations are relatively safe and easy to perform, even on very large scales, tolerate a wide variety of solvents, including water, and are suitable for a large number of monomers. However, most free radical polymerizations are unsuitable for preparing block copolymers or polymers with narrow molar mass distributions. 

Many substituted cthylenes also undergo radical chain polymerization, yielding polymers with substituent groups regularly spaced at alternating carbon atoms along the chain. Propylene, for example, yields polypropylene (although a different rnethod of polymerization is used in practice), and styrene yields polystyrene (p. 258). 

Ford and collaborators reported a comparison between polymethyl methacrylate (PMMA)/Cgo and polystyrene (PS)/Cgo systems, generated by radical-chain polymerization [68]. Whereas high molecular PS contains up to one hundred fullerene units, high molecular PMMA contains only one Cgg unit/polymer. 

Different processes are nsed in industry for the manufacture of polymers by fi-ee-radical chain polymerization. Among them homogeneous bulk polymerization is economically the most attractive and yields products of higher purity and clarity. But it has problems associated with the heat of polymerization, increases in viscosity, and removal of unreacted monomer. This method is nevertheless used for the manufacture of PVC, polystyrene, and poly(methyl methacrylate). More common processes are homogeneous solution polymerization and heterogeneous suspension polymerization. 

The familiar polymers polyvinyl chloride (PVC), polystyrene (PS), and polymethylmethacrylate (PMMA), which were produced in the 1930s and 1940s in large-scale production plants, are examples of so-called radical chain polymerization. One way of replacing the high-pressure polymerization method used for ethylene (ICI), which involved radical catalysts, with a low-pressure process, was provided by anionic coordinative catalysts, for example titanium tetrachloride plus aluminum triethyl as a cocatalyst in the method according to K. Ziegler (1953). 

Polystyrene Polyisoprene (cis) Polyisobutylene CH2=CHPh CH, CH2=CH-C=CH2 CHj / CH2=C CHa peroxide-catalyzed radical-chain polymerization coordination or butyllithium-catalyzed anionic polymerization low-temperature cationic polymerization, BF3 and AIQ3 catalysis... 

Using the multiple reactive double bonds, G o can be employed as a co-monomer in an in situ polymerization process with various vinyl based monomers through addition polymerization. For example, Mourey et al. have used radical chain polymerization with Al BN as initiator in 1,2-dichlorobenzene to produce branched structures of Ca) and both PMMA [poly(methyl methacrylate)] and PS (polystyrene)... 

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... 

Initiation of a free radical chain takes place by addition of a free radical (R ) to a vinyl monomer (Equation 6.8). Polystyrene (PS) will be used to illustrate the typical reaction sequences. (Styrene, like many aromatic compounds, is toxic, and concentrations that come into contact with us should be severely limited.) It is important to note that the free radical (R ) is a companion of all polymerizing species and is part of the polymer chain acting as an end group and hence should not be called a catalyst even though it is often referred to as such. It is most properly referred to as an initiator. 

So far, there have been only few reports about the synthesis of amphipolar polymer brushes, i.e. with amphiphilic block copolymer side chains. Gna-nou et al. [115] first reported the ROMP of norbornenoyl-endfunctionalized polystyrene-f -poly(ethylene oxide) macromonomers. Due to the low degree of polymerization, the polymacromonomer adopted a star-like rather than a cylindrical shape. Schmidt et al. [123] synthesized amphipolar cylindrical brushes with poly(2-vinylpyridine)-block-polystyrene side chains via radical polymerization of the corresponding block macromonomer. A similar polymer brush with poly(a-methylstyrene)-Wocfc-poly(2-vinylpyridine) side chains was also synthesized by Ishizu et al. via radical polymerization [124]. Using the grafting from approach, Muller et al. [121, 125] synthesized... 

With the recent development of living radical polymerization, the problem of gel formation during radical polymerization possibly can be controlled. This is because termination by radical chain coupling is virtually eliminated. Thus Hawker reported the preparation of soluble hyperbranched polystyrene using alkoxyamine IV as a living radical polymerization initiator [12]. 

When such comparisons are made it becomes clear that the reactivities of radicals, monomers, or transfer agents depend on the particular reaction being considered. It is not possible to conclude, for example, that polyfvinyl acetate) radical will always react x times more rapidly than polystyrene radical in addition reactions or y times as rapidly in the atom abstraction reactions involved in chain transfer. Similarly the relative order of efficiency of chain transfer agents will not be the same for all radical polymerizations. This is because resonance, sleric, and polar influences all come into play and their effects can depend on the particular species involved in a reaction. 

Poly(styrene)s containing acylperoxide groups are thus obtained by selective photolysis of the azo moieties at 350 or 371 nm. These prepolymers are successively used as macronitiators for the free radical polymerization of vinyl chloride at 70 °C. Styrene/vinyl chloride block copolymers are thus produced [55] by the above two-step route, although relevant amounts (50-60%) of poly(styrene) and poly(vinyl chloride), due to both low peroxide content ( 0.6 groups per macromolecule of polystyrene) and chain transfer with solvent and monomer, are also pre t. 

A similar well-defined graft copolymer consisting of polystyrene main chain and branches (G-7) can be prepared simply via repetition of copper-catalyzed living radical polymerizations.209 Thus, the synthesis starts with the copolymerization of styrene and />(acetoxymethy 1)styrene or />(methoxymethyl)sty-rene, followed by bromination of the substituent into the benzyl bromide moiety, which then initiates the copper-catalyzed radical polymerization of styrene to give graft polymers with 8—14 branches. 

A combination of metallocene-catalyzed syndiospe-cific styrene polymerization and the metal-catalyzed radical polymerization affords various graft copolymers consisting of syndiotactic polystyrene main chains (G-8).433 The reactive C—Br bonds (7—22% content) were generated by bromination of the polystyrene main chain with AZ-bromos uccimid e in the presence of AIBN. 

Another very important elass of ehain reaetions, perhaps the most important from a commereial viewpoint, ineludes those involved in polymerization. Materials such as polyethylene and polystyrene are formed in chain reactions with free radical chain carriers. These addition polymerization ehains are similar in substance to those we have been discussing, but differ in three important respects. First, the monomer, particularly when purified, is often quite unreaetive and it is necessary to use small quantities of separate substanees (initiators) that essentially trick the monomer into... 

Free-radical polymerization results when a suitable alkene is heated with a radical initiator. For example, styrene polymerizes to polystyrene when it is heated to 100° C in the presence of benzoyl peroxide. This chain-growth polymerization is a free-radical chain reaction. Benzoyl peroxide cleaves when heated to give two carboxyl radicals, which quickly decarboxylate to give phenyl radicals. 

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