Polymerization copolymerization: Free radical

Random copolymers of vinyl chloride and other monomers are important commercially. Most of these materials are produced by suspension or emulsion polymerization using free-radical initiators. Important producers for vinyl chloride—vinyUdene chloride copolymers include Borden, Inc. and Dow. These copolymers are used in specialized coatings appHcations because of their enhanced solubiUty and as extender resins in plastisols where rapid fusion is required (72). Another important class of materials are the vinyl chloride—vinyl acetate copolymers. Principal producers include Borden Chemicals Plastics, B. F. Goodrich Chemical, and Union Carbide. The copolymerization of vinyl chloride with vinyl acetate yields a material with improved processabihty compared with vinyl chloride homopolymer. However, the physical and chemical properties of the copolymers are different from those of the homopolymer PVC. Generally, as the vinyl acetate content increases, the resin solubiUty in ketone and ester solvents and its susceptibiUty to chemical attack increase, the resin viscosity and heat distortion temperature decrease, and the tensile strength and flexibiUty increase slightly. 

Polystyrene (PS) is the fourth big-volume thermoplastic. Styrene can be polymerized alone or copolymerized with other monomers. It can be polymerized by free radical initiators or using coordination catalysts. Recent work using group 4 metallocene combined with methylalumi-noxane produce stereoregular polymer. When homogeneous titanium catalyst is used, the polymer was predominantly syndiotactic. The heterogeneous titanium catalyst gave predominantly the isotactic. Copolymers with butadiene in a ratio of approximately 1 3 produces SBR, the most important synthetic rubber. 

One of the great surprises of fluorothiocarbonyl chemistry is the ease with which these compounds undergo free-radical polymerization. This behavior is unique among thiocarbonyl compounds. Though thioacetone polymerizes in free-radical systems, it does not do so with anything like the avidity of fluorothiocarbonyl compounds. Thioacetone does not copolymerize with compounds containing carbon-carbon unsaturation, which is a most important property of fluorothiocarbonyl compounds. 

VEC is a sluggish monomer having free radical polymerization toward free radical polymerization. Due to the electronic structure of the double bond, VEC copolymerizes with vinyl ester monomers over a wide compositional range. However, VEC cannot be completely incorporated into an acrylic copolymer. When copolymerization with styrene is attempted, VEC is barely incorporated. 

It has been found in practice that a number of monomers that normally do not polymerize by free radical processes in the temperature range 10-50°C can be block copolymerized by cold mastication techniques, indicating ionic initiation via heterolytic scission. 

The acid chlorides of both acrylic and methacrylic acids polymerize by free-radical mechanism in dry aromatic and aliphatic solvents. Molecular weights of the products, however, are low, usually under 10,000 [273, 274]. Polyacrylic and polymethacrylic acids are used industrially as thickeners in cosmetics, as flocculating agents, and when copolymerized with divinyl benzene in ion-exchange resins. 

Free-radical polymerization n. A reaction initiated by a free radical derived from a polymerization catalyst. Polymerization proceeds by the chain-reaction addition of monomer molecules to the free-radical ends of growing chain molecules. Major polymerization methods such as bulk, suspension, emulsion, and solution polymerization involve free radicals. The free-radical mechanism is also useful in copolymerization, in which alternating monomeric units are promoted by the presence of free radicals. Lenz RW (1967) Organic chemistry of high polymers. Interscience Publishers, New York. Odian G (2004) Principles of polymerization, 4th edn. Wiley-Interscience, New York. 

Three events are involved with chain-growth polymerization catalytic initiation, propagation, and termination [3], Monomers with double bonds (—C=C—R1R2—) or sometimes triple bonds, and Rj and R2 additive groups, initiate propagation. The sites can be anionic or cationic active, free-radical. Free-radical catalysts allow the chain to grow when the double (or triple) bonds break. Types of free-radical polymerization are solution free-radical polymerization, emulsion free-radical polymerization, bulk free-radical polymerization, and free-radical copolymerization. Free-radical polymerization consists of initiation, termination, and chain transfer. Polymerization is initiated by the attack of free radicals that are formed by thermal or photochemical decomposition by initiators. When an organic peroxide or azo compound free-radical initiator is used, such as i-butyl peroxide, benzoyl peroxide, azo(bis)isobutylonitrile, or diazo- compounds, the monomer s double bonds break and form reactive free-radical sites with free electrons. Free radicals are also created by UV exposure, irradiation, or redox initiation in aqueous solution, which break the double bonds [3]. 

Copolymerization. Acrylic and methacrylic acids readily copolymerize free radically with many vinyl monomers. This versatility results from a combination of their highly reactive double bonds and their miscibility with a wide variety of water- and solvent-soluble monomers. Reactivity ratios derived from copolymerizations with many monomers are tabulated in many books on polymerization, for example in Wiley s Polymer Handbook (14) (see also Wiley s Database of Polymer Properties). Q and e values are parameters that may be established for a monomer based on a large number of reactivity ratios with other monomers. These parameters are associated with interactions between the monomer and the growing chain via resonance (Q) and polar effects (e). 

Little has been published on techniques for achieving the homopolymerization of mono or disubstituted MA monomers (see Chapter 3). Thamm and Hensinger recently showed that y irradiation of dichloromaleic anhydride (DCMA) in benzene produced a polymer as the main product. The yield and molecular weight of the polymer increased with radiation dose. The polymer backbone contained chlorophenylsuccinic anhydride residues. It was shown that chlorophenylmaleic anhydride was produced during the reaction. Under the same conditions, dimethylmaleic anhydride (DMMA) failed to polymerize with free-radical, ionic, and UV initiation with sensitizers.Presumably, techniques may be found for the homopolymerization of chloro or phenyl-maleic anhydride. Chloromaleic anhydride (CMA) reacts with methyl radicals more readily than MA and much more readily than DCMA. " This, coupled with halogen activation and ring coplanarity, should allow CMA to be homopolymerized. It is known that CMA will copolymerize with styrene, methyl methacrylate, butadiene, cyanoacrylates, and other olefins. 

A conventional branched polymer results when a dimethacrylate is copolymerized in a random fashion with monofunctional monomers. However it is necessary to keep the dimethacrylate/initiator ratio less than 1/1 in order to prevent gelation. When the dimethacrylate/lnitiator ratio equals or exceeds a 1/1 ratio, there should be an infinite number of branches with each chain connected to at least two other chains, resulting in a gel. Table,1 shows that gels result when a 2/1 dimethacrylate/initiator ratio is used and the dimethacrylate is randomly copolymerized with a monofunctional monomer. This is true whether the polymerization is free radical or GTP. 

When ethylene is polymerized by free-radical mechanism, high pressure and high temperature are required. Organic initiators and oxygen are used as free-radical generators. The general kinetic scheme for free radical ethylene copolymerization is represented as follows ... 

In Refs. 125, 126 the viscosity is measured with a magnetic sphere rheometer at decreasing shear rate (its lowest value is 10" s ) in different polymeric systems free-radical copolymerization of styrene-meta-divinylbenzene with solvent and polycondensation of hexamethyl diisocyanate with polyoxypropylene with and without solvent. These experiments yield k = 0.79 in both systems. The standard deviation of experiments performed at different shear rates at the gel points is 0.07 in k. Since t and E are measured in Ref. 126 using the same apparatus and the same sample, T is determined as the time vriiere 9 and E are equal to zero, the precision in Tc being higher than 10" %. 

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