Free radical Polyethylene Polystyrene

Addition polymers, which are also known as chain growth polymers, make up the bulk of polymers that we encounter in everyday life. This class includes polyethylene, polypropylene, polystyrene, and polyvinyl chloride. Addition polymers are created by the sequential addition of monomers to an active site, as shown schematically in Fig. 1.7 for polyethylene. In this example, an unpaired electron, which forms the active site at the growing end of the chain, attacks the double bond of an adjacent ethylene monomer. The ethylene unit is added to the end of the chain and a free radical is regenerated. Under the right conditions, chain extension will proceed via hundreds of such steps until the supply of monomers is exhausted, the free radical is transferred to another chain, or the active site is quenched. The products of addition polymerization can have a wide range of molecular weights, the distribution of which depends on the relative rates of chain grcnvth, chain transfer, and chain termination. 

Materials that are constructed from organic polymers such as polyethylene, polystyrene, polyisoprene (natural rubber and a synthetic elastomer) and poly(vinyl chloride) are common features of our daily lives. Most of these and related organic polymers are generated from acyclic precursors by free radical, anionic, cationic or organometallic polymerisation processes or by condensation reactions. Cyclic precursors are rarely used for the production of organic polymers. 

Free radicals are also produced by chain scission during deformation of polyethylene and FT-IR has been used to follow this process 237). The polyethylene samples were unaxially drawn and the resultant spectra corrected for orientation. An increase in the vinyl and methyl end groups created by decay of the free radicals occured in going from draw ratios of 5 to 20 44). A similar study involving deformation was made of polystyrene 246) and a comparison demonstrated between the results of thermal and mechanical degradation 24S. ... 

Oxygen free irradiation of solid polymers was also carried out in the absence of monomer in this case free radicals with relatively long life time are produced which are able to initiate the polymerization of a second monomer (56,113). Several irradiated polymers have been used as initiators of polymerization, and, evidently, if the polymerization of the second monomer is initiated only by these frozen-in radicals, the product will undoubtedly be hundred per cent grafted. Bevington using 14C-acrylonitrile was able to estimate the amount of acrylonitrile grafted on preirradiated polystyrene, polyacrylonitrile, polyethylene terephtha-Iate and nylon yams (36,37). 

Chain Free radical Polybutadiene Polyethylene (branched) Polyisoprene Polymethylmethacrylate Polyvinyl acetate Polystyrene... 

A majority of commercial polymers are produced by free-radical polymerization. Foremost among these are polystyrene, polyethene (i.e., polyethylene), poly(vinyl chloride), poly(vinyl alcohol), poly (vinyl acetate), and poly (methyl methacrylate). In each of these, polymerization involves an olefinic double bond. However, free-radical polymerization is not restricted to such monomers. 

The majority of packaging plastic materials consists of polyolefins and vinyl polymers, namely polyethylene (PE), polypropylene (PP), polystyrene (PS) and poly(vinyl chloride) (PVC). Obviously, these polymers have many other applications not only as packaging materials. Chemically they are all composed of saturated hydrocarbon chains of macro-molecular size their typical thermal decomposition pathway is free radical one initiated by the homolytic scission of a backbone carbon-carbon bond. In spite of the basic similarity of the initial cleavage, the decomposition of the hydrocarbon macroradicals is strongly influenced by fhe nafure of the side groups of the main chain. 

Molecular weight distributions in commercial polymers are characterized by ratios of about 3 for substances like polystyrene in which transfer to polymer does not appear to be important. Where long branches can be formed by chain transfer to polymer, the molecular weight distribution will be even broader and M /M ratios of 50 and more are observed in some polyethylenes made by free-radical syntheses. 

The polyethylene latexes obtained in the different emulsion polymerization procedures using the various aforementioned nickel(II) complexes display average particle diameters of 100 to 600 nm. A number of anionic surfactants or neutral stabilizers are suitable, i.e. compatible with the catalysts and capable of stabilizing the latex. Solids contents of up to 30% have been reported to date. A typical TEM image is shown in Fig. 7.2. By comparison to smooth, spherical latex particles of amorphous polystyrene as a well studied hydrocarbon polymer prepared by free-radical emulsion polymerization, the ruggedness of the particles shown can be rationalized by their high degree of crystallinity. 

A polymer is a giant molecule composed of a repeating structural unit called a monomer. Addition polymers result from the addition of alkene molecules to one another. The polymerization occurs by cationic, free-radical, and anionic reaction mechanisms. Examples of addition polymers include polyethylene, polystyrene, PVC, and Teflon. 

As isotactic polystyrene has a similar crystal structure to that of polyethylene, the concept of the intramolecular cydization may be applied to the degradation of the former to account for the formation of the products of degradation. Let the three-unit one turn radical form the cyclic compound (VIII) by the interaction of the Cn and CK atoms. The cyclic compound (VIII) may lose hydrogens to other free radicals to form 1,3,5-triphenyl-benzene (IX). 

Wojtczak have found that the photosensitized degradation of polyethylene glycols decreases in the order triethylene glycol > polyethylene glycol 400 mol. wt. > polyethylene glycol 4000 mol. wt. Sastre and Gonzalez have shown that bromoalkanes are powerful sensitizers for the photo-oxidation of polystyrene, and Rabek and Ranby have found that polynuclear aromatics are photosensitizers for polybutadiene. Aromatic carbonyls have been shown to induce free-radical formation in cellulosic materials. 

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... 

In bulk polymerization, the only components of the formulation are monomers and the catalyst or initiator. When the polymer is soluble in the monomer, the reaction mixture remains homogeneous for the whole process. Examples of homogeneous bulk polymerization are the production of low-density polyethylene (LDPE), general purpose polystyrene and poly(methyl methacrylate) produced by free-radical polymerization, and the manufacture of many polymers produced by step-growth polymerization including poly(ethylene terephthalate), polycarbonate and nylons. In some cases (e.g., in the production of HIPS and acrylonitrile-butadiene-styrene (ABS) resins), the reaction mixture contains a preformed... 

In spite of the great discoveries by Ziegler and Natta, most synthetic polymers are still made by free-radical reactions. Some of the important homopolymers are poly (vinyl chloride), poly (methyl methacrylate), polystyrene, and low-density polyethylene. Other important polymers made by free-radical reactions contain two or more monomers, for example, the styrene-butadiene rubbers, and the acrylonitrile-butadiene-styrene plastics. Most of these polymers are not stereoregular. A few that are represent the subject of this section. 

The production and applications of polymers have gradually developed, gaining ground in many fields. The main classes of polymers, namely polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene and polyethylene terephthalate are produced in millions of tonnes annually [1]. There are many methods of polymer synthesis free-radical polymerisation (bulk, solution, emulsion and suspension), condensation polymerisation, ethoxylation, polymer compounding and formulations involving solvents, fillers, pigments and so on. Besides the high volume consumption of these common plastics, the demand for polymers with specific end-use properties has increased. 

The chain addition polymerizations require monomers with double bonds. They require free radical or ionic initiators to open the double bond and form the polymerization path in the manufacture of polymers such as polyethylene, polypropylene, polystyrene, and polyvinyl chloride which together constitute the majority of polymers, about 70 % of all polymers produced. A wide range of copolymers or terpolymers are produced by chain addition polymerization of two or three different monomers with double bonds. 

This will be recognized as the mechanism for chain polymerization which was described in Chapter 5 for the formation of polyethylene. However here, after quite a short polystyrene chain is formed, the free radical end encounters polyester unsaturation and copolymerization occurs ... 

During this early period, Bailey developed a very clever free radical route to polyesters which he used to introduce weak linkages into the backbones of hydrocarbon polymers and render them susceptible to biodegradability (Bailey 1975, 1979, 1985, 1991). Copolymerization of ketene acetals with vinyl monomers incorporates an ester linkage into the pohmier backbone by rearrangement of the ketene acetal radical as illustrated below. The ester is a potential site for biological attack. The chemistr) has been demonstrated with polyethylene (Bailey), poly (acrylic acid) (Bailey, 1990), and polystyrene (Tokiwa). 

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