Polyethylene chains, growth

Scheme 27.2 Neodymium-catalyzed polyethylene chain growth on magnesium with [Nd](BH4)/MgR2 catalyst.

Britovsek GJP, Cohen SA, Gibson VC, Maddox PJ, van Meurs M (2002) Iron-catalyzed polyethylene chain growth on zinc linear a/pfia-olefins with a Poisson distribution. Angew Chem Int Ed 41(3) 489 91... 

Van Meurs, M., Britovsek, G. J.P., Gibson, V.C., and Cohen, S.A. (2005) Polyethylene chain growth on zinc catalyzed by olefin polymerization catalysts A comparative investigation of highly active catalyst systems across the transition series. Journal of the American Chemical Society, 127,9913-9923. 

Addition or chain-growth polymerization involves the opening of a double bond to form new bonds with adjacent monomers, as typified by the polymerization of ethylene to polyethylene ... 

Chain-growth polymer (Section 31.1) A polymer w hose bonds are produced by chain reactions. Polyethylene and other alkene polymers are examples. 

The most common backbone structure found in commercial polymers is the saturated carbon-carbon structure. Polymers with saturated carbon-carbon backbones, such as polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polyacrylates, are produced using chain-growth polymerizations. The saturated carbon-carbon backbone of polyethylene with no side groups is a relatively flexible polymer chain. The glass transition temperature is low at -20°C for high-density polyethylene. Side groups on the carbon-carbon backbone influence thermal transitions, solubility, and other polymer properties. 

Chain growth polymerization. Important polymers manufactured hy chain growth are polyethylene, polystyrene, polyacrylonitrile, and polymethacrylates. 

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. 

We can create thermoplastic polymers by chain growth or step growth reactions. In either case the polymer chains consist of a string of monomer residues, each of which is attached to two other monomer residues. The polyethylene molecule shown in Fig. 1.1 is an example of a thermoplastic polymer made via chain growth polymerization, as shown in Fig. 1.7,... 

Chain growth polymers comprise most of the commodity polymers found in consumer products. Common examples include the polyethylene used in trash can liners, the polyvinyl chloride used as wire insulation, and the polypropylene used in food storage containers. ... 

Figure 2,3 Chain growth polymerization exemplified by free radical polymerization of polyethylene a) initiation, b) propagation, c) chain transfer, and d) termination...

Secondary processes are normally employed to crosslink chain growth polymers. In one example a linear thermoplastic, such as polyethylene, is compounded with an organic peroxide that is thermally stable at standard processing temperatures but decomposes to chemically react with the polymer chain at higher temperatures creating crosslinks. 

We make polyethylene resins using two basic types of chain growth reaction free radical polymerization and coordination catalysis. We use free radical polymerization to make low density polyethylene, ethylene-vinyl ester copolymers, and the ethylene-acrylic acid copolymer precursors for ethylene ionomers. We employ coordination catalysts to make high density polyethylene, linear low density polyethylene, and very low density polyethylene. 

An example of a backbiting reaction that creates the short chain branches is shown in Fig. 18.5. In this example the growing end of a polyethylene chain turns back on itself and abstracts a hydrogen atom from the carbon atom located four bonds away from the chain end, as shown in Fig. 18.5 a). Chain growth proceeds from the newly formed unpaired electron, leaving a pendant butyl group, as shown in Fig. 18.5 b). There are many variants of backbiting, which create a variety of short chain branches. 

Characterization results have been reported for dendrigraft-polylpthylene ox-ide)s of generation Gl, prepared by adding different amounts of ethylene oxide to the initiator core in the side chain growth reaction (Table 9.8). Because of the grafting from method used, the molecular weight of the polyethylene oxide) branches cannot be accurately determined, and hence it is impossible to confirm... 

Although several variations occur, depending on the reactive intermediate that is present, all addition polymers are formed by chain mechanisms, in which one initiator molecule causes a laige number of monomers to react to form one polymer molecule. For this reason these polymers are also known as chain-growth polymers. To better understand how this process occurs, let s examine a specific case, the formation of polyethylene by a radical chain mechanism, as shown in the following equation ... 

A Whole New Approach to Chain-Growth Polymers How the Hula Hoop Saved the Day From Polyethylene to Polypropylene— Stereoregularity Other Significant Advances Chain-Growth Copolymers Conducting Polymers... 

Ethylene is also polymerized by free-radical chain-growth polymerization. With ethylene, the free-radical intermediates are less stable, so stronger reaction conditions are required. Ethylene is commonly polymerized by free-radical initiators at pressures around 3000 atm and temperatures of about 200 °C. The product, called low-density polyethylene, is the material commonly used in polyethylene bags. 

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