Polymers chain-growth polymerization

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

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. 

Polymerization reactions are classified as being either chain growth or step growth. In chain-growth polymerization, a small molecule reacts with a growing polymer chain to form an incrementally longer chain ... 

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

Conditions that are important to all chemical reactions such as stoichiometry and reactant purity become critical in polymer synthesis. In step growth polymerization, a 2% measuring and/or impurity error cuts the degree of polymerization or the molecular weight in half. In chain growth polymerization, the presence of a small amount of impurity that can react with the growing chain can kill the polymerization. 

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 gro tvth polymerization begins when a reactive species and a monomer react to form an active site. There are four principal mechanisms of chain growth polymerization free radical, anionic, cationic, and coordination polymerization. The names of the first three refer to the chemical nature of the active group at the growing end of the monomer. The last type, coordination polymerization, encompasses reactions in which polymers are manufactured in the presence of a catalyst. Coordination polymerization may occur via a free radical, anionic, or cationic reaction. The catalyst acts to increase the speed of the reaction and to provide improved control of the process. 

We can create crosslinks during chain growth polymerization by copolymerizing dienes with vinyl monomers. When the two vinyl functions of the diene are incorporated into separate chains, a crosslink is formed. This process is shown in Fig. 2.18. When we use a low concentration of dienes, we produce a long chain branched polymer, while high concentrations of dienes create a highly crosslinked polymer network... 

Scheme 5. Structural and chronological overview of the macromonomers used for synthesis of dendronized polymers chain growth (a) and step growth polymerizations (b)...

Chain-growth polymerization. A 1,2-polybutadlene polymer is crosslinked with t-butylstyrene, utilizing a free radical initiator. Reaction rates include... 

Chapters 5 through 7 deal with polymers formed from chain-growth polymerization. Chain-growth polymerization is also called addition polymerization and is based on free radical, cationic, anionic, and coordination reactions where a single initiating species causes the growth of a polymer chain. 

The present book contains about 110 detailed polymer recipes. Yet, for quite a number of common polymers recipes are missing. The following Tables 2.2,2.3, 2.4, and 2.5 attempt to fill this gap. The information provided includes the name of the monomer, the formula of the basic unit of the polymer, and references for detailed recipes. Table 2.2 lists polymers prepared by chain growth polymerization, Tables 2.3 and 2.4 those prepared by step growth polymerization, and Table 2.5 contains polymers obtained by chemical modifications of (natural) macromolecules. 

In chain growth polymerization reactions the average molecular weight, the molecular weight distribution and in some cases the type of terminal group of the polymer can be varied within certain limits by proper choice of reaction conditions and/or the addition of low-molecular-weight compounds (regula-... 

In general, there are two distinctively different classes of polymerization (a) addition or chain growth polymerization and (b) condensation or step growth polymerization. In the former, the polymers are synthesized by the addition of one unsaturated unit to another, resulting in the loss of multiple bonds. Some examples of addition polymers are (a) poly(ethylene), (b) poly(vinyl chloride), (c) poly(methyl methacrylate), and (d) poly(butadiene). The polymerization is initiated by a free radical, which is generated from one of several easily decomposed compounds. Examples of free radical initiators include (a) benzoyl peroxide, (b) di-tert-butyl peroxide, and (c) azobiisobutyronitrile. 

Commodity Chain-Growth Polymers. Two of the largest commodity wafer-soluble polymers are poly(vinyl alcohol) (PVA) and polyacrylamide (PAM). They are prepared by the free-radical initiation of vinyl monomers, a chain-growth polymerization technique. 

In chain-growth polymerization, propagation is caused by the direct reaction of a species bearing a suitably generated active center with a monomer molecule. The active center (a free radical, an anion, a cation, etc.) is generated chainwise by each act of growth the monomer itself constitutes the feed (reactive solvent) and is progressively converted into the polymer. 

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