Monomers chain-growth polymerization

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 ... 

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... 

A final example of homogeneous catalysis is the use of metallocene catalyst systems in chain growth polymerization processes. The metallocene, which consists of a metal ion sandtviched between two unsaturated ring systems, is activated by a cocatalyst. The activated catalyst complexes with the monomer thereby reducing the reaction s energy of activation. This increases the rate of the reaction by up to three orders of magnitude. 

PHA is produced in Alcaligenes eutrophus from acetyl CoA in three steps and the last step is the chain growth polymerization of hydroxyalkanoate CoA esters catalyzed by PHA polymerase (synthase), yielding PHA of high molecular weight. Kinetics and mechanism of the polymerization of hydroxyalkanoyl CoA monomers with this bacterial polymerase have been investigated. 

Chain growth polymerizations (also called addition polymerizations) are characterized by the occurrence of activated species (initiators)/active centers. They add one monomer molecule after the other in a way that at the terminus of each new species formed by a monomer addition step an activated center is created which again is able to add the next monomer molecule. Such species are formed from compounds which create radicals via homolytic bond scission, from metal complexes, or from ionic (or at least highly polarized) molecules in the initiating steps (2.1) and (2.2). From there the chain growth can start as a cascade reaction (propagation 2.3) upon manifold repetition of the monomer addition and reestablishment of the active center at the end of the respective new product ... 

Monomers appropriate for chain-growth polymerizations either contain double or triple bonds or are cyclic, having a sufficiently high ring strain ... 

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. 

Chain-growth polymerizations are characterized by chains that propagate by adding one monomer molecule at a time, ie, rr-mer + monomer — x + l)-mer. There are, however, several mechanisms by which this occurs. 

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. 

The reaction is called an addition reaction because two monomers are added to each other with the elimination of a double bond. This is also called a chain growth polymerization reaction. However, the reaction as such does not go without the help of an unstable molecule, called an initiator, that starts the reaction. Benzoyl peroxide or i-butyl benzoyl peroxide are such initiators. Benzoyl peroxide splits into two halves under the influence of heat or ultraviolet light and thus produces two free radicals. A free radical is a molecular fragment that has one unpaired electron. Thus, when the central bond was broken in the benzoyl peroxide, each of the shared pair of electrons went with one half of the molecule, each containing an unpaired electron. 

To name a copolymer prepared by chain-growth polymerization, we simply link the names of the two (or more) monomers with co and precede the names with poly. The set of names is usually placed in parentheses to avoid confusion. 

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