Growth Addition Polymerization

Monomers contain double bonds and add end to end Requires an initiator 

Very fast reaction — up to 10 times faster than condensation polymerization 

Chain polymerization involves three steps. To start the reaction, a catalyst that can generate an active site, such as a free radical (R ), is used. In the initiation step, the radical adds to the double bond, and the radical site is moved to the end carbon. This new radical reacts with another molecule to give a larger radical, and the propagation reaction is imderway. Usually, the number of monomers in the chain is greater than 1000. In the above formulae. 

X indicates a small substituent, which may be an atom such as hydrogen (H) or chlorine (Cl) or it may he a group such as methyl (CH3), cyano (CN), carhoxyl (COOH), carbomethoxy (COOCH3), etc. The growing chain is terminated by collision with another chain or other radical source or by one of several other mechanisms. The number of monomer units in the polymer chain is the degree of polymerization, abbreviated DP. If the degree of polymerization is very low, the product is sometimes referred to as an oligomer. 

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Table 2.4. Synthesis of macromolecules by step growth addition polymerization (polyaddition)...

Polymerization reactions can proceed by various mechanisms, as mentioned earlier, and can be catalyzed by initiators of different kinds. For chain growth (addition) polymerization of single compounds, initiation of chains may occur via radical, cationic, anionic, or so-called coordinative-acting initiators, but some monomers will not polymerize by more than one mechanism. Both thermodynamic and kinetic factors can be important, depending on the structure of the monomer and its electronic and steric situation. The initial step generates... 

Table 3.1. Initiators for chain growth (addition) polymerizations ...

The newly formed R may also add to another alkene molecule to give a trimer. The process whereby simple molecules, or monomers, are merged can continue, eventually giving high-molecular-weight molecules called polymers. This reaction of alkenes is called chain-growth (addition) polymerization. The repeating unit in the polymer is called the mer. If a mixture of at least two different monomers polymerizes, there is obtained a copolymer. 

Table 5.1 Comparison of Step-Growth and Chain-Growth Polymerizations Step-growth/condensation polymerization Chain-growth/addition polymerization...

Step-growth/condensation polymerization Chain-growth/addition polymerization... 

In addition to the three fundamental processes described above for chain-growth (addition) polymerization, another important process, chain transfer, may occur. Chain transfer involves the transfer of radical reactivity to another species, which may be a monomer, a polymer, a solvent, an initiator, or an impurity. This creates a new species that is capable of further propagation but terminates the growth of the original chain. The transfer reaction involves the transfer of an atom between the radical and the molecule. For a saturated molecule like a solvent an atom is transferred to the radical ... 

Chain growth addition polymerization based on active carbene (e.g., styrene, vinyl carbazole, isobutylene) is a fast reaction that is most difficult to control since termination by combination or disproportionation is not possible. The initiator for cationic polymerization is a proton from a donor such as water, in the presence of a Lewis acid or sulfuric acid. Polymerization tends to be uncontrollable when undiluted monomers are used, and so these reactions are carried out at low monomer levels, usually in chlorinated... 

Synthetic polymers used to form fibers are often classified on the basis of their mechanism of polymerization--step growth (condensation) or chain growth (addition) polymerization. Step growth polymerization involves multifunctional monomers which undergo successive condensation with a second monomer or with itself to form a dimer, which in turn condenses with another dimer to form a tetramer, etc., usually with loss of a small molecule such as water. Chain growth involves the instantaneous growth of a long molecular chain from unsaturated monomer units, followed by initiation of a second chain, etc. The two methods are outl ined below schematically ... 

Free radical chain-growth addition polymerization of vinyl monomers is an important route to commercial polymers. The chain-growth mechanism involves three steps initiation of a chain, propagation of the growing chain, and termination of the reactive intermediates. 

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