Polymerizations by chain reactions

Chain. A linear or branched macromolecule is often called a chain because the repeating units are joined together like links in a chain. Many polymers are polymerized by chain reactions, which arc characterized by a scries of successive reactions initiated by a single primary event. Here the term chain is used to designate a kinetic sequence of reaction events which results in the production of a molecular chain composed of linked repeating units. 

Tetrafunctlonal monomers or polymers with vinyl side chains offer the unique possibility of quantum amplification by spatial polymerization. This reaction Is also shown In Figure 6. Multlcentered polymerization by chain reaction provides high speed and the mechanical-physical strength of a crossllnked polymer. However, because of the rapid Increase In viscosity, the amplification may not be as large as expected. 

F and MF are typical precursors to furan monomers bearing a moiety which can be polymerized by chain-reaction mechanisms. Scheme 6.11 provides a non-exhaustive array of entries into such structures, which have all been synthesized, characterized and polymerized [4], A major exception to this general postulate is constituted by furfuryl alcohol (2-hydroxymethylfuran, FA), which is in fact still today the most important commercially available furan compound, obtained by the catalytic reduction of F involving more than 80 per cent of its world production. FA is widely used as a polycondensation monomer and does not therefore belong to the class of coirpounds shown in Scheme 6.11. 

Synthetic polymers are classified by their method of synthesis as either chain-growth or step-growth. The categories ate somewhat imprecise but nevertheless provide a useful distinction. Chain-growth polymers are produced by chain-reaction polymerization in which an initiator adds to a carbon-carbon double bond of an unsaturated substrate (a vinyl monomer) to yield a reactive inter-... 

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. 

Addition polymerizations of unsaturated monomers leading to the formation of products of high molecular weight invariable proceed by chain reaction mechanisms. Primary activation of a monomer M (or a pair of monomers) is followed by the addition of other monomers in rapid succession... 

What percentage of polymer is usually found when a polymer produced by chain-reaction polymerization is heated above its ceiling temperature ... 

Although the mechanism of copolymerization is similar to that discussed for the polymerization of one reactant (homopolymerization), the reactivities of monomers may differ when more than one is present in the feed, i.e., reaction mixture. Copolymers may be produced by step-reaction or by chain reaction polymerization. It is important to note that if the reactant species are Mi and M2, then the composition of the copolymer is not a physical mixture or blend, though the topic of blends will be dealt with in this chapter. 

Polymers produced by chain-reaction polymerization Copolymerization Kinetics... 

The third major method for achieving difunctionality involves the ring-opening polymerization of a cyclic monomer, typified for example by the synthesis of polyamides from cyclic lactams. Reactions of this type proceed by chain-reaction mechanisms but yield polymers more typical of step-reactions, in that they contain functional groups within the chain. 

In contrast to polymers produced by chain reactions, those derived from step-reaction routes are expected to have lower molecular weights. Naarman327 has used ir spectroscopy to estimate the phenyl end-groups, and so the molecular weights, of polyphenylenes produced by the Kovacik method. By increasing the reaction temperature from 5 to 36 °C, he was able to increase the degree of polymerization from 10 to 45. Kovacik and co-workers 328) used a similar approach and have estimated... 

Cationic and anionic chain-growth polymerizations occur by chain reactions similar to those for free-radical polymerizations but involving charged intermediates (14.3 14.4). 

The application chain reaction is here somewhat confusing. The reaction sequence (XVI.10.2) does not correspond to a chain reaction in the strict sense, since at no point is the original chain earner reproduced. Only by generalizing the definition of chain carrier in the case of polymers to the point of identifying radical Mn- with Mn-i can we include these polymerizations under chain reactions. Undoubtedly part of the confusion arises from the fact that chains are produced as products. 

Inhibition [77]. An inhibitor is itself being consumed as it traps free radicals. To be effective, it must therefore be present in an excess over the initiator. In practice, this limits effective inhibition to chain reactions apt to be set off by small amounts of an initiator other than the bulk reactant. The most common application of inhibition is for protection of sensitive chemicals whose decomposition or polymerization by chain mechanisms may easily be triggered. 

Chain-growth polymers, also called addition polymers, are prepared by chain reactions. These compounds are formed by adding monomers to the growing end of a polymer chain. The conversion of vinyl chloride to poly(vinyl chloride) is an example of chain-growth polymerization. These reactions were introduced in Section 15.14. 

Copolymers may be produced by step reaction or by chain reaction polymerization in similar mechanisms to those of homopolymerization. The most widely used synthetic rubber (SBR) is a copolymer of styrene (S) and butadiene (B). Also, ABS, a widely used plastic, is a copolymer or blend of polymers of acrylonitrile, butadiene, and styrene. A special... 

Figure 8, Chain reaction photoinitiation. The efficiency of photoinitiation can be increased by chain reactions. Hydrogen abstraction by triplet excited benzophenone forms a THF free radical. Subsequent oxidation by the aryliodonium salt produces the THF cation capable of initiating polymerization, and a phenyl radical. Hydrogen abstraction by the phenyl radical produces the THF free radical completing the...

Ionic routes such as formation of the polymer anion by reaction with a strong base or the direct reaction of a polyamide with sodium are less likely to be used in reactive processing than is free-radical initiation. The process of self-graft polymerization by chain transfer to polymer, when it occurs in a single monomer/polymer system during polymerization, is an example of chain branching that is discussed in the next section. 

The new radical A, which results from chain transfer, can reinitiate polymerization by the reaction... 

Chain-growth polymers are made by chain reactions— by the addition of monomers to the end of a growing chain. These reactions take place by one of three mechanisms radical polymerization, cationic polymerization, or anionic polymerization. Each mechanism has an initiation step that starts the polymerization, propagation steps that allow the chain to grow at the propagating site, and termination steps that stop the growth of the chain. The choice of mechanism depends on the stmcture of the monomer and the initiator used to activate the monomer. In radical polymerization, the initiator is a radical in cationic polymerization, it is an electrophile and in cationic polymerization, it is a nucleophile. Nonterminated polymer chains are called living polymers. 

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