Alkenes, chain polymerization

Radical Polymerization of Alkenes Chain-Growth Polymers... 

A wide range of carbon-carbon double bonds undergo chain polymerization. Table 3-1 shows monomers with alkyl, alkenyl, aryl, halogen, alkoxy, ester, amide, nitrile, and heterocyclic substituents on the alkene double bond. 

Chain polymerizations of alkenes are exothermic (negative AH) and exoentropic (negative AS). The exothermic nature of polymerization arises because the process involves the exothermic conversion of re-bonds in monomer molecules into CT-bonds in the polymer. The negative AS for polymerization arises from the decreased degrees of freedom (randomness) for the polymer relative to the monomer. Thus, polymerization is favorable from the enthalpy viewpoint but unfavorable from the entropy viewpoint. Table 3-15 shows the wide range of... 

Strictly speaking, some kind of coordination is a prerequisite for any ionic polymerization. Some active centres can bind the monomer prior to its controlled attachment to the end of a propagating macromolecule. Chains of a regular or tactic polymer are thus formed. Such processes are designated as coordination polymerizations proper. At the present time, the centres of alkene coordination polymerizations and the precursors of such centres are of greatest importance. 

Initiation is a prerequisite to any chain polymerization it generates the active sites capable of propagation. Cationic initiation of alkenes is often a... 

Alkenyl- and alkynyl-triazoles have received little attention. By analogy with the behaviour of other azoles they are expected to polymerize but to be less reactive in addition reactions than alkenes or alkynes. Although the most promising polymers derived from triazoles are obtained by different methods (see Section 4.12.5.2.3), some information is available on potentially polymerizable vinyltriazole (63MI4120i). The styryltriazole (134) could be oxidized to 3-methyl-l-phenyl-l,2,4-triazole, i.e. without affecting either the triazole or iV-phenyl ring, but hydroxylation of the alkene chain failed (s4JCS4256). 

S25.5 The polymerization of mono-substituted alkenes introduces sterogenic centres along the carbon chain at every other position. Without R groups attached to the Zr centre there is no preference for specific binding of new alkenes during polymerization and thus the repeat is random or atactic (shown below). 

Many examples of such eliminations have now been seen for the f-block and for d° metals. This type of jS-alkyl elimination is recognized as an important chain transfer step in Ziegler-Natta and metallocene polymerization catalysis. When it occurs the polymer chain terminates in a C=C bond (equation 2) and in certain cases the alkene chain end can undergo reinsertion and get back into the polymer growth... 

The steps involved in the chain polymerization of alkenes using this type of catalyst are shown in Eqs. (2.81)-(2.85) (Ver Strate, 1986). ... 

As in the case of intramolecular hydrogen abstraction, branching by chain transfer is not a problem when alkenes are polymerized under Ziegler-Natta conditions because free radicals are not intermediates in coordination polymerization. 

The types of compounds that can be polymerized readily by the radical-chain mechanism are the same types that easily undergo free-radical addition reactions. Alkenes with aryl, ester, nitrile, or halide substituent groups that can stabilize the intermediate radical are most susceptible to radical polymerization. Terminal alkenes are generally more reactive toward radical-chain polymerization than more highly substituted isomers. The dominant mode of addition in radical-chain polymerization is head-to-tail. The reason for this orientation is that each successive addition of monomer takes place in such a way that the most stable possible radical intermediate is formed. For example, the addition to styrene occurs to give the phenyl-substituted radical to acrylonitrile, to give the cyano-substituted radical ... 

Chain polymerization of alkenes can also be catalyzed by anionic reagents. Anionic polymerization occurs most readily when the alkene carries a carbanion stabilizing substituent. Even ethylene can be polymerized anionically, however, if a... 

RADICAL POLYMERIZATION OF ALKENES CHAIN-GROWTH POLYMERS... 

Alkenes also polymerize when they are treated with strong acids. The growing chains in acid-catalyzed polymerizations are cations rather than radicals. The following reactions illustrate the cationic polymerization of isobutylene ... 

Undoubtedly, the most important reaction of the alkenes is polymerization, in which the double bond reacts to produce long chains of the once-alkenes bonded together. This is the process used to produce plastics (see Chapter IQ. 

The characteristic feature of a chain-initiation step is the formation of radicals from a molecule with only paired electrons. In the case of peroxide-initiated polymerizations of alkenes, chain initiation is by (1) heat cleavage of the O—O bond of a peroxide to give two alkoxy radicals and (2) reaction of an alkoxy radical with a molecule of alkene to give an alkyl radical. In the general mechanism shown, the initiating catalyst is given the symbol In-In and its radical is given the symbol In. ... 

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