Chain polymerization of alkenes

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... 

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). ... 

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... 

The corresponding reactions of transient Co(OEP)H with alkyl halides and epoxides in DMF has been proposed to proceed by an ionic rather than a radical mechanism, with loss of from Co(OEP)H to give [Co(TAP), and products arising from nucleophilic attack on the substrates. " " Overall, a general kinetic model for the reaction of cobalt porphyrins with alkenes under free radical conditions has been developed." Cobalt porphyrin hydride complexes are also important as intermediates in the cobalt porphyrin-catalyzed chain transfer polymerization of alkenes (see below). 

Stereochemistry Coordination Polymerization. Stereoisomerism is possible in the polymerization of alkenes and 1,3-dienes. Polymerization of a monosubstituted ethylene, such as propylene, yields polymers in which every other carbon in the polymer chain is a chiral center. The substituent on each chiral center can have either of two configurations. Two ordered polymer structures are possible — isotactic (XII and syndiotactic (XIII) — where the substituent R groups on... 

Radical Polymerization of Alkenes Chain-Growth Polymers... 

In general, the catalysts may be classified as acids and metal halides. As will be explained below, both types of catalysts are acid-acting catalysts in the modern sense of the term. Some metals (e.g., sodium, copper, and iron) are catalysts for the polymerization of alkenes, especially ethylene. They are active probably because they can combine with one of the pi electrons of the alkene and form a free radical which can then initiate a chain reaction (p. 25). 

Isomerism is observed in the polymerization of alkenes when one of the carbon atoms of the double bond is monosubstituted. The polymerization of a monosubstituted ethylene, CH2=CHR (where R is any substituent other than H), leads to polymers in which every tertiary carbon atom in the polymer chain is a stereocenter (or stereogenic center). The... 

Cationic polymerization of alkenes involves the formation of a reactive carbo-cationic species capable of inducing chain growth (propagation). The idea of the involvement of carbocations as intermediates in cationic polymerization was developed by Whitmore.5 Mechanistically, acid-catalyzed polymerization of alkenes can be considered in the context of electrophilic addition to the carbon-carbon double bond. Sufficient nucleophilicity and polarity of the alkene is necessary in its interaction with the initiating cationic species. The reactivity of alkenes in acid-catalyzed polymerization corresponds to the relative stability of the intermediate carbocations (tertiary > secondary > primary). Ethylene and propylene, consequently, are difficult to polymerize under acidic conditions. 

Most technically important polymerizations of alkenes occur by chain mechanisms and may be classed as anion, cation, or radical reactions, depending upon the character of the chain-carrying species. In each case, the key steps involve successive additions to molecules of the alkene, the differences being in the number of electrons that are supplied by the attacking agent for formation of the new carbon-carbon bond. For simplicity, these steps will be illustrated by using ethene, even though it does not polymerize very easily by any of them ... 

Mixtures of alkyl halides and Lewis acids are well-known initiating systems for the polymerizations of alkenes, and the mechanism suggested for these reactions by Kennedy [55] appears to be generally accepted (Scheme 9), although the importance of the chain transfer step from initiator has been questioned [56]. 

Radical polymerization of alkenes was first discussed in Section 15.14, and is included here to emphasize its relationship to other methods of chain-growth polymerization. The initiator is often a peroxy radical (RO-), formed by cleavage of the weak 0-0 bond in an organic peroxide, ROOR. Mechanism 30.1 is written with styrene (CH2=CHPh) as the starting material. 

Anionic polymerization (Section 30.2C) Chain-growth polymerization of alkenes substituted by electron-withdrawing groups that stabilize intermediate anions. 

Radical polymerization (Section 15.14B) A radical chain reaction involving the polymerization of alkene monomers by adding a radical to a 7t bond. 

One such process is the Cossee-Arlman mechanism,proposed for the Ziegler-Natta polymerization of alkenes (also discussed in Section 14-4-1). According to this mechanism, a polymer chain can grow as a consequence of repeated 1,2 insertions into a vacant coordination site, as follows ... 

There are a number of others in common industrial use, such as those used for polymerization of alkenes. One example of an organometallic homogeneous catalyst is the Zr(IV) complex [Zr(CH3)(T 5-Cp)2X], which operates by binding alkene monomers to the metal prior to addition to a growing carbon chain. A similar coordination of substrate is involved in the use of [Co(CO)4H] as catalyst in the hydroformylation of alkenes to aldehydes. Likewise, the [Rh(CO)2l2] ion, formed in situ, catalyses the carbonylation of methanol to acetic acid. 

Chain Transfer and Termination There are a variety of reactions by which a propagating cationic chain may terminate by transferring its activity. Some of these reactions are analogous to those observed in cationic polymerization of alkenes (Chapter 8). Chain transfer to polymer is a common method of chain termination. Such a reaction in cationic polymerization of tetrahydrofuran is shown as an example in Fig. 10.1. Note that the chain transfer occurs by the same type of reaction that is involved in propagation described above and it leads to regeneration of the propagating species. Therefore, the kinetic chain is not affected and the overall effect is only the broadening of MWD. 

Chain walking is a typical side reaction in the polymerization of alkenes, producing branching, and many examples can be found [76],... 

---