Alkenes, chain polymerization initiation

The chiral sites which are able to rationalize the isospecific polymerization of 1-alkenes are also able, in the framework of the mechanism of the chiral orientation of the growing polymer chain, to account for the stereoselective behavior observed for chiral alkenes in the presence of isospecific heterogeneous catalysts.104 In particular, the model proved able to explain the experimental results relative to the first insertion of a chiral alkene into an initial Ti-methyl bond,105 that is, the absence of discrimination between si and re monomer enantiofaces and the presence of diastereoselectivity [preference for S(R) enantiomer upon si (re) insertion]. Upon si (re) coordination of the two enantiomers of 3-methyl-l-pentene to the octahedral model site, it was calculated that low-energy minima only occur when the conformation relative to the single C-C bond adjacent to the double bond, referred to the hydrogen atom bonded to the tertiary carbon atom, is nearly anticlinal minus, A- (anticlinal plus, A+). Thus one can postulate the reactivity only of the A- conformations upon si coordination and of the A+ conformations upon re coordination (Figure 1.16). In other words, upon si coordination, only the synperiplanar methyl conformation would be accessible to the S enantiomer and only the (less populated) synperiplanar ethyl conformation to the R enantiomer this would favor the si attack of the S enantiomer with respect to the same attack of the R enantiomer, independent of the chirality of the catalytic site. This result is in agreement with a previous hypothesis of Zambelli and co-workers based only on the experimental reactivity ratios of the different faces of C-3-branched 1-alkenes.105... 

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

Although carboxylic acids generally form 1 1 adducts with alkenes, the resulting esters are easily ionized in the presence of either Lewis or protonic acids. The higher efficiency of chlorinated acetic acids relative to hydrogen halides is ascribed to the ability of their 1 1 adducts to coordinate with excess acid. Alkyl halides are eventually formed when carboxylic acids are used to initiate polymerization in the presence of a Lewis acid due to migration of the carboxylate moiety to the Lewis acid [Eq. (25)]. Similarly, styrene and isobutene polymerizations initiated by preformed alkyl acetate adducts in the presence of BC13 always produce Cl-terminated chains [104,105]. 

Table 27.2 reviews alkene polymerizations that proceed by free radicals and by coordination complexes of the Ziegler-Natta type. Both are chain-growth processes their propagation steps were outlined in Mechanism 6.11 (page 265) and Mechanism 14.5 (page 635), respectively. The present section examines two other significant factors in alkene polymerization initiation and termination. 

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. ... 

The type of monomers suitable for cationic polymerization are those containing an electron-donating substituent such as 1,1-dialkyl, alkene, alkoxy, and phenyl that stabilize the propagating cationic centers. Successftil industrial examples include polyisobutylene and its copolymers with dienes such as butyl mbber. In ionic polymerization, initiator is conventionally called a catalyst. However, by definition, catalyst and initiator are two different types of reagents. Catalyst takes part in reactions but can be removed from the final product if necessary. On the other side, initiator molecules or their fragments become a part of the produced chains after polymerization. In cationic polymerization, a single catalyst is not sufficient and a cocatalyst is required. Typical catalysts are Lewis acids such as BF3, AICI3, and TiCU that must be used with a protonic cocatalyst such as H2O and methanol ... 

Conjugated dienes can be polymerized just as simple alkenes can (Section 7.10). Diene polymers are structurally more complex than simple alkene polymers, though, because double bonds remain every four carbon atoms along the chain, leading to the possibility of cis-trans isomers. The initiator (In) for the reaction can be either a radical, as occurs in ethylene polymerization, or an acid. Note that the polymerization is a 1,4-addition of the growing chain to a conjugated diene monomer. 

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. 

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). 

The mechanism for the stereoselective polymerization of a-olefins and other nonpolar alkenes is a Ti-complexation of monomer and transition metal (utilizing the latter s if-orbitals) followed by a four-center anionic coordination insertion process in which monomer is inserted into a metal-carbon bond as described in Fig. 8-10. Support for the initial Tt-com-plexation has come from ESR, NMR, and IR studies [Burfield, 1984], The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond together with an electrophilic attack by the cationic counterion on the alkene Ti-electrons. 

Siloxane polymerization differs mechanistically from the formation of hydrocarbon polymers in that it is essentially an acid-base process, as might be expected from the strong alternation of electronegativites along the het-eroatomic chain, and the radical initiators that catalyze the homocatenation of alkenes do not work for siloxanes. Long, unbranched polysiloxane chains are favored by higher condensation reaction temperatures and basic catalysts such as alkali metal hydroxides. Acidic condensation catalysts tend to produce polymers of lower molar mass, or cyclic oligomers. 

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. 

Initiation of alkene polymerization by the anion-chain mechanism may be formulated as involving an attack by a nucleophilic reagent Y e on one end of the double bond and formation of a carbanion ... 

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