Alkenes, chain polymerization propagation

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

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. 

According to present ideas, the initiation mechanism of diene polymerization on ZN centres does not substantially differ from that of alkene initiation. Experiments support the idea according to which the chain is propagated on the transition metal atom it may be bound to the metal either by a a, or by a n allylic bond... 

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... 

As a simple computational model for the catalysis of alkene polymerization, let us consider some aspects of the general chain-propagation reaction... 

Intermolecular hydride transfer to polymer probably accounts for the short-chain branching found in the polymerizations of 1-alkenes such as propene. The propagating carbocations are reactive secondary carbocations that can abstract tertiary hydrogens from the polymer... 

For ethylene polymerization, the iso- (syndio-) specific catalytic activity is not needed. For higher 1-alkene polymerizations it is, of course, indispensable crystalline polymers are in greater demand. For some time, third-generation catalysts could not be widely used because of their insufficient ability to induce tactic chain propagation. This drawback was overcome by the revamped discovery of the donor effect of Lewis bases [285]. 

The polymer chain grows by propagation with regeneration of the same type of acti ve species. Thus, propagation is an Ads reaction in carbo-cationic alkene polymerizations [Eq. (49)], and an Sn2 reaction in most ring-opening polymerizations,... 

The degenerative nature of propagation results in reformation of the same active species, but with monomer consumption and chain growth. Although the monomer s thermodynamic polymerizability is independent of the mechanism, the mechanism and structure of the active species determines the rate of monomer conversion. The structure of the active species involved in carbocationic polymerizations was discussed in Section II detailed information on the reactivities of model species was presented in Chapter 2, with the conclusion that covalent precursors do not react directly with alkenes, but must first ionize to sp2-hybridized carbenium ions. Only the resulting carbenium ions can add to double bonds. 

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