Isobutylene polymerizations, controlled

The first paper of this series concerns the effects of f-BuX, Me3Al, Et2AlX and EtAlCl2, and MeX on PIB yields and polymerization rates. The second paper1 will survey and discuss the effects of reaction variables on molecular weights of PIB and molecular weight control in isobutylene polymerization. 

A relation between A jjv and molecular weight controlling mechanisms was discovered and the effect of initiator system, solvent and temperature on a mv was explained. The present work has led to an understanding of the effect of counteranion on PIB molecular weight. These studies provide better insight into the detailed mechanism of isobutylene polymerization, in particular into the initiation and the molecular weight controlling events. 

V. The Relationship Between AEmv and Molecular Weight Control in Isobutylene Polymerization... 

Table 8 is a comprehensive compilation of AEmv values obtained in this research together with those available from the literature. An examination of the data led to a definition of the effect of initiating system, solvent and temperature, and to a general hypothesis on molecular weight control in isobutylene polymerization. 

Another consequence of this hypothesis is that for every isobutylene polymerization system, there must exist three temperature regions over which molecular weight control is, respectively by termination, a combination of transfer to monomer and termination, and transfer to monomer alone although due to experimental limitations all three regions may not be possible to detect. 

Importantly, also, isobutylene polymerizations initiated by 7-radiation in bulk are characterized by AEjjv = —6-6 kcal/mole in the range from +29° to —78 °C52. Since counteranion is absent in these systems, termination by counteranion must also be absent and molecular weight control can only occur by transfer to monomer. 

In sum, a relation of counteranion nucleophilicity and the molecular weight in isobutylene polymerization is discovered, according to which an increase in G nucleophility leads to an increase in the rate of termination but a decrease in the rate of chain transfer to monomer. Thus, an increase in G6 nucleophilicity leads to increased termination and hence decreased molecular weight for systems in which termination is molecular weight governing. Similarly, it leads to a decrease in rate of transfer and hence to an increase in molecular weights for systems in which chain transfer controls molecular weight. The nucleophilicity of G is determined by the... 

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. 

Early Developments till the Discovery of Controlled Initiation. Under suitable conditions any electrophilic species may induce cationic polymerizations (1). As a practical matter, the most convenient cationogens are Bronsted acids alone or in conjunction with Frie-del-Crafts acids (1). Systematic research on the initiation of carbocationic polymerization became possible by the discovery of coinitiation by British investigators (2-4). These workers found that the strong Lewis acid BF3 alone is unable to initiate isobutylene polymerization but in the presence of suitable cationogens, i.e., H2O, immediate and vigorous polymerization ensues. Their formalism ... 

Kennedy and Squires investigated termination and transfer processes in isobutylene polymerization initiated by AICI3 at -78°C in n-pentane (160) and devised a scale of termination or poison and transfer coefficients. Most compounds acted as both poisons and chain-transfer agents. In industry, chain-transfer agents are often deliberately used to control polymerization processes (161). 

Derivatives of polyisobutylene (6. in Figure 9.1) offer the advantage of control over the molecular weight of the polyisobutylene obtained by cationic polymerization of isobutylene. Condensation on maleic anhydride can be done directly either by thermal activation ( ene-synthesis reaction) (2.1), or by chlorinated polyisobutylene intermediates (2.2). The condensation of the PIBSA on polyethylene polyamines leads to succinimides. Note that one can obtain mono- or disuccinimides. The mono-succinimides are used as... 

Polybutenes. Polybutenes are produced by controlled polymerization of butenes and isobutene (isobutylene) (see Butylenes). A typical polyisobutylene stmcture is... 

Butyl rubber is produced at very low temperature (below — 90°C) to control the rapid exotherm, and to provide high molecular weight. The process consists of charging isobutylene along with isoprene (2-4%) with an inert diluent such as methyl chloride to a reactor to which a Friedel-Crafts catalyst is added. The polymerization is very rapid, and the polymer forms in a crumb or slurry in the diluent. Heat is removed via the reactor jacket. The slurry is steam-stripped to remove all volatiles. The catalyst is neutralized, and antioxidants are added to the slurry prior to drying.53 The halogenated derivatives are produced by the direct addition of the halogen to a solution of the isobutylene-isoprene polymer. 

Account for the fact that 1-butene can be used to control polymer molecular weight in cationic polymerization of isobutylene. 

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