Kinetic Investigation of the Living Cationic Polymerization of Isobutylene Using a t-Bu-m-DCC/TiCl₄/2,4-DMP Initiating System

Abstract

A kinetic and mechanistic study of the living cationic polymerization of isobutylene was conducted using 5-tert-butyl-1,3-bis(2-chloro-2-propyl)benzene (t-Bu-m-DCC)/TiCl4 initiating system with 2,4-dimethylpyridine (DMP) as an electron donor in 60/40 (v/v) hexane/methyl chloride cosolvents. The complex between TiCl4 and DMP was observed to be largely soluble in this medium in the temperature range −50 to −80 °C. Over the same temperature range using a [t-Bu-m-DCC]o:[TiCl4]o ratio of 1:20, polymerizations were free of termination and chain transfer reactions, up to monomer conversions in excess of 95%, and molecular weight distributions were narrow. It was observed that the concentration of DMP had to be maintained in excess of the concentration of protic impurities to obtain living polymerizations. At −80 °C, the kinetic order of the rate of polymerization was unity with respect to both monomer and initiator concentrations, approximately 2 with respect to [TiCl4]eff, defined as [TiCl4]o − [DMP], and −0.25 with respect to DMP. These results were similar to those obtained using pyridine as an electron donor, which forms an insoluble complex with TiCl4. Similar polymerization rates were obtained for various polymerizations in which the same [TiCl4]eff was obtained from different combinations of [TiCl4]o and [DMP], demonstrating that the fractional, negative kinetic order of the rate with respect to DMP reflects the reduction in [TiCl4]o caused by complexation. The apparent activation energy for the rate of polymerization was observed to be negative, attributed to an equilibrium between dormant and active growing chains. It was proposed that the observed second-order kinetic dependency on the TiCl4 concentration results from the fact that propagation takes place predominantly through chains possessing dimeric gegenions and that the latter must form by reaction of additional TiCl4 with monomeric gegenions, rather than direct ionization of chains by neutral, dimeric Ti2Cl8.

TiCl4 Reaction Order in Living Isobutylene Polymerization at Low [TiCl4]:[Chain End] Ratios

“…4 It seems reasonable that a similar reaction would occur in the formation of onium salts. If [TiCl 4 ] 0 .…”

“…Under the most common reaction conditions, i.e., [TiCl 4 ] > the concentration of polymer chain ends (CE), a secondorder dependence on the [TiCl 4 ] has been consistently reported in a number of papers by at least two different groups. [3][4][5][6] The sole contrary report under these reaction conditions is that of Kennedy and co-workers, 19 who claimed a first-order [TiCl 4 ] dependence. A polymerization mechanism ( Figure 2) involving dimeric counterions of the type Ti 2 Cl 9 -has been advanced to account for the observed second-order dependency on [TiCl 4 ].…”

“…A polymerization mechanism ( Figure 2) involving dimeric counterions of the type Ti 2 Cl 9 -has been advanced to account for the observed second-order dependency on [TiCl 4 ]. 3,4 Figure 2 indicates that the formation of dimeric gegenions proceeds exclusively by reaction of monomeric gegenions with additional TiCl 4 (as opposed to direct ionization by Ti 2 Cl 8 ) 4 and that propagation can theoretically occur through chains carrying either monomeric or dimeric gegenions, or both. However, the experimental observation of a second-order dependency on [TiCl 4 ] suggests that propagation via monomeric gegenions is insignificant.…”

Macromolecular Engineering via Carbocationic Polymerization: Branched Structures, Block Copolymers and Nanostructures

“…All three methods yielded polymers with M -n A 400 000 g/mol. [24 -30] Kinetic investigations showed that living isobutylene polymerization co-initiated by TiCl 4 proceeds via ion pairs [19,31,32] and that the ionization equilibrium is strongly shifted towards covalent species (see Equation (6)). A similar mechanism was suggested for the living polymerization of isobutyl vinyl ether initiated by the monomer chloride/ZnCl 2 system.…”

“…[65] Fiber-optic real-time FTIR monitoring demonstrated that N,N-dimethylacetamide (DMA), a strong electron pair donor, complexes with TiCl 4 under polymerization conditions as postulated by Storey. [32] Figure 5 contains FTIR spectra of DMA before and after the addition of TiCl 4 . DMA has a strong carbonyl absorption at 1 659 cm -1 , and another strong band at 1 187 cm -1 , assigned to the amide groups.…”