Living cationic polymerization of isobutylene

The synthesis of A2B miktoarm star polymers has been discussed and exemplified using PIB as a component. The synthesis involves a quasi living cationic polymerization of isobutylene from a monofunctional cationic initiator. This initiator also contains a blocked hydroxyl group. Eventually, the blocked hydroxyl group of the initiator is deblocked, and functionalized with a branching agent. This activated reagent is then used for an atom transfer radical polymerization process of /erf-butyl acrylate (18). 

The INBFER technique has been used for quasi living cationic polymerization of isobutylene. Bi- and tnfunctional initiators yield telechelics with two or three chlorine end groups (Scheme 6). ... 

It is also possible to carry out living cationic polymerization of isobutylene, initiated by a difiinctional initiator." This results in a formation of bifunctional living segments of polyis-obutylene that are soft and rubbery. Upon completion of the polymerization, another monomer, one that yields stiff segments and has a high Tg value, like indene, is introduced into the living charge. Polymerization of the second monomer is initiated from both ends of the formed polyisobutylene. When the reaction is complete, the polymerization is quenched. Preparations of a variety of such triblock and star block polymers have been described." ... 

Studies of the living cationic polymerization of isobutylene and copolymerization with isoprene have begun (36,37). The living copolymerization of isobutylene and isoprene has so far produced a random copolymer with narrow molecular weight distribution and a well-defined structin-e. For example, the BClg/ciunyl acetate polymerization system in methyl chloride or methylene chloride at —30°C provides for copolymers with 1-8 mol% trans-l,4-isoprene units and Afn between 2000 and 12,000 with a M /Mn of imder 1.8. The advent of living polymerization... 

Figure 5.14. Iniferter-based living cationic polymerization of isobutylene.

As seen in Scheme 2 (A), the most of the syntheses have been carried out with the HI/I2 and HX/ZnX2 (X = halogen) initiating systems, because these systems can effectively polymerize a large variety of vinyl ethers, including those with pendant functions, into well-defined living polymers [1]. In this way, the sequential living cationic polymerizations of two vinyl ethers are mostly "reversible i.e., both A - B and B - A polymerization sequences are operable. This is in sharp contrast to the block copolymerization of a vinyl ether with a styrene derivative or isobutylene (see below), where such reversibility often fails to work. 

A new technique was developed recently, by introducing cationic to anionic transformation. A living carbocationic polymerization of isobutylene is carried out first. After it is complete, the ends of the chains are transformed quantitatively to polymerization-active anions. The additional blocks are then built by an anionic polymerization. A triblock polymer of poly(methyl methacrylate)-polyisobutylene-poly(methyl methacrylate) can thus be formed. The transformation involves several steps. In the first, a compound like toluene is Friedel-Craft alkylated by a,6t>"di-rerr-chloro-polyisobutylene. The ditolylpolyisobutylene which forms is lithiated in step two to form a,cu-dibenzyllithium polyisobutylene. It is then reacted with 1,1-diphenylethylene to give the corresponding dianion. After cooling to -78 °C and dilution, methyl methacrylate monomer is introduced for the second polymerization in step three. 

The first living cationic polymerization of isobutylvinyl ether was reported with HI/I in 1984 [29]. Sawamoto and coworkers suggested the Hl/vinyl ether adduct acts as an initiator while the acts as an activator [30]. In 1986, complexes of BQ3 with esters, such as cumyl acetate, were reported to initiate the living polymerization of isobutylene [31]. Then, a living cationic polymerization of A-vinylcarbazole with HI in toluene (-40 °C) and methylene chloride (-78 °C) was reported in 1987 [30]. 

In view of the great structural similarity between the propagating sites in the cationic polymerization of P-PIN and isobutylene and their respective polymers (4), and our considerable experience accumulated with the LC Pzn of isobutylene [1-3], efforts have been made to adapt LC Pzn conditions found to yield living polyisobutylenes for the polymerization of p-PIN. 

As the range of styrene derivatives for living cationic polymerization expands (Chapter 4, Section V.C), a variety of block copolymers with sty-renic segments have been synthesized. Most of the reported examples involve combinations of styrene derivatives with vinyl ethers or isobutene. Some examples of styrene derivative-vinyl ether block copolymers are listed in Fig. 6 [16,87-89]. Monomers that can form similar block copolymers with isobutylene are listed in Fig. 7 (Section III.B.3). 

The discovery of living cationic polymerization has provided methods and technology for the synthesis of useful block copolymers, especially those based on elastomeric polyisobutylene (Kennedy and Puskas, 2004). It is noteworthy that isobutylene can only be polymerized by a cationic mechanism. One of the most useful thermoplastic elastomers prepared by cationic polymerization is the polystyrene-f -polyisobutylene-(>-polystyrene (SIBS) triblock copolymer. This polymer imbibed with anti-inflammatory dmgs was one of the first polymers used to coat metal stents as a treatment for blocked arteries (Sipos et al., 2005). The SIBS polymers possess an oxidatively stable, elastomeric polyisobutylene center block and exhibit the critical enabling properties for this application including processing, vascular compatibility, and biostability (Faust, 2012). As illustrated below, SIBS polymers can be prepared by sequential monomer addition using a difunctional initiator with titanium tetrachloride in a mixed solvent (methylene chloride/methylcyclohexane) at low temperature (-70 to -90°C) in the presence of a proton trap (2,6-dt-f-butylpyridine). To prevent formation of coupled products formed by intermolecular alkylation, the polymerization is terminated prior to complete consumption of styrene. These SIBS polymers exhibit tensile properties essentially the same as those of... 

Living cationic polymerizations have been carried out with a number of monomers, such as isobutylene, styrene, p-methylstyrene, p-methoxystyrene, A/-vinyl caibazole, and others. To achieve living conditions, it is necessary to match the propagating carbon cation with the counterion, the solvent polarity, and the reaction temperature. Some examples are presented in Table 3.2. 

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