Copolymers of isobutene

In addition there is the possibility that other olefins may generate polymers with low Tg s which show little or no crystallinity at room temperature and are therefore potentially elastomeric. One commercial example is butyl rubber (designated HR), a copolymer of isobutene with a small amount of isoprene. 

Copolymers of isobutene with styrene (< 10 %) and isoprene (< 3 %) may be used for the manufacture of food contact materials. The following polymerizates and polymer mixes can be added to these polymerizates polyethylene, polypropylene, styrene-acrylonitrile mixed polymers, mixed polymers of ethylene, propylene, butylene, vinyl esters and unsaturated aliphatic acids as well as salts and esters and polybutene-1. 

Considerable efforts have been directed, primarily in Kennedy s group [3], to synthesize a series of block copolymers of isobutene with isoprene [90,91], styrene derivatives [92-104], and vinyl ethers [105-107]. Figure 7 lists the monomers that have been used for the block copolymerizations with isobutene. The reported examples include not only AB- but also ABA- and triarmed block copolymers, depending on the functionality of the initiators (see Chapter 4, Section V.B, Table 3). Obviously, the copolymers with styrene derivatives, particularly ABA versions, are mostly intended to combine the rubbery polyisobutene-centered segments with glassy styrenic side segments in attempts to prepare novel thermoplastic elastomers. These styrene monomers are styrene, p-methylstyrene, p-chlorostyrene, a-methylstyrene, and indene. 

Figure 7 Block copolymers of isobutene a list of comonomers.

The butyl rubber (HR, as abbreviated in the ISO nomenclature for isobutene-isoprene rubber) random copolymer of isobutene with 0.5-2.5 wt% isoprene, and its haloderivatives (XIIR). 

As previously noticed, butyl rubber (HR), poly(methylpropene-co-2-methyl-1,3-butadiene), is a random copolymer of isobutene and 0.7-2.2 mol% of isoprene. The industrial slurry process used all over the world consists in a low-temperature copolymerization initiated by A1C13 in meth-ylchloride. In contrast to 1,3-butadiene, isoprene copolymerizes readily with the more reactive isobutene. Reactivity ratios of the pair isobutene-isoprene, ri = 2.5 0.5 and r2 = 0.4 0.1, measured at the conditions of industrial process [10], show that the copolymerization behaves ideally (ri-r2 = 1), and, at the used low concentration of isoprene, isolated units of this latter comonomer are randomly distributed along the chain with 90% M-p-aiw-enchainment [52,53] ... 

Systematic studies were carried out on copolymers of isobutene with 1,3,5-hexatriene (HTI) and 2,4,6-octatriene (OTI). The structure of the triene units present in the copolymer chains was determined by riKans of C-NMR by making reference to the cationic triene homopolymers and to the same copolymers previously hydrogenated. In the case of HTI there are only traces of repeat units descending from the 1,2- and 1,4-opening of the trienic system, the 1,6-enchainment being prevalent some cyclic structures are also present The cationic homopolymer of 2,4,6-octatriene results from the preval t (ca. S)%) 2,7-opening of the monomer, while the remainder unit results from 2,5-addition. A similar situation also seems to be present in OTI ... 

The only important commercial elastomer prepared by a cationic polymerization is butyl rubber, i.e., a copolymer of isobutene and isoprene. The latter... 

Although, there are many more cationicaUy polymerizable monomers than anionically polymerizable ones, relatively few cationic polymerizations (e.g., isobutene polymerization to produce polyisobutene and butyl rubber - copolymer of isobutene with small fractions of isoprene [18]) are performed industrially because macrocations are highly reactive and prone to suffer termination and chain transfer reactions. 

The only important commercial elastomer prepared by a cationic polymerization is butyl rubber, i.e., a copolymer of isobutene and isoprene. The latter monomer is incorporated in relatively small proportions (-1.5 mole % [76]) in order to introduce sufficient unsaturation for sulfur vulcanization. The slurry process with aluminum chloride at -98 to -90°C in methyl chloride diluent can be described by the accompanying flow sheet [115,116,124]. In this process the polymerization is almost instantaneous and extensive cooling by liquid ethylene is required to control the reaction. 

More precise control over the concentration of active centres may be obtained by the use of well-defined copolymers of isobutene and para-(chloromethyl)-styrene, which may be treated with EtjAlCl to yield benzylic cations as in reaction (9). Very high grafting efficiencies were obtained with indene, styrene, or a-methyl styrene as the graft monomer. 

Block copolymers of isobutene and styrene can be produced by homopolymerizing styrene in dichloromethane with titanium(IV)chloride and 2-chloro-2-phenylpropane as initiator system to a desired chain length followed by addition of isobutene [597]. At a reaction temperature of —50 °C the molecular weight of the block copolymer is = 45 000 and that of the styrene block is = 29 000. Homopolystyrene and homopoly(isobutene) are removed by extraction with pentane and butanone [598],... 

In addition to isobutene-styrene, di- and triblock copolymers can be synthesized by isobutene/a-methylstyrene. Further, block copolymers of isobutene can be made by polyaddition reactions with hydroxy- and dihydroxytelechelics of poly(isobutene). 

It is possible to graft isobutene onto activated polymer molecules when they can be alkylated in an electrophilic reaction [602-604]. Polystyrene, for instance, is alkylated with Friedel-Crafts catalysts. The grafting is simplified by the presence of halogen atoms in the main polymer chain. Addition of diethylaluminumchloride or triethylaluminum leads to the formation of cations. The graft polymers usually contain between 10 and 60% of isobutene units and feature outstanding elasticity and dimensional stability at higher temperatures. Also, copolymers of isobutene and isoprene, butadiene, styrene, and so on, can be grafted in this manner [605-607]. 

As mentioned above, a major limitation of polyisobutene is its tendency to cold flow. It is to be expected that this limitation would be substantially overcome by cross-linking the polymer. However, vulcanization by conventional techniques using sulphur is not possible since the polymer is saturated and heating with peroxides leads to extensive chain scission. A method of overcoming this difficulty was discovered by Thomas and Sparks of Standard Oil Development Company in 1937. It was found that copolymers of isobutene containing small amounts of conjugated dienes can be vulcanized with sulphur. The copolymer with isoprene, which is the preferred diene, is known as butyl rubber (or HR) and has become an important commercial material. The first commercial butyl plant came on stream in 1943 in the U.S.A. 

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