Polyisoprene—polybutadiene star-block

Although the model studies indicated that selectivity of sulfonation was considerably less than desired, it was decided to proceed with the synthesis of the polyisoprene/polybutadiene star-block copolymer, anyway. It was felt that selectivity was good enough so that an appreciable fraction of the isoprene units could be sulfonated while limiting sulfonation of the butadiene units to some negligible level. For example, the data in Table II indicates that at 21 C, with a 1 1 mole ratio of sulfonating reagent, one should obtain about 50% sulfonation of the isoprene units with virtually no sulfonation of the butadiene units. This was deemed acceptable since the number of isoprene units at the chain ends could be simply doubled, and... 

Figure 7. Synthesis scheme for three-arm star polyisoprene/ polybutadiene (IBD) block copolymers.

Figure 8. Gel permeatioii chromatograms of polyisoprene/ polybutadiene block copolymers (IBD-2) arm (top) and star.

It is interesting to note that some of their electron micrographs exhibit features similar to those of a linear polystyrene-block-poly((4-vinylbenzyl)di-methylamine)-block-polyisoprene triblock copolymer with almost equal amounts of the three components, when cast from benzene [168] (Fig. 16c,d). Also a quaternary star copolymer consisting of polystyrene, polyisoprene, polybutadiene, and poly(4-methylstyrene) was reported, but no morphological characterization was given [79]. 

The anionic arm-first methods can also be applied to the synthesis of star block copolymers [59]. The procedure is identical except that living diblock copolymers (arising from sequential copolymerization of two appropriate monomers, added in the order of increasing nucleophilicity) are used as living precursor chains. The active sites subsequently initiate the polymerization of a small amount of a bis-unsaturated monomer (DVB in most cases) to generate the cores. If polystyrene and polyisoprene (or polybutadiene) are selected, the resulting star block copol)miers behave as thermoplastic elastomers because of their different glass transition temperatures. 

With the exception of a few commercial polymers such as polyisobutylene, polybutadiene and styrene-butadiene block copolymers, living polymers are prepared in small quantities under stringent conditions. Larger amounts can only be prepared by repeating the synthesis many times, and this is a costly and time-consuming process. In the case of hydrogenated polybutadiene, to prepare samples that resemble polyethylene, the need for a secondary reaction step renders the preparation even more costly. This has so far limited the extent to which it has been possible to use these materials to test models. Cell et al. [ 18] prepared asymmetric stars with structures similar to ethylene-propylene copolymers by hydrogenation of star-branched polyisoprene. The reactions to produce these materials took up to three weeks, and... 

By means of anionic polymerization, it is possible to produce in the laboratory linear polymers that are nearly monodisperse and have many types of branching such as multi-armed stars and combs and H-shaped molecules. For example, there have been reports of studies of anionically polymerized polystyrene, polybutadiene, and polyisoprene. An example of the anionic polymerization of a branched polymer is the technique of Roovers and Toporowski [22] for making comb polystyrenes. The varieties of model branched polymer that can be produced today by means of block polymerization and coupling chemistries include stars, H-shaped molecules, super-H molecules (multi-armed stars at both ends of a backbone segment), and combs of various types [23]. So-called pom-pom polymers are of special interest, because their rheological behavior has been modeled by McLeish and Larson [24]. These molecules have several arms at each end of a central crossbar, and polybutadienes having this structure have been synthesized [25,26]. 

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