Polyisoprene center blocks

One way to approach the problem with the hysteretic loss associated with the onset of the polystyrene glass transition in vulcanized S-I-S polymers is with block copolymers having small random styrene-isoprene copolymer end-blocks, and a pure polyisoprene center block, IS-I-IS. The copolymer composition is chosen to produce a Tg of 40-45 C, at about 60% styrene content. At temperatures experienced in "green processing or tire building the block copolymer would be below Tg and strong, while at the elevated temperatures typically experienced by the vulcanized product in actual use, the end-blocks would be above their glass transition temperature... 

Table 13.12 also delineates triblock copolymers based on polystyrene-l)toc/c-polybutadiene-6tocA -polystyrene. Polyisoprene may be substituted for the polybutadiene, and/or the center block may be hydrogenated. The hydro-... 

In this paper we describe the preparation and the properties of the title triblock with a low vinyl-1,2 (or 3,4 in the case of polyisoprene) polydiene center block. Two different solvent systems were used as the media of polymerization. In the first system, the polydiene center block was prepared in cyclohexane. Alpha-methylstyrene (AMS) and a polar solvent tetrahydrofuran (THF) were then added. This was followed by a slow and continuous styrene addition to complete the end block preparation. In the second system, AMS itself was used as the solvent with no other solvent added. The second solvent system enabled us to use several different polymerization schemes. The center block could be prepared first to form a tapered or untapered triblock. The end block copolymer also could be prepared first and then the diblock and then coupled to form a tri- or a radial block polymer. Instead of coupling, more styrene could be added to complete the triblock. All these different routes of preparation were used in this work. 

Two other triblock polymers were prepared in cyclohexane. Run no. 13 used polyisoprene in place of polybutadiene as the center block. Run no. 14 used vinyltoluene in place of styrene in the SAMS end block copolymer. The polymerization conditions and results are shown in Table 5. 

McGrath and coworkers [217] demonstrated that the use of the dilithium initiator, 90, provides a very versatile method with respect to the ability to vary the chemical composition of the end blocks after first polymerizing a diene such as butadiene or isoprene. The a,to-dilithiumpolydiene center block can be used to initiate the polymerization of polar monomers to form both end blocks simultaneously. This type of triblock copolymer with polar end blocks cannot be prepared by other block copolymer synthesis procedures. Thus, they prepared poly(tert-butyl methacrylate)-btocfc-polyisoprene-Wock-poly(fcrf-butyl methacrylate) triblock copolymers. However, the resulting triblock copolymers did not exhibit well-defined structures. The molecular weight distributions tended to be broad (M /M =1.10-1.25) or bimodal. 

Thermoplastic elastomers with unsaturated center blocks, such as polybutadiene (SBS) or polyisoprene (SIS), are much more oxidation-prone than those with saturated elastomer segments such as ethylene/butylene (SEES) or ethylene/propylene (SEPS), because the elastomer component is more oxidation-prone than the thermoplastic component. UV stabilization thus has to be adapted to the elastomer component. Eor outdoor use, stabilization is recommended either with UV stabilizers or with carbon black filler. 

The different chemical structures in SBS and SIS center blocks result in different effects while butadiene crosslinks in SBS, leading to embrittlement in the material, the polyisoprene in SIS degrades, making the material soft and tacky [83]. 

A new star—block copolymer architecture, the inverse star—block copolymer, was recently reported.87 These polymers are stars having four polystyrene-/risoprene) copolymers as arms. Two of these arms are connected to the star center by the polystyrene block, whereas the other two are connected through the polyisoprene block. The synthetic procedure is given in Scheme 32. The living diblocks (I) were prepared by anionic polymerization and sequential addition of monomers. A small quantity of THF was used to accelerate the initiation of the polymerization of styrene. The living diblock copolymer (I) was slowly added to a solution of SiCL. The reaction was monitored by SEC on samples with-... 

A typical triblock copolymer may consist of about 150 styrene units at each end of the macromolecule and some 1,000 butadiene units in the center. The special physical properties of these block copolymers are due to inherent incompatibility of polystyrene with polybutadiene or polyisoprene blocks. Within the bulk material, there are separations and aggregations of the domains. The polystyrene domains are dispersed in continuous matrixes of the polydienes that are the major components. At ambient temperature, below the Tg of the polystyrene, these domains are rigid and immobilize the ends of the polydiene segments. In effect they serve both as filler particles and as cross-links. Above Tg of polystyrene, however, the domains are easily disrupted and the material can be processed as a thermoplastic polymer. The separation into domains is illustrated in Fig. 6.4. 

Polystyrene (PS) and polyisoprene (PI) form block copolymers with PA in the presence of Ti(OBu)4 catalysts [83-85]. The two copolymers are prepared similarly. For instance, styrene is first initiated by n-BuLi (typically 0.05 M) in an anionic polymerization. A lithiated polystyiyl anion can then displace one butoxy group from the titanium center to form a new Ti-C bond, which serves as the active site for the subsequent acetylene polymerization. However, before acetylene was added, this polymeric catalyst was often aged for 1 day (PS) or 2 days (PI). The acetylene polymerization was then carried out under dilute conditions so as to minimize side reactions. In this manner, acetylene can be polymerized through the Ti(UI) catalyst, forming an AB diblock copolymer. In the case of polystyrene, less than 20 wt% of PA in the copolymer renders the copolymer soluble. Gels that were not soluble could be pressed into thin films for characterization. 

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