Styrene-Butadiene Sequence Copolymers

Butadiene-Styrene Copolymers from Ba-Mg-Al Catalyst Systems. Figure 13 shows the relationship between copolymer composition and extent of conversion for copolymers of butadiene and styrene (25 wt.7. styrene) prepared in cyclohexane with Ba-Mg-Al and with n-BuLi alone. Copolymerization of butadiene and styrene with barium salts and Mg alkyl-Al alkyl exhibited a larger initial incorporation of styrene than the n-BuLi catalyzed copolymerization. A major portion of styrene placements in these experimental SBR s are more random however, a certain fraction of the styrene sequences are present in small block runs. 

It is evident that reactions of unsaturated polymers with bisnitrile oxides lead to cross-linking. Such a procedure has been patented for curing poly(butadiene), butadiene-styrene copolymer, as well as some unsaturated polyethers and polyesters (512-514). Bisnitrile oxides are usually generated in the presence of unsaturated polymers by dehydrochlorination of hydroximoyl chlorides. Cross-linking of ethylene-propylene-diene co-polymers with stable bisnitrile oxides has been studied (515, 516). The rate of the process has been shown to reduce in record with the sequence 2-chloroterephthalonitrile oxide > terephthalonitrile oxide > 2,5-dimethylterephthalonitrile oxide > 2,3,5,6-tetramethylterephthalo-nitrile oxide > anthracene-9,10-dicarbonitrile oxide (515). 

Winston A, Wichacheewa P. Sequence distribntion in 1-chloro-1,3-butadiene styrene copolymers. Macromolecnles 1973 6 200-205. 

Another area in which both proton and carbon-13 nmr have proved very powerful is the determination of the structure of copolymers. This has a long history (ref. 6, Chap. X 15,16)9 beginning with the observations of butadiene-styrene copolymers in 1959, Again, the information content of the spectra has increased remarkably since these early reports. Although compositional sequence lengths and probabilities can be calculated from the copolymer equation using the traditional data of polymer composition vs. monomer feed composition, nmr allows direct measurement of the sequences and gives in addition much structural detail not available from overall composition alone. 

C13-0006. In a 3 1 copolymer of butadiene and styrene, the placement of butadiene and styrene fragments along the chain is random. Draw a line stmeture for a portion of the copolymer that has this sequence -butadiene-styrene-styrene-butadiene-. 

Another important class of copolymers synthesized by chain polymerisation are block (or sequenced) copolymers diblock and triblock copolymers being the most important ones. They are very useful as compatibilisers (emulsifiers) in immiscible polymer blends. Another major use is as thermoplastic elastomers. Both uses are best explained through the example of butadiene-styrene block copolymers. 

Order-disorder transitions and spinodals were computed for linear multi block copolymers with differing sequence distributions by Fredrickson et al. (1992). This type of copolymer includes polyurethanes, styrene-butadiene rubber, high impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene (ABS) block copolymers. Thus the theory is applicable to a broad range of industrial thermoplastic elastomers and polyurethanes. The parameter... 

The work was planned on the basis of a model of a dispersed solid particle onto which one type of sequences of a BG copolymer is adsorbed selectively while the other type sequence is dissolved in the dispersion medium. A sketch of this model is shown in Figure 1. The model is the result of applying the same arguments which had been advanced (12) in discussing the mechanism of stabilization of polymeric oil-in-oil emulsions by BG copolymers to the problem of stabilization of dispersions of solid particles in organic media. Previously, essentially the same arguments had led to the demonstration of micelle formation of styrene-butadiene block copolymers in organic media under certain conditions (15). 

In addition to the relative ratio of the monomers, the arrangement of the units in the chain is important. This arrangement is referred to as the copolymer sequence distribution. In the previous discussion, the assumption was made that the comonomer units were well mixed in the polymer chain. If this is not the case, parts of the chain can reflect properties of the corresponding homopolymer. It is thus possible to produce polymers that have significantly different properties in different parts of the polymer chain. A most dramatic example of this can be found in styrene-butadiene-styrene or styrene-isoprene-styrene thermoplastic elastomers. The properties of these unique materials will be discussed in the section Thermoplastic Elastomers. ... 

The structures listed in Table 1.6 are divided into three categories Short sequences, Long sequences, and Networks. Within the first category a sequence of placement of individual CRU is considered, within the second the placement of long sequences of CRU defines the copolymer type, while to the third belong crosslinked networks, crosslinked polymers, and chemical-type interpenetrating polymer networks. The network is a crosslinked system in which macromolecules of polymer A are crosslinked by macromolecules of polymer B [Sperling, 1992]. The composition can be expressed as, e.g., Woc -co-poly(butadiene/styrene) (75 25 wt%), or gra/i-co-poly[isoprene/ (isoprene acrylonitrile)] (85 15 mole %). 

Among the commercially available families of PSA one distinguishes three large groups solvent-borne acrylic adhesives emulsion acrylic adhesives and block copolymer based adhesives [27]. The block copolymer based adhesives are typically formulated as a blend of one or several block copolymers and one or several tackifying resins. The block copolymers are based on a glassy monomer such as styrene and an elastomeric monomer such as isoprene or butadiene. The glassy sequence is the minority phase (15-30 wt%) and diblock and tri-... 

When the sequences making up the segments are random copolymers, the prefix CO may be introduced, with the major component monomer preceding the minor constituent. A backbone polymer of butadiene-styrene rubber grafted with styrene containing a small percentage of acrylic acid would be described as poly[(butadiene-co-styrene)-(styrene-co-acrylic acid)] and could be schematically represented as... 

The melt rheology of amorphous block copolymers, e.g., styrene-butadiene block copolymers (Arnold and Meier, 1968 Holden et al, 1969a Meier, 1969), has been described and interpreted already (Section 4.11). It is interesting to compare the amorphous block copolymers with block copolymers that have the additional feature of crystallizable sequences. A basic study of block copolymer rheology was carried out by Erhardt et al (1970), who determined the complex modulus and tan 6, and studied melt behavior at temperatures between about 60 and 200°C. A report on dielectric behavior by Pochan (1971) is also significant. 

The same dependence of tensile elongation characteristics on block copolymer structure that was observed in this work has been reported for styrene/butadiene block copolymers. Table 3. The importance of the hard/soft/hard sequence is evident since the hard/ soft (A/B) sequence has very low elongation. This corresponds in this work to incompletely extended polymer. Table 2. 

The methods of both papers assume that the hydrodynamic volumes of the block copolymer sequences are additive, implying a negligible interaction between unlike segments. Chang s method [5] gave lower molecular masses than Runyon s method [4], but in the case of styrene-butadiene block copolymers in tetrahydrofuran (THF), the difference was negligible [5]. 

Block copolymers or graft copolymers made up of soft and rigid polymer sequences. Styrene block copolymers like polystyrene (PS) blocks [styrene-butadiene-styrene (SBS), styrene-isoprene-styrene (SIS), and styrene-ethylene-butylene-styrene (SEBS)], and polyester TPEs belong to this family. Structurally, the thermoplastic blocks form physical network knots within the polydiene. 

Early results with cryomicrotomes were described by Cobbold and Mendelson [80]. Polyurethane elastomer, a blend of crystalline and noncrystalline polymers, showed spherulitic textures after sectioning at about -70°C. Injection molded polypropylene (PP) was also sectioned at about -70°C, while polytetrafluoroethylene (PTFE) was sectioned at much lower temperatures. The authors concluded that the technique, though difficult, had potential. Extruded styrene-butadiene-styrene (SBS) copolymer was prepared by cryosectioning with a diamond knife in liquid air at —85 to —115°C, followed by osmium tetroxide vapor staining for one hour [81]. This method revealed the alternating sequence of the polystyrene and polybutadiene lamellae. Odell et al. [82] prepared extruded triblock copolymer by first chemically hardening the polybutadiene, with osmium tetroxide, followed by cryoultramicrotomy to produce 30 nm thick sections which showed fine structure details. Parallel polystyrene rods were observed in the SBS copolymer. Ultramicrotomy and selective staining with osmium tetroxide was also used in the preparation of a binary blend of PP and thermoplastic rubber [83]. 

Material properties of polymers are determined by their chain miaostmctures. For polymers made from a single monomer type, the above-discussed molecular weight and distribution, chain stereoregularity, head-tail and trans-cis configurations, and so on all play important roles. For copolymers that contain multiple monomer types, chain composition, sequence, as well as their distributions, are added to the important microstmc-ture property list. With these new parameters, almost unlimited number of polymer types can be produced for better balance of properties for commercial applications. Outstanding commercial examples include acrylonittile-butadiene-styrene (ABS), SBS, Acrylan (acrylonittile-vinyl acetate), styrene-butadiene (SBR), butyl mbber (isobutylene-isoprene), Vinylite (vinyl chloride-vinyl acetate), and styrene-maleic anhydride (SMA). 

Ozonisation followed by gel permeation chromatography (GPC) has been used by Tanaka and co-workers [78,79] to study sequencing of vulcanised styrene copolymers. Tanaka and co-workers [78, 79] carried out the ozonolysis in methylene dichloride and examined the fractions obtained following GPC by H-NMR. These workers found nonad, diad and triad styrene sequences flanked by 1,4-butadiene units and long styrene sequences ... 

---