Anionic polymerization tapered block copolymers

In addition to the triblock thermoplastic elastomers, other useful copolymers of styrene with a diene are produced commerically by living anionic polymerization. These include di-and multiblock copolymers, random copolymers, and tapered block copolymers. A tapered (gradient) copolymer has a variation in composition along the polymer chain. For example, S-S/D-D is a tapered block polymer that tapers from a polystyrene block to a styrene-diene random copolymer to polydiene block. (Tapered polymers need not have pure blocks at their ends. One can have a continuously tapered composition from styrene to diene by... 

The possibilities inherent in the anionic copolymerization of butadiene and styrene by means of organolithium initiators, as might have been expected, have led to many new developments. The first of these would naturally be the synthesis of a butadiene-styrene copolymer to match (or improve upon) emulsion-prepared SBR, in view of the superior molecular weight control possible in anionic polymerization. The copolymerization behavior of butadiene (or isoprene) and styrene is shown in Table 2.15 (Ohlinger and Bandermann, 1980 Morton and Huang, 1979 Ells, 1963 Hill et al., 1983 Spirin et al., 1962). As indicated earlier, unlike the free radical type of polymerization, these anionic systems show a marked sensitivity of the reactivity ratios to solvent type (a similar effect is noted for different alkali metal counterions). Thus, in nonpolar solvents, butadiene (or isoprene) is preferentially polymerized initially, to the virtual exclusion of the styrene, while the reverse is true in polar solvents. This has been ascribed (Morton, 1983) to the profound effect of solvation on the structure of the carbon-lithium bond, which becomes much more ionic in such media, as discussed previously. The resulting polymer formed by copolymerization in hydrocarbon media is described as a tapered block copolymer it consists of a block of polybutadiene with little incorporated styrene comonomer followed by a segment with both butadiene and styrene and then a block of polystyrene. The structure is schematically represented below ... 

Figure 4. Living anionic polymerization of one-step tapered block copolymers.

SBR prepared by anionic polymerization in solution exhibits a quite different distribution of the comonomeric units due to very different reactivity ratios and a higher content of 1,4-cis units to the detriment of 1,2 units. When anionic statistical polymerization is carried out in batch and in absence of polar additives, the chains formed are similar to tapered block copolymers with one block mainly constituted of polybutadiene, then a tapered block with an increasing content in styrene and a block containing almost exclusively styrene units. 

With this in mind, the anionic polymerization of styrene (PS), styrene-butadiene (SB), or SI is typically carried out by feeding distilled and dried monomer and solvent and purified n-butyl-lithium to a CSTR operating in semicontinuous mode. The reaction mix is heated to the reaction temperature and the second monomer is fed at a programmed feed rate that allows the desired copolymer microstructure to be built, whether a block SB or SI, or tapered or random copolymers. Solution SBR is commonly... 

The NIR in situ process also allowed for the determination of intermediate sequence distribution in styrene/isoprene copolymers, poly(diene) stereochemistry quantification, and identification of complete monomer conversion. The classic one-step, anionic, tapered block copolymerization of isoprene and styrene in hydrocarbon solvents is shown in Figure 4. The ultimate sequence distribution is defined using four rate constants involving the two monomers. NIR was successfully utilized to monitor monomer conversion during conventional, anionic solution polymerization. The conversion of the vinyl protons in the monomer to methylene protons in the polymer was easily monitored under conventional (10-20% solids) solution polymerization conditions. Despite the presence of the NIR probe, the living nature of the polymerizations was maintained in... 

In 1965 Shell Oil Co. introduced styrenic TPEs, under the trade name Kraton, which are polystyrene-b-polyisoprene(or polybutadiene)- -polystyrene (PS-b-PI(or PBd)- -PS linear triblock copolymers, made by anionic polymerization. Later, BASF introduced styrenic PTEs having tapered blocks (i.e., Styroflex). The typical styrene content of TPEs is between 25 and 40wt.%. When more stable TPEs, toward oxygen. 

Block Copolymerization. A polymerization with long chain lives can be used to make block copolsrmers (qv). An important commercial example is styrene/butadiene blocks produced by anionic polymerization (qv). A solution polymerization is done in a batch reactor, starting with one of the two monomers. That monomer is reacted to completion and the second monomer is added while the catalytic sites on the chains remain active. This produces a block copolymer of the AB form. Early addition of the second monomer produces a tapered block. If the second monomer is reacted to completion and replaced by the first monomer, an ABA triblock is obtained. This process is not easily converted to continuous operation because polsrmerizations inside tubes rarely approach the piston-flow environment that is needed to react one monomer to completion before adding the second monomer. Designs using static mixers (also known as motionless mixers) are a possibility. 

---