Emulsion polybutadiene-polystyrene-styrene

We have considerable latitude when it comes to choosing the chemical composition of rubber toughened polystyrene. Suitable unsaturated rubbers include styrene-butadiene copolymers, cis 1,4 polybutadiene, and ethylene-propylene-diene copolymers. Acrylonitrile-butadiene-styrene is a more complex type of block copolymer. It is made by swelling polybutadiene with styrene and acrylonitrile, then initiating copolymerization. This typically takes place in an emulsion polymerization process. 

Berlin and coworkers (5,56) desired to obtain a material with an increased mechanical strength. They carried out a plasticization of bulk ami emulsion polystyrene molecular weight 80000 and 200000 respectively at 150-160° C, with polyisobutylene, butyl rubber, polychloroprene, polybutadiene, styrene rubber (SKS-30) and nitrile rubber (SKN 18 and SKN 40). The best results were obtained with the blends polystyrene-styrene rubber and polystyrene-nitrile rubber. An increase of rubber content above 20-25% was not useful, as the strength properties were lowered. An increase in the content of the polar comonomer, acrylonitrile, prevents the reaction with polystyrene and decreases the probability of macroradical combination. This feature lowers the strength, see Fig. 14. It was also observed that certain dyes acts as macroradical acceptors, due to the mobile atoms of hydrogen of halogens in the dye, AX ... 

HIPS) is produced commercially by the emulsion polymerization of styrene monomer containing dispersed particles of polybutadiene or styrene-butadiene (SBR) latex. The resulting product consists of a glassy polystyrene matrix in which small domains of polybutadiene are dispersed. The impact strength of HIPS depends on the size, concentration, and distribution of the polybutadiene particles. It is influenced by the stereochemistry of polybutadiene, with low vinyl contents and 36% d5-l,4-polybutadiene providing optimal properties. Copolymers of styrene and maleic anhydride exhibit improved heat distortion temperature, while its copolymer with acrylonitrile, SAN — typically 76% styrene, 24% acrylonitrile — shows enhanced strength and chemical resistance. The improvement in the properties of polystyrene in the form of acrylonitrile-butadiene-styrene terpolymer (ABS) is discussed in Section VILA. 

Method of synthesis MBS consists of an elastomeric core and a glass shell. The elastomeric core is polybutadiene or styrene-butadiene rubber (SBR), and the shell is poly(methyl methacrylate) and polystyrene. The MBS copolymers are synthesized by emulsion polymerization method. In the preparation process PB polymer or SBR have to be synthesized first and then St and MMAare polymerized on rubber particles. Zhou, C Chen, M Tan, Z Y Sun, S L Ao, Y H Zhang. M Y Yang, H D Zhang, H X, Eur. Polym. J., 42, 1811-18,2006. 

Chem and Poehlein [52] developed a kinetic model based on the nonuniform free radical distribution function to predict the grafting efficiency of the emulsion emulsion polymerization of styrene in the presence of polybutadiene seed latex particles. The predominant grafting reaction appears to be the attack of growing polystyrene chains on the allyl hydrogen atoms of... 

Tuzar and Kratochvil (23) have reported that styrene-butadiene block copolymers mlcellise in selective solvents for polystyrene and solubilise large amounts of polybutadiene homopolymer. Sinc.e the surface active grades of polyvinyl alcohol are polyvinyl alcohol-acetate block copolymers and water is a selective solvent for polyvinyl alcohol a similar effect may be expected which could affect the course of the vinyl acetate emulsion polymerisation. 

While studying polymer distribution between the emulsion phases it was found that in the systems mentioned above obtained both by copolymerization of styrene with polybutadiene rubber and mixing styrene solutions of polymers when the composition is far enough from the critical mixing point, thermodynamic equilibrium is reached.At this thermodynamic equilibrium the ratio of polymer concentration (Cp) in rubber (index ) as well as in polystyrene (index ) phases is practically constant (table II),... 

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 ... 

Styrene butadiene n. A group of thermo plastic elastomers. They are linear co-polymers of styrene and butadiene, produced by lithium catalyzed solution polymerization, with a sandwich molecular structure containing a long Polybutadiene center surrounded by shorter polystyrene ends. A co-polymer of styrene and butadiene made by emulsion polymerization for use in latex paints. 

Acrylonitrile is also commonly found in impact modifiers, such as the acrylonitrile-butadiene-styrene (ABS) type, produced by emulsion polymerisation. Polybutadiene seed latex particles are grafted onto styrene and acrylonitrile in a seeded emulsion polymerisation process. As the styrene-acrylonitrile copolymer shell forms, polybutadiene domains are spontaneously separated within. The resulting impact modifier particles are subsequently compounded with polystyrene to product high impact polystyrene (HIPS). The impact modification properties of the latex particles may be optimised through varying the butadiene content, the particle size and structure, and the shell molecular weight. A basic formulation for an ABS impact modifier is given in Table 6. 

Developments in the anionic polymerization of butadiene were adopted for manufacture of solution SBR. While the emulsion process gave primarily 1,4-cis microstructure in the final product, the solution process gave a lower level of 1,4-cis level, typically around 45%. Furthermore the cis content as well as 1,2-vinyl content could be modified. In addition, better control of branching and molecular weight distribution attainable with anionic process made solution SBR suitable for tire applications, challenging the established use of cold SBR. Developments in the anionic process also led to new copolymer structures in which blocks of polybutadiene can be coupled to blocks of polystyrene, generating a imique class of polymers. Developments in SB block copolymers led to new materials which were thermoplastic in character, unlike SBR which is an elastomer. Solution-processes-based thermoplastic SB block copolymers form the basis of the transparent impact polystyrene (TIPS) as well as the other block copolymers used in plastics modification. The block copolymers of styrene and butadiene are the subject of the second part of this article. 

A common material made by anionic polymerization is the random copolymer composed of a roughly 1 3 mixture of polystyrene and polybutadiene known as SBR (for styrene butadiene rubber). Both of these monomers are compatible with anionic polymerization, and indeed SBR is made this way as a popular substitute for natural rubber, often used in the automotive industry. Styrene and butadiene are also both compatible with radical polymerization, and SBR can be made under emulsion polymerization conditions using radical initiators. 

SBRs are produced by addition copolymerization of styrene and butadiene monomers in either an emulsion or a solution polymerization process. The ratio of styrene to butadiene controls the glass transition temperature (Tg) of the copolymer and thus the stiffness of the elastomer. The higher the styrene content, the higher the Tg and stiffness. Polymers can be made over the whole range of Tg from about — SO C for polybutadiene to about H-100°C for polystyrene. 

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