AB block copolymers of polystyrene

Stabilization in Nonaqueous Radical Dispersion Polymerization with AB Block Copolymers of Polystyrene and Poly(dimethyl siloxane)... 

This ABA block copolymer consists of stiff polystyrene (PS) and resilient polybutadiene blocks. The domains of these TPEs have characteristic Tt values of 100 and —80 C, respectively. The polybutadiene blocks retain their flexibility at low temperatures, and the polystyrene blocks lose their stiffness when the polymer is heated above 110 C. A related thermoplastic is a transparent AB block copolymer of styrene and butadiene (K-resin). 

Dawkins and Taylor109 dispersed poly(methyl methacrylate) (PMMA) or polystyrene (PS) particles in n-alkanes stabilized by AB block copolymers of styrene and dimethyl-siloxane. In these cases, styrene blocks act as anchors and dimethylsiloxane blocks give a surface layer. The thickness 6 of the dimethylsiloxane layer was determined by viscosity measurements as a function of the molecular weight of dimethylsiloxane blocks. 

The critical block sizes needed for domain formation are greater than those needed for phase separation in physical mixtures of the corresponding homopolymers. This is because the conformational entropy of parts of molecules in the block domains is not as high as in mixtures, since placement of segments is restricted by the unlike components to which they are linked. Thus the minimum molecular weights of polystyrene and c/i-polybutadiene for domain formation in AB block copolymers of these species are about 5,000 and 40,000, respectively [29]. 

Scheme 33 Synthesis of (AB)3-type star block copolymer of polystyrene and polyether...

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... 

A gel permeation chromatogram obtained by Heller (13) indicated that Kraton 101 contained 1% polystyrene, 22% polystyrene/1,4-poly-butadiene, and 77% polystyrene/1,4-polybutadiene/polystyrene, apparently because the material is manufactured by coupling of the diblock. The diblock would be present if the coupling reaction is not 100% efficient. Kraton 101 would thus be essentially a blend of ABBA and AB block copolymers. Assuming that Kraton 102 is manufactured in the same way, the molecular weight of the ABBA block copolymer results as about 72,000 from the above data, the polystyrene blocks having a molecular weight of about 12,000, and the polybutadiene center block one of about 48,000. 

The use of polysilanes as photoinitiators of radical polymerization was one of the hrst means whereby they were incorporated within block copolymer structures [38 0], albeit in an uncontrolled fashion. However the resulting block copolymer structures were poorly defined and interest in them principally lay in their application as compatibilisers for polystyrene (PS) and polymethylphenylsilane blends PMPS. The earliest synthetic strategies for relatively well-defined copolymers based on polysilanes exploited the condensation of the chain ends of polysilanes prepared by Wurtz-type syntheses with those of a second prepolymer that was to constitute the other component block. Typically, a mixture of AB and ABA block copolymers in which the A block was polystyrene (PS) and the B block was polymethylphenylsilane (PMPS) was prepared by reaction of anionically active chains ends of polystyrene (e.g. polystyryl lithium) with Si-X (X=Br, Cl) chain ends of a,co-dihalo-polymethylphenylsilane an example of which is shown in Fig. 2 [43,44,45]. Similar strategies were subsequently used to prepare an AB/ABA copolymer mixture in which the A block was poly(methyl methacrylate) (PMMA) [46] and also a multi- block copolymer of PMPS and polyisoprene (PI) [47]. 

Figure 6.14. Comparison of the temperature dependence of viscosity above the polystyrene for AB block copolymer 63 and that predicted from the WLF expression (Erhardt et aL, 1969).

In the first case, the arms are grown from a single core with a given number of potentially active sites or a well-defined multifunctional initiator. In contrast to anionic multifunctional initiators, weU-defined soluble multifunctional cationic initiators are readily available. These multifunctional initiators with 3-8 initiating sites have been successfully applied for the synthesis of 3-8 arm star homo- and block copolymers of vinyl ethers, styrene and styrene derivatives, and IB. For example, six-arm star polystyrenes were prepared using initiator with six phenylethylchloride-type functions emanating from a central hexa-substituted benzene ring [250]. By subsequent end functionalization, a variety of end-functionaUzed A or (AB) (see above) star-shaped structures can also be obtained. 

Preparation of styrene-diene block copolymers involves sequential addition of the diene monomer to "living" polystyrenes. The resulting polymer is an AB block copolymer consisting of segments of styrene mers and diene mers. 

Block copolymers consisting of segments with widely separated solubility characteristics have generated considerable interest because of their unusual surfactant properties. In fact, one of the earliest commercial block copolymers were the Wyandotte "Pluronics." These were poly(propylene oxide-b-ethylene oxide) prepared by sequential addition of ethylene oxide to sodium alkoxide initiated propylene oxide (37,38). Szwarc (39) and others (40,41) prepared poly(styrene-b-ethylene oxide) by addition of ethylene oxide to polystyrene anions in tetrahydrofuran. Other syntheses of AB or ABA block copolymers of styrene-ethylene oxide include sequential addition in various solvents, and coupling reactions (42,43). 

We have studied the structure of AB block copolymers polybutadiene-poly (y-benzyl-L-glutamate) (BG), polybutadiene-poly(e-carbo-benzoxy-L-lysine) (BCK), polystyrene-poly(y-benzyl-L-glutamate) (SG), polystyrene-poly(e-carbobenzoxy-L-lysine)(SCK) with polypeptide compositions between 18 and 84 % and ABA block copolymers poly(y benzyl L-glutamate)-polybutadiene-poly(y-benzyl-L-glutamate) (GBG) with polypeptide compositions between 55 and 86 %, by X ray diffraction, electronmicroscopyinfrared spectroscopy and circular dichroism. 

Decomposition of the azo function in the presence of styrene yields AB block copolymers, which can be converted into Q, -dihydroxyl polystyrene by a reductive cleavage of the ester bonds. 

After numerous experimental studies it now seems to have been reasonably well established that for AB- and ABA-type block copolymers containing polystyrene and polyisoprene (or polybutadiene) the microdomain size, Dj, and lattice repeat distance d, of periodically organized microdomains have the approximate molecular weight dependence " ... 

Evidently, if R is polymeric the product (29) will be a block copolymer, of AB or ABA type according to the functionality of the activator. For the reaction to proceed satisfactorily the ring = chain equilibrium between lactam and polyamide must be favourable for polymerization in the conditions employed, and there must be an absence of side reactions causing loss of propagative anionicity or transfer leading to the formation of free lactam homopolymer. Successful polymerizations have been achieved giving products with polyether, " polystyrene SBR, polydiene or polyisobutene " blocks, usually in combination with nylon 6 as the polyamide segment(s) or, in some cases, with nylon 4 or nylon 8. 

Barker MC, Vincent B (1984) The preparation and characterization of polystyrene-poly (ethylene oxide) AB-block copolymers. Colloid Surf 8 289-296... 

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