Synthesis, styrene block polymer

Triblock copolymers of ABA type, where B is the central elastomeric block and A is the rigid end-block, are well-known commercially available polymers [7,8]. The chemical structures of some common TPEs based on styrenic block copolymers are given in Eigure 5.1. Synthesis of such ABA-type polymers can be achieved by three routes [9] ... 

In the foregoing examples the synthesis of block copolymers was based on the solubility differences between two monomers, of which one is water soluble while the other is emulsified. Another polymerization technique is based on the kinetics of the emulsion polymerization. When a water emulsion of a monomer, such as styrene, is irradiated during a short time, the reaction, continues at a nearly steady rate until practically all the monomer is used up. If a second monomer is then added, it will polymerize, being initiated by the radicals occluded in the polymer particles. Although in this case also the yields of block copolymers are low, nevertheless the physical properties of the final product are markedly different from those of statistical copolymers (4, 5, 151, 176). 

If the diazonium groups result from the diazotation of poly-/>-amino-styrene, the macroradicals will initiate grafting. Contrarily, if >-(N-acetyl) phenylenediamine is diazotized and used as initiator of a first monomer, a polymer is obtained with an acetamino. phenyl end group (-CGH4-NH-Ac). After hydrolysis of this last and diazotation of the free amine group, the polymeric terminal diazonium salt can be used with ferrous ions for the synthesis of block copolymers. 

Korotkov, A. A., L. A. Shibaev, L. M. Pyrkov, V. G. Aldoshin and S. Ya. Frenkel The synthesis and investigation of hybrid polymers. I. Styrene and isoprene block polymers formed by the catalytic polymerization in solution under the action of butyllithium. Vysokomolekulyarnye Soedineniya 1, 433... 

The first workable capping agents for controlled radical polymerization were discovered by Rizzardo et al. [77, 78] who used nitroxides. The nitroxide reacts reversibly with radical chain ends but itself does not initiate the monomer. They called their new system Stable Free Radical Polymerization (SFRP). Scheme 32a depicts an example of SFRP using TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy). SFRP was developed independently by Georges at Xerox for the synthesis of styrene block polymer as dispersing agents [79]. 

The controlled free-radical miniemulsion polymerization of styrene was performed by Lansalot et al. and Butte et al. in aqueous dispersions using a degenerative transfer process with iodine exchange [91, 92]. An efficiency of 100% was reached. It has also been demonstrated that the synthesis of block copolymers consisting of polystyrene and poly(butyl acrylate) can be easily performed [93]. This allows the synthesis of well-defined polymers with predictable molar mass, narrow molar mass distribution, and complex architecture. 

An interesting synthesis of block copolymers by cationic polymerization of vinyl compounds was described by Kennedy and Melby [277] who used 2-chloro-6-bromo-2,6-dimethylheptane as coinitiator. Br- is eliminated by triethylaluminium, and styrene can be polymerized, without transfer, on the generated carbocation. After all the styrene has reacted, diethylaluminium chloride is added to eliminate Cl- from the coinitiator and thus produce new carbocations on the polymer chain. In the presence of 2-methylpropene, the two-block copolymer poly(styrene)-6/ock-poly(2-methylpropene) is formed. 

The living ends of a suitable polymer may initiate polymerization of another monomer, and thus lead to the synthesis of block polymers free of homopolymers. For sample, one prepares living polystyrene then adds pure methyl methacrylate to its solution and produces in this way a block polymer of styrene and methyl methacrylate (22). Actually, it is possible to produce living polymers with two active ends which can form a block polymer containing three segments—ABA. 

Two related procedures have been developed to effect this transformation. Both Involve the Initial synthesis of mono- or dlfunctlonal living anionic polymers of styrene, butadiene, or block copolymers of both. They are then reacted via Grlgnard Intermediates (7 ) with either excess bromine or with excess m-xylylyl dlbromlde (8-10) to yield polymers with reactive halide terminal groups (benzyllc or allyllc depending upon the polymer and terminating agent). The reactions for polystyrene are shown In equations 2 and 3. 

Similar to the synthesis of the difunctional polysulfone macroinitiator, a polyester was used in the synthesis of block copolymers by ATRP. The a, co-dihydroxy terminal polymer was synthesized by the transesterification of 1,6-hexanediol with dimethyl adipate [237]. The end groups were then esterified with 2-bromo-propionyl bromide and the ATRP of styrene yielded the ABA triblock copolymers. 

The direct introduction of peroxide groups into the backbone of polymers, such as poly(methyl methacrylate), has been used to produce macro-molecular initiators for the synthesis of block copolymers for example, poly(methyl methacrylate- -acrylonitrile) and poly(methyl methacrylate-Z -styrene). Ozonization can also be used, with careful control of the degree of ozonolysis, to introduce epoxy ring structures into natural rubber ... 

The catalytic system used to make OBCs uses a chain-shuttling agent (CSA) to shuttle or transfer growing chains between two distinct catalysts with different comonomer (alpha-olefm) selectivity." This is shown in Figure 9. Synthesis of olefin block polymer via chain shuttling requires the chain transfer to be reversible. OBCs are produced in a continuous solution polymerization process more economically favorable than the batch processes employed to make styrenic block copolymers. 

R. F. Storey, T. L. Maggio and L. B. Brister, Synthesis of poly(styrene-b-isobutylene-b-styrene) block copolymers using real-time in situ ATR-FTIR monitoring, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 40 964 (1999). 

Block Copolymers. Well defined block copolymers of 4-hydroxystyrene and styrene were prepared by firstly, the synthesis of low molecular weight blocks of styrene capped with TEMPO. The molecular weights of these styrene blocks were controlled by the ratio of unimolecular initiator (4) relative to styrene monomer (2). For example, 1-phenyl-l-(2, 2, 6, 6 -tetramethyl-r-piperidinyloxy)ethane (4) (1.67g, 0.0064 mol) was added to styrene (2) (20.0g, 0.192 mol) and heated to 125-130°C, under N2, for 48 hours. The reaction was then cooled to room temperature and the polymer dissolved in tetrahydrofuran (100 mL), and isolated by precipitation into methanol (1000 mL). The TEMPO terminated polystyrene (7) was then filtered, washed with methanol and dried in a vacuum oven overnight at 50°C. Isolated yield 90% of theory. M = 2764, M = 3062, PD = 1.10 (Theoretical A.M.U = 3120). 

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