Styrenic block copolymer synthesis

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] ... 

The additional complexity present in block copolymer synthesis is the order of monomer polymerization and/or the requirement in some cases to modify the reactivity of the propagating center during the transition from one block to the next block. This is due to the requirement that the nucleophilicity of the initiating block be equal or greater than the resulting propagating chain end of the second block. Therefore the synthesis of block copolymers by sequential polymerization generally follows the order dienes/styrenics before vinylpyridines before meth(acrylates) before oxiranes/siloxanes. As a consequence, styrene-MMA block copolymers should be prepared by initial polymerization of styrene followed by MMA, while PEO-MMA block copolymers should be prepared by... 

Shi, Z. Q., and Holdcroft, S. 2005. Synthesis and proton conductivity of partially sulfonated poly([vinylidene difluoride-co-hexafluoropropylene]-b-styrene) block copolymers. Macromolecules 38 4193-4201. 

Various block copolymers have been synthesized by cationic living polymerization [Kennedy and Ivan, 1992 Kennedy, 1999 Kennedy and Marechal, 1982 Puskas et al., 2001 Sawamoto, 1991, 1996]. AB and ABA block copolymers, where A and B are different vinyl ethers, have been synthesized using HI with either I2 or Znl2. Sequencing is not a problem unless one of the vinyl ethers has a substituent that makes its reactivity quite different. Styrene-methyl vinyl ether block copolymer synthesis requires a specific sequencing and manipulation of the reaction conditions because styrene is less reactive than methyl vinyl ether (MVE) [Ohmura et al., 1994]. Both monomers are polymerized by HCl/SnCLj in the presence of (n-CrikjtiNCI in methylene chloride, but different temperatures are needed. The... 

More recently, Kennedy reported another initiating system that controls styrene polymerization with an added nucleophile 2,2,4-trimeth-ylpentyl chloride (TMP-Cl)/TiCl4 with N,N-dimethylacetamide (DMA) in CH3Cl/methylcyclohexane (4 6 v/v) mixture at -80° C [165]. The use of another additive, 2,6-di-ferf-butylpyr idine (proton trap), is described as beneficial. The molecular weight and MWD are controlled in this system, but the role of the added DMA is still ambiguous [166]. This system with the aliphatic (erf-chloride was designed to extend to the synthesis of isobutene-styrene block copolymers via sequential cationic polymerization (Chapter 5). 

A final example of the use of the macroinitiator approach for block copolymer synthesis is the use of a macro azo initiator to initiate NMRP of styrene (Scheme 8.5). This process was used to make styrene-W-siloxanes [17]. 

The synthesis of styrenic block copolymers (SBCs) has been discussed in a number of books and review articles concerning block copolymers [1] and anionic polymerization [2]. A comprehensive review of the field is beyond the scope of this chapter, the objective of which is to provide an overview of the technology, with particular emphasis on processes currently used for commercial production. 

Since shortly after its discovery by Szwarc et al. [5] in the mid-1950s, living anionic polymerization has been recognized as an ideal route to styrenic block copolymers [6]. To date, living anionic polymerization remains the only commercially important technology for SBC synthesis. The anionic polymerization of styrene and common dienes such as butadiene and isoprene satisfies the criteria outlined above, particularly when carried out in a hydrocarbon solvent and initiated by an appropriate lithium alkyl. 

Such living conditions are found principally In anlonlcally Initiated systems and Involve common monomers such as styrene, ormethylstyrene, butadiene and Isoprene (1,22). They are far less common In catlonlcally Initiated systems, there being virtually no established example Involving vinyl monomers, but some cyclic monomers such as tetrahydrofuran (THF) and the oxetanes may be polymerized under carefully specified conditions to yield living polymers ( ). Although living free radical systems have also been described In which radicals have been preserved on surfaces. In emulsion, or by precipitation before termination occurs, these are special conditions not easily adapted for clean block copolymer synthesis. 

Handlin DL Jr Trenor S, Wright K. Applications of thermoplastic elastomers based on styrenic block copolymers. In Matyjaszewski K, Gnanou Y, Leibler L, editors. Macro-molecular Engineering Precise Synthesis, Materials Properties, Applications. Volume 4. Weinheim, Germany Wiley-VCH 2007. p 2001. 

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). 

In the case of AB diblock copolymers prepared by the RAFT technique, the order of monomer addition must be taken into account. A characteristic example of such a block copolymer synthesis is demonstrated in Scheme 19. Initially, a poly(N, N-dimethylacrylamide) (PDMA) macro-CTA was prepared, followed by the use of PDMA-CTA as an initiator to polymerize successfully the second monomer N,N-dimethyl vinyl benzy-lamide (DMVBA). The final diblock copolymer is not contaminated with homopolymer. It has been discovered that the reverse approach is impossible, probably due to the slow fragmentation of the intermediate radical or due to the slow initiation efficiency of the intermediate radical (styrenic macroradical). 

Navarro, A., del Rio, C. and Acosta, J.L. 2008. Pore-filling electrolyte membranes based on microporous polyethylene matrices activated with plasma and sulfonated hydrogenated styrene butadiene block copolymer. Synthesis, microstructural and electrical characterization. J. Pohm. Sci. B Polvm. Phvs. 46 1684-1695. 

Similarly, such AB diblocks may be prepared based on the acrylamido family of monomers. For example, the synthesis of novel AB diblock copolymers comprised of the two anionic monomers sodium 2-acrylamido-2-methylpropanesulfonate (AMPS) and sodium 3-acrylamido-3-methylbutanoate (AMBA) have been reported (42,44). By analogy with the styrenic block copolymers, these AMPS-AMBA species also exhibit reversible pH-induced self-assembly by virtue of the fact that the AMBA residues may be reversible protonated, switching the residues from a hydrophilic (high pH) to a hydrophobic (low pH) state. Similar AB diblocks of AMPS with sodium 6-acrylamidohexanoate which also exhibit pH-induced micellization have been reported by Yusa and co-workers (45) RAFT has... 

As we describe these topics we shall include in our discussion a brief description of the synthesis, morphology, and properties of styrenic block copolymers, especially as they relate to their uniqueness for the particular functional use. 

Berlin, D., and Boutevin, B. (1996). Controlled radical polymerization. Synthesis of chloromethylstyrene/styrene block copolymers. Polym. Bull., 37(3) 337-344. 

Lacroix-Desmazes, P., et al. (2000). Synthesis of poly(chloromethylstyrene-b-styrene) block copolymers by controlled free-radical polymerization. J. Polym. Sci., Part A Polym. Chem., 38(2 ) 3845-3854. 

Ramireddy C, Tuzar Z, Prochazka K, Webber SE, Munk P (1992) Styrene tert-butyl methacrylate and styrene methacrylic-acid block copolymers— synthesis and characterization. Macromolecules 25(9) 2541-2545. doi 10.1021/ma00035a037... 

According to the second method of carbonate block copolymer synthesis, sequential monomer polymerization is proceeded with transformation of the active center. The block copolymers are prepared in three steps. First, the polymerization of one monomer is carried out. After complete conversion of the first monomer the transformation of active centers is performed, and the initiation of the polymerization of the second monomer is proceeded. For example, this approach was applied for obtaining poly(styrene-l7-neopentyl carbonate).After completion of the styrene living polymerization, carbanionic centers were transformed into alkoxide ones via reaction with EO and then the ROP of neopentyl carbonate polymerization was performed. In the case of block copolymers of methyl methacrylate with neopentyl carbonate living PMMA, prepared according to GTP, was used as a macroinitiator for DTC polymerization. A silyl keteneacetal active center was transformed to an alkoxide one. Depending on the functionality of the macroinitiator (A) used for cyclic carbonate polymerization, two types of block copolymers can be obtained A-B or B-A-B. 

Another example for block copolymer synthesis is the preparation of poly (methylene-fc-styrene), which was achieved by using a hydroxyl-terminated living polystyrene obtained by TEMPO-mediated living radical polymerization [75] (Scheme 55). The chain end hydroxyl group was transformed into an allyl ether moiety, which was subjected to hydroboration with BH3 to afford polystyrene macroinitiator for the polymerization of 11. After chain elongation with 11 oxi-dation of the C-B chain ends furnished poly(methylene- -styrene) bearing TEMPO and hydroxyl group at chain ends. 

Somewhat limited work has been reported over the last decade. There are several reports on the synthesis and physical and structural characterization of styrene-dimethylsiloxane 141 144) and methylmethacrylate-dimethylsiloxane145> diblock, triblock and multiblock copolymers. Several reports are also available on the thermal223), solution 224,2251 and surface196 2261 characterization of various styrene-dimethyl-siloxane block copolymers synthesized by anionic techniques. 

By employing anionic techniques, alkyl methacrylate containing block copolymer systems have been synthesized with controlled compositions, predictable molecular weights and narrow molecular weight distributions. Subsequent hydrolysis of the ester functionality to the metal carboxylate or carboxylic acid can be achieved either by potassium superoxide or the acid catalyzed hydrolysis of t-butyl methacrylate blocks. The presence of acid and ion groups has a profound effect on the solution and bulk mechanical behavior of the derived systems. The synthesis and characterization of various substituted styrene and all-acrylic block copolymer precursors with alkyl methacrylates will be discussed. 

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