Copolymers, ABA block

Certain block copolymers have also found appHcation as surfactants (88). Eor example, AB or ABA block copolymers in which one block is hydrophilic and one block is hydrophobic have proven useful for emulsifying aqueous and non-aqueous substances and for wetting the surface of materials. Examples of such surfactants are the poly(propylene oxide- /oi / -ethylene oxide) materials, known as Pluronics (BASC Wyandotte Co.). 

Erwins, E.E., St. Clair, D.J., Erickson, J.E. and Korez, W.H., Thermoplastic rubbers ABA block copolymers. In Satas, D. (Ed.), Handbook of Pressure Sensitive Adhesive Technology. Van Nostrand Reinhold, New York, 1989, pp. 317-373. 

If (P ) is terminated by a chain transfer to a solvent or a monomer, a graft copolymer is formed, or, if the termination is from a combination, a crosslinked network polymer is formed. If the pre-existing polymer (B) contains an end group that itself is photosensitive (or can produce a radical by interacting with photoinitiator) and in the presence of a vinyl monomer (A), block copolymer of type AB can be produced if the photosensitive group is on one end of the polymeric chain. Type ABA block copolymer can be produced if the polymer chain (B) contains a photosensitive group on both ends. 

The quantum yield of polymerization is 6.72 and for photoinitiation < / = 2.85 x 10 . The polystyrene produced with this initiator shows photosensitivity when irradiated with UV light (A = 280 nm). This polymer, which carries two photosensitive end groups of - SC(S) N(CH3)2, behaves as a telechelic polymer and it is useful for production of ABA block copolymer. 

A very special type of ABA block copolymer where A is a thermoplastic (e.g., styrene) and B an elastomer (e.g., butadiene) can have properties at ambient temperatures, such as a crosslinked rubber. Domain formations (which serves as a physical crosslinking and reinforcement sites) impart valuable features to block copolymers. They are thermoplastic, can be eaisly molded, and are soluble in common solvents. A domain structure can be shown as in Fig. 2. 

Depending on the termination reaction of the vinyl monomer, termination by disproportionation or termination by combination occurs. As a result, AB or ABA block copolymers might be obtained. 

Synthesis of Poly(2-ethyl-2-oxazoline)-b-PDMS (ABA) Block Copolymers 2911... 

Synthesis of PDMS-b-(e-caprolactam) ABA block copolymers was reported 343 In these reactions, anhydride-terminated PDMS oligomers were used to initiate the polymerization of -caprolactam in the presence of a catalytic amount of sodium hydride in melt at 130 °C. Under these conditions, the reaction was reported to be completed in... 

Fig. 16 Various type of elastin-based side-chain polymers (a) ABA block copolymer produced via ATRP, (b) homopolymer produced via ATRP, (c) homopolymer produced via RAFT polymerization...

Figure 4. Contact angle vs. time for PB treated with PhTD and neutralized with 3,3-ionene hydroxide ( ), styrene-butadiene ABA block copolymer treated with PhTD and neutralized with NaHCC>3 ( ), and PI treated with PhTD and neutralized with CaO (A).

In their efforts to construct stimuli-responsive, supramolecular amphiphiles, Frechet et al. [126-129] reported the synthesis of a novel series of AB and ABA block copolymers via the Williamson ether synthesis (e.g., 47, Fig. 21). Polyethylene glycols (PEGs) of different lengths were used as the linear hydrophilic B block while polyaryl ether dendrons of different sizes were used as the hydro-phobic A block. These copolymers were characterized by optical microscopy,... 

The nature of the hard domains differs for the various block copolymers. The amorphous polystyrene blocks in the ABA block copolymers are hard because the glass transition temperature (100°C) is considerably above ambient temperature, i.e., the polystyrene blocks are in the glassy state. However, there is some controversy about the nature of the hard domains in the various multiblock copolymers. The polyurethane blocks in the polyester-polyurethane and polyether-polyurethane copolymers have a glass transition temperature above ambient temperature but also derive their hard behavior from hydrogen-bonding and low levels of crystallinity. The aromatic polyester (usually terephthalate) blocks in the polyether-polyester multiblock copolymer appear to derive their hardness entirely from crystallinity. 

A similar technique was applied to the synthesis of AB and ABA block copolymers containing random and alternating copolymer sequences [178-180]. For example poly(St-ra dora-MMA)-hZock-poly(VAc), poly(VAc-hZock-poly(St-ran-dora-MMA)-hZock-poly(VAc), poly(St)-foZoc/c-poly(DiPF-aZMBVE), poly(IBVE-aZf-MAn)-hZock-poly(St)-hZock-poly(IBVE-aZf-MAn), poly(St)-hZock-poly(EA)-random-AA), and poly(St)-hZock-poly(EA-ra dora-AA-random-MMA) were synthesized [178]. 

In this section the emphasis will be on assemblies prepared from amphiphilic dendrimers. Amphiphilic dendrimers, carrying both hydrophobic and hydrophilic regions within one molecule, tend to self-assemble into a large variety of different aggregates depending on their structure. The dendritic amphiphiles investigated so far include unimolecular micelles, bolaamphiphiles, superam-phiphiles and various other AB and ABA block copolymers. 

No formal termination is given in structure 5.42 because in the absence of contaminants the product is a stable macroanion. Szwarz named such stable active species living polymers. These macroanions or macrocarbanions have been used to produce block copolymers such as Kraton. Kraton is an ABA block copolymer of styrene (A) and butadiene (B) (structure 5.43). Termination is brought about by addition of water, ethanol, carbon dioxide, or oxygen. 

Although block copolymers do not occur naturally, synthetic block copolymers have been prepared by all known classical polymerization techniques. The first commercial block copolymer was a surfactant (Pluronics) prepared by the addition of propylene oxide to polycarbanions of ethylene oxide. While neither water-soluble PEO nor water-insoluble poly(propylene oxide) exhibits surface activity, the ABA block copolymer consisting of hydrophilic and lyophilic segments, is an excellent surfactant. Each block has 20 plus repeat units of that variety. 

The most widely used chain reaction block copolymers are those prepared by the addition of a new monomer to a macroanion. AB and ABA block copolymers called Soprene and Kraton, respectively, are produced by the addition of butadiene to styryl macroanions or macrocarbanions (Equation 7.32). This copolymer is normally hydrogenated (Equation 7.33). 

ABA Block copolymer with three sequences of mers in the order shown. 

Kraton Trade name for an ABA block copolymer of styrene and butadiene. 

Which sequence in the ABA block copolymer of ethylene oxide and propylene oxide is more lyophilic ... 

The product obtained by the partial hydrogenation of polybutadiene (Hydropol) has been used as a wire coating and a saturated ABA copolymer (Kraton) is produced by the hydrogenation of the ABA block copolymer of styrene and butadiene. 

Tri- and higher block copolymers, such as ABA, ABC, ABCB, can be synthesized by a continuation of the processes with the successive additions of the appropriate monomers. A symmetric block copolymer such as ABA or CABAC can be made efficiently by using a difunctional initiator, such as, a, a-dichloro-p-xylene or dimethyl 2,6-dibromoheptanedioate, instead of the monofunctional initiator. For the ABA block copolymer only two, instead of three, monomer charges are needed ... 

Group transfer polymerization allows the synthesis of block copolymers of different methacrylate or acrylate monomers, such as methyl methacrylate and allyl methacrylate [Hertler, 1996 Webster and Sogah, 1989]. The synthesis of mixed methacrylate-acrylate block copolymers requires that the less reactive monomer (methacrylate) be polymerized first. The silyl dialkylketene acetal propagating center from methacrylate polymerization is more reactive for initiation of acrylate polymerization than the silyl monoalkylketene acetal propagating center from acrylate polymerization is for initiation of methacrylate polymerization. Bifunctional initiators such as l,4-bis(methoxytri methyl si loxymethylene)cyclohexane (XXXIII) are useful for synthesizing ABA block copolymers where the middle block is methacrylate [Steinbrecht and Bandermann, 1989 Yu et al., 1988]. 

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... 

Gauthier S, Eisenherg A. Vinylpyridinium ionomers. 2. Styrene-based ABA block copolymers. Macromolecules 1987 20 760-767. 

The ABA block copolymer of styrene (12.5%)-butadiene (75%)-styrene (12.5%) (Kraton), is a thermoplastic elastomer CTPE) with the multiple repeating units shown below ... 

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). 

The first approach Is to polymerize small amounts of 4-vlnyl pyridine on to the ends of anionic living polybutadiene, mono- or difunctional, to produce what are essentially AB or ABA block copolymers (equation 5). Materials possessing values of n typically averaging about 3 have been prepared and shown to produce solids when quaternlzed with benzyl bromide. The result of... 

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