ABA copolymer

Photoinitiators provide a convenient route for synthesizing vinyl polymers with a variety of different reactive end groups. Under suitable conditions, and in the presence of a vinyl monomer, a block AB or ABA copolymer can be produced which would otherwise be difficult or impossible to produce by another polymerization method. Moreover, synthesis of block copolymers by this route is much more versatile than those based on anionic polymerization, since a wider range of a monomers can be incorporated into the blocks. 

JH NMR) studies confirmed the presence of the AB diblocks in the product. This determination was facilitated by the fact that the dendritic nitroxide could be differentiated from the nonnitroxide-bearing dendron by NMR spectroscopy. Careful analytical studies confirmed that the pure ABA copolymers could be separated by column chromatography and that the undesired diblock impurity resulted mainly from the loss of the dendritic nitroxide during the course of the reaction. Obviously, this approach to ABA triblocks has rather limited practical value since the thermal stability of the final product is quite low. 

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. 

The addition of large linear blocks to dendrons with opposite polarity creates a desymmetrized structure predisposed to sequester insoluble components by aggregation rather than intramolecular hydrogen-bonding. Amphiphilic, linear-dendritic diblock (AB) and triblock (ABA) copolymers self-assemble into multimolecular micelles with CMC values that are well below those of low molecular weight surfactants. Typically, a hydrophilic linear block such as PEG is attached to the focal point... 

These ABA copolymers have an index of refraction of 1.5 and water absorption of about 0.2%. Unless hydrogenated to saturated block copolymers, these unsaturated unstabilized plastics are degraded in sunlight. The polybutadiene domains are attacked by aliphatic hydrocarbon solvents, such as hexane, and the polystyrene domains arc attacked by aromatic hydrocarbon... 

Jumaa, M. and Muller, B.W. (1998) The stabilization of parenteral fat emulsion using non-ionic ABA copolymer surfactantlnt. J. Pharm., 174 29-37. 

A broader study of the dilute solution behavior of ABA copolymers was reported by Beezer et al. 

Zhou, Z. and B. Chu. 1988. Anomalous micellization behavior and composition heterogeneity of a triblock ABA copolymer of (a) ethylene oxide and (b) propylene oxide in aqueous solUM laaromolecules 21 2548-2554. 

For example, simple dilithioalkanes (e.g., 1,4-dilithiobutane) are not purely difunctional, and are insoluble when prepared in hydrocaibon solvents (81). Monofunctional impurities are especially undesirable because they lead to AB diblocks that reduce the material properties of the ABA copolymers. 

A variety of dilithio initiators have been homogeneously prepared in polar solvents [e.g., THF, (C2H5)20, etc.] (82, 83) yet, polymerizations in such solvents result in higher vinyl (lower 1,4) content for the B blocks and poorer performance for the ABA copolymers. 

High 1,4 B-block ABA copolymers having good physical properties have been reported from l,4-dilithio-l,l,4,4-tetraphenylbutane (84) and 1,4-dilithio-l,4-dimethyl-1,4-tetraphenyIbutane (84) and 1,4-dilithio-1,4-dimethyl-1,4 diphenylbutane (85) initiation. These initiators have been prepared with a minimum of ether (preferably anisole). However, both of these compounds have shown a gradual loss of solubility as a result of association of the alkyllithiums. Attachment of solubilizing oligomeric polydienes apparently alleviated this problem (84, 85). 

The Lithcoa Co. has commercialized a series of dilithium reagents, DiLi-l (1A) (17), DiLi-3 (18), and DiLi-4 (19) (86) that also depend upon dimerization or oligomerization of hydrocarbon monomers for increased solubility. These materials have been applied to the synthesis of ABA copolymers. DiLi-l and DiLi-lA have the following structures (17). 

Fetters and co-workers (89, 90) reported a difunctional initiator (20) from lithium and 2,4-hexadiene which, presumably, could be used for the synthesis of ABA copolymers. This initiator is not only oligomeric but also has bulky chain ends for reduced Li association and increased solubility. Although a purely hydrocarbon-soluble initiator was claimed, most of the data were based upon catalysts in benzene-amine solvents. Also, the authors mentioned that this initiator is stable at room temperature, but no detailed aging data were given. 

Foss and co-workers (88) reported ABA copolymers obtained from a new dilithium reagent this organolithium initiator was formed by the addition of sec-butyllithium to m-diisopropenylbenzene in the presence of a small proportion of triethylamine, followed by reaction with isoprene to improve the hydrocarbon solubility. Unfortunately, the starting ma-... 

In addition, compound 21 precipitates from hexane solution after 3 days at room temperature. Oligomerization with isoprene furnishes a more soluble dilithium reagent from which ABA copolymers having good molecular-weight control have been made. 

We shall examine successively the behaviour of AB copolymers of butadiene and styrene, butadiene and a-methylstyrene, butadiene and vinylnaphtalene and of BAB and ABA copolymers of butadiene and styrene. 

ABA copolymers poly(methyl methacrylate)-polyisoprene-poly(methyl methacrylate) having polyisoprene with a high vinyl content as central block have been synthetized by Cole et al. 2I°. Dynamic mechanical properties of films of these ABA copolymers have been studied as a function of the copolymer composition, the temperature and the nature of the solvent (carbon tetrachloride, toluene, ethyl acetate, methylethyl ketone, dioxane) used for film preparation210. ... 

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]. 

Bifunctional Initiation. The bifunctional initiators like alkali metal complexes of polycyclic aromatic compounds can be used to produce ABA triblock copolymers even when the A anion is not sufficiently basic to initiate polymerization of B monomers. In these cases polymerization would be started with monomer B to produce a polymeric dianion which could initiate polymerization of the A monomer which is added later. These initiators can be prepared only in aliphatic ethers, however. This precludes their use for the synthesis of useful styrene-diene ABA copolymers because polydienes made anionically in such solvents have low 1,4 contents and are not good rubbers. 

Coupling Reactions. In this technique, a living AB block copolymer is made by monofunctional initiation and is then terminated with a bifunctional coupling agent likeadihaloalkane. ABA copolymers can be madeby joining AB polymeric anions ... 

Reaction (9-23) could be used to produce an ABA copolymer of styrene and methyl methacrylate even though the latter anion is too weakly basic to initiate the polymerization of styrene. If the presence ofa linking group, like the—(—CH2—) in the above reaction, is undesirable a coupling agent like I2 can be used. 

Jumaa M, Muller BW. The stabilization of parental fat emulsion using nonionic ABA copolymer surfactant. International J Pharm 1998 174 29-37. 

Of the various surface active chemistries currently available, this paper will mainly concentrate on a class of materials called Silicone Polyethers. This family of copolymers is used to provide multifunctional benefits in water borne systems. The main uses of silicone polyethers in inks and coatings include de-foaming, de-aerating, improved substrate wetting, levelling and enhanced slip properties (1,2). The three most common molecular structures for silicone surfactants are rake type copolymers, ABA copolymers and trisiloxane surfactants. These are illustrated in Figs 1,2 and 3 respectively and the performance of these structures will be described in two types of coatings ... 

Further reaction with styrene produces triblock ABA copolymers. 

There are two important types of polymer in which an elastomeric and a glassy or non-crystalline rigid component are copolymerised. The simpler thermoplastic elastomers are usually triblock ABA copolymers with B as the elastomeric component and A as the glassy component, whereas the segmented polyurethane elastomers are multiblock copolymers in which alternate blocks are hard , i.e. relatively inflexible, and soft , i.e. relatively flexible. 

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