With AB block copolymers

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

In blends of ABC with AB block copolymers a mixing at the molecular level of both blocks of the diblock copolymer with the corresponding blocks of the triblock terpolymer leads to a centrosymmetric superstructure. However, in the case of a blend of ABC and AC block copolymers a noncentrosymmetric structure is obtained when all diblock and triblock terpolymer molecules prefer to form common domains with the corresponding block of the other species. Before discussing this situation in some detail, other general possibilities for blend formations need to be considered. Besides the trivial case of macrophase separation and the random sequence of ABC and AC block copolymers due to equal energetic situations between the different A blocks and C blocks, two different centrosymmetric double-layered structures are also possible. 

In blends of ABC with AB block copolymers a mixing at the molecular level of both blocks of the diblock copolymer with the corresponding blocks of the triblock copolymer leads to a centrosymmetric superstructure. However, in... 

Fig. 4. Structure of an AB block copolymer with, from left to right, an increasing chain length of the A block 1, spheres of A in a continuous matrix of B ...

Multiblock Copolymers. Replacement of conventional vulcanized mbber is the main appHcation for the polar polyurethane, polyester, and polyamide block copolymers. Like styrenic block copolymers, they can be molded or extmded using equipment designed for processing thermoplastics. Melt temperatures during processing are between 175 and 225°C, and predrying is requited scrap is reusable. They are mostiy used as essentially pure materials, although some work on blends with various thermoplastics such as plasticized and unplasticized PVC and also ABS and polycarbonate (14,18,67—69) has been reported. Plasticizers intended for use with PVC have also been blended with polyester block copolymers (67). 

The hindered carbon-centered radicals are most suited as mediators in the polymerization of 1,1-disubstituled monomers e.g. MMA,78,95 other methacrylates and MAA,06 and AMS97). Polymerizations of monosubstituted monomers are not thought to be living. Dead end polymerization is observed with S at polymerization temperatures <100°C.98 Monosubstituted monomers may be used in the second stage of AB block copolymer synthesis (formation of the B block).95 However the non-living nature of the polymerization limits the length of the B block that can be formed. Low dispersities are generally not achieved. 

A porphinatoaluminum alkoxide is reported to be a superior initiator of c-caprolactone polymerization (44,45). A living polymer with a narrow molecular weight distribution (M /Mjj = 1.08) is ob-tmned under conditions of high conversion, in part because steric hindrance at the catalyst site reduces intra- and intermolecular transesterification. Treatment with alcohols does not quench the catalytic activity although methanol serves as a coinitiator in the presence of the aluminum species. The immortal nature of the system has been demonstrated by preparation of an AB block copolymer with ethylene oxide. The order of reactivity is e-lactone > p-lactone. 

The transformation of the chain end active center from one type to another is usually achieved through the successful and efficient end-functionalization reaction of the polymer chain. This end-functionalized polymer can be considered as a macroinitiator capable of initiating the polymerization of another monomer by a different synthetic method. Using a semitelechelic macroinitiator an AB block copolymer is obtained, while with a telechelic macroinitiator an ABA triblock copolymer is provided. The key step of this methodology relies on the success of the transformation reaction. The functionalization process must be 100% efficient, since the presence of unfunctionalized chains leads to a mixture of the desired block copolymer and the unfunctionalized homopolymer. In such a case, control over the molecular characteristics cannot be obtained and an additional purification step is needed. 

In this section, we would like to briefly describe the micellization behavior of hybrid AB copolymers, leading to remarkable structures with new potential applications. By hybrid AB block copolymers we mean copolymers in which at least one of the constituent blocks is not a classical poly-... 

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... 

In the 1980s, a synthetic method to produce AB block copolymers of propylene and tetrahydrofurane (THF) was proposed [29]. Polypropylene-fi/ock-poly(THF) was prepared by a combination of living polymerization of propylene with a V(acac)3 catalyst and the living polymerization of THF. Its synthesis was based on the transformation of living polypropylene chain ends to cationic ones, which initiated the living polymerization of THF. 

When e-CL and l-LA are block copolymerized, the monomer addition sequence is very important. AB block copolymers can be prepared by ROP with SnOct2 as catalyst and ethanol as initiator provided that e-CL is polymerized first [47]. If the l-LA block is synthesized first and the hydroxy-terminated mac-romer formed is used to initiate polymerization of e-CL, the polymer formed is totally randomized. 

AB)n-type star corresponds to a star with n similar AB block copolymer branches... 

By the use of the polymer-linking method with 20a, a variety of starshaped poly(vinyl ethers) have been synthesized (Scheme 12) [208-212]. A focus of these syntheses is to introduce polar functional groups, such as hydroxyl and carboxyl, into the multiarmed architectures. These functionalized star polymers include star block (23a,23b) [209,210], heteroarm (24) [211], and core-functionalized (25) [212] star polymers. Scheme 12 also shows the route for the amphiphilic star block polymers (23b) where each arm consists of an AB-block copolymer of 1BVE and HOVE [209] or a vinyl ether with a pendant carboxyl group [210], Thus, this is an expanded version of triarmed and tetraarmed amphiphilic block copolymers obtained by the multifunctional initiation (Section VI.B.2) and the multifunctional termination (Section VI.B.3). Note that, as in the previously discussed cases, the hydrophilic arm segments may be placed either the inner or the outer layers of the arms. 

One example of a 1 1 AB block copolymer with poly-(IX-8) and poly-(X-S) blocks showed a smectic A phase the mesophase of the homopolymer of X-8. The more ordered smectic A phase dominated the nematic phase of the polynorbornene block. 

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 ... 

It is well known that block copolymers in a selective solvent (a good solvent for one block but a non-solvent for the other) form a micellar structure through the association of the insoluble segments. In contrast with micelles formed from low molecular weight surfactants, block copolymer micelles dissociate slowly to free polymeric chains. They have a greater capacity for solubilizing aromatic molecules and express lower CMCs. The AB block copolymers are considered useful vehicles for hydrophobic drugs. 

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