AB or ABA block copolymers

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.). 

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. 

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... 

For completeness it should be pointed out that many interesting morphologies of the type described above can be obtained by polymerizing a solution of AB or ABA block copolymers in styrene monomer, analogous to the process of HIPS production in which polybutadiene homopolymer is the dissolved species. This approach has been described in detail by Echte... 

Block copolymers consisting of segments with widely separated solubility characteristics have generated considerable interest because of their unusual surfactant properties. In fact, one of the earliest commercial block copolymers were the Wyandotte "Pluronics." These were poly(propylene oxide-b-ethylene oxide) prepared by sequential addition of ethylene oxide to sodium alkoxide initiated propylene oxide (37,38). Szwarc (39) and others (40,41) prepared poly(styrene-b-ethylene oxide) by addition of ethylene oxide to polystyrene anions in tetrahydrofuran. Other syntheses of AB or ABA block copolymers of styrene-ethylene oxide include sequential addition in various solvents, and coupling reactions (42,43). 

AB or ABA block copolymers are a second type of steric stabilizers that can be used in dispersion polymerization [104,105]. Poly(styrene-Z)-ethylene oxide) was recently used by Winnik and coworkers in the dispersion polymerization of styrene in methanol [106]. Provided that a selective solvent for the block copolymer is used as the continuous phase, these copolymers can be... 

TPE-A containing a-amino acid co-monomers, e.g., glycine and phenylalanine [19] though in most cases, only AB or ABA block copolymers are prepared. 

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). 

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... 

Fig. 1 Phase diagram of self-assembled structures in AB diblock copolymer melt, predicted by self-consistent mean field theory [31] and confirmed experimentally [33]. The MesoDyn simulations [34, 35] demonstrate morphologies that are predicted theoretically and observed experimentally in thin films of cylinder-forming block copolymers under surface fields or thickness constraints dis disordered phase with no distinct morphology, C perpendicular-oriented and Cy parallel-oriented cylinders, L lamella, PS polystyrene, PL hexagonally perforated lamella phase. Dots with related labels within the areal of the cylinder phase indicate the bulk parameters of the model AB and ABA block copolymers discussed in this work (Table 1). Reprinted from [36], with permission. Copyright 2008 American Chemical Society...

Both the 2,2-diphenyl vinyl and the l-methoxy-l,l-diphenylethyl chain ends are potential endgroups for the anionic polymerization of a variety of monomers by metalation. Our earlier results indicate that quantitative metalation of the 2,2-diphenylvinyl endgroups with alkyllithium cannot be achieved, most likely because of steric hindrance. However, as described recently, the ether cleavage of 1-methoxy-l,l-diphenyl-3,3,5,5-tetramethylhexane or electron transfer to 3,3,5,5-tetra-methyl-l,l-diphenylhex-l-ene by K/Na alloy, Cs or Li led to quantitative metalation resulting in nearly quantitative initiation of the polymerization of methacrylic monomers. Both precursors led to identical (macro)initiators verified by H NMR. These compounds can be considered as models of PIB chain ends formed by LCCP of IB and subsequent end-capping with DPE. The present study deals with the application of this method to the synthesis of different AB and ABA block copolymers by the combination of LCCP and living anionic polymerization. 

The synthesis of AB and ABA block copolymers with a polyvinyl and one or two polypeptide blocks is performed in 3 steps. At first the polyvinyl block is obtained by anionic or radical polymeriza-... 

Some of the AB and ABA polymers of PBD and PDMS are shown in Tables 1 and 2 respectively. Isolation of the precursor PBD or PMMA by protonation of a portion of the living PBD or PMMA allowed direct determination of the precursor block length in all of the copolymers. The mol. weight of the PMMA blocks for the PBD copolymers was obtained by H NMR integration (Table 1). Inspection of Tables 1 and 2 shows that the AB and ABA block copolymers are quite monodisperse (Mw/Mn < 1.20) and that the polydispersity of the star-block copolymers is also quite reasonable (Mw/Mn < 1.30) if the molecular weights are not too high. ... 

The term amphiphile implies an affinity to two different media. Familiar amphiphilic molecules incorporate two incompatible components that give rise to this behavior. Similarly, in AB and ABA block copolymers there are two incompatible blocks of different solubility. However, ABC triblock copolymers incorporate three chemically different blocks. When the three blocks are mutually incompatible and of different solubilities the ABC surfactants can exhibit affinity to three different media rather than two. The consequences of this higher functionality have not been explored in detail. For example, little attention has been given to their behavior at interfaces. Linear ABC triblock copolymers or the corresponding star copolymers may be able to form two-dimensional mesophases [56], This can occur at the interface between two fluids I and II such that the B block is selectively solubilized in I while A and C are only soluble in II. In this situation, the A and C blocks are constrained to the surface and bound to each other. A two-dimensional amphiphile is obtained when the A and C blocks are incompatible. A dense monolayer of this type should undergo microphase separation leading to the formation of circular and striped mesophases. Note that cylindrical and lamellar mesophases are indistinguishable in this case. A mixed monolayer comprised of BC, BA, and ABC block copolymers will mimic the behavior of amphiphiles in the presence of two two-dimensional and incompatible fluids. When the ABC copolymers are a minority component, they should straddle the boundary line between the two-dimensional A and C phases. 

For nitroxide-mediated radical polymerizations and in the RAFT process, the same synthetic strategy as for ATRP can be used in the synthesis of AB and ABA block copolymers. The first step is coupling a functionalized alkoxyamine with a telechelic or monofunctional nonvinylic polymer to give a macroinitiator. This macroinitiator can be used in standard controlled free-radical polymerization procedures. This approach is best illustrated by the preparation of PEO-based block copolymers [81-84]. One example is the preparation of macroinitiator LMI-7 by the reaction of a monohydroxy-terminated PEO with sodium hydride followed by reaction with the chloromethyl-substituted alkoxy amine as shown in Scheme 3.16. 

In previous articles [45,47], it has been reported that AB and ABA block copolymers are easily synthesized through living radical polymerization using polymeric photoiniferters obtained from BDC and XDC as monofunctional and bifunctional photoiniferters. This technique was applied to the synthesis of a variety of block copolymers consisting of three- or four-component systems [53]. The use of a tetrafunctional photoiniferter [i.e., 1,2,4,5-tetrakis (iV,iV-diethyl dithiocarbamyl (methyl)benzene (DDC)] has been reported [49] to synthesize star polymers of MMA. The structure of DDC is given in the following scheme. 

Integrating block copolymer-directed nanostmcture control of hybrid materials with other stmcture formation processes is one future direction that this research will most likely take. A more fundamental direction is the transition from AB or ABA diblock copolymers to ABC terblock copolymers or even higher-order multiblock copolymers as stmcture-directing agents for inorganic materials. The introduction of a third block in ABC terblock copolymer self-assembly leads to a... 

In aqueous solutions of amphiphilic polymers, which contain both hydrophilic and hydrophobic sequences, strong interpolymer hydrophobic association often leads to bulk-phase separation or gelation. However, there are classes of amphiphilic polymers that form well-organized associated structures in aqueous solution without accompanying macroscopic phase separation. This is generally characteristic of amphiphilic AB and ABA block copolymers, where A and B represent hydrophilic and hydrophobic sequences, respectively. These types of block copolymers and low molecular weight surfactant molecules have common features in their associating behavior. 

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. 

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. 

Using the same method a block copolymer of polypeptides and vinyl monomers was also prepared. As mentioned in Section II, Bamford and Mullik [22] introduced an interesting method of photoinitiation of vinyl monomers by the Mn2(CO)io or the Re2(CO)io/C2F4 system. By these methods polymeric molecule with (CO)sMn—CF2CF2—terminals is produced (see Scheme [12]). If a polymer of this kind is heated to 100°C in the presence of vinyl monomer, a block copolymer AB or ABA with Cp2- F2 linkage is produced [ ] ... 

The mode of chain termination affects the type of block copolymer formed. For example, if a MAI (based essentially on the first monomer A) possessing one central azo bond is decomposed in the presence of monomer B, the growing chain Bn can terminate either by disproportionation or combination, leading to AB and ABA type copolymers, respectively. 

Polymer micelles are nanometer sized (usually several tens of nanometers) self-assembled particles having a hydrophobic core and hydrophilic outer shell composed of amphiphilic AB- or ABA-type block copolymers, and are utilized as drug delivery vehicles. The first polymer micelle-type drug delivery vehicle was made of PEG-b-poly(aspartic acid) (PEG-b-PAsp), immobilizing the hydro-phobic anticancer drugDXR [188-191]. After this achievement by Kataoka et al., a great amount of research on polymer micelles has been carried out, and there are several reviews available on the subject [192-194]. 

Difunctional Iniferter X-X > X-QA X Telechelic polymer, AB- or ABA-type block copolymer... 

Previous sections discussed the micellization behavior of AB or ABA linear block copolymers. With the recent progress achieved in the field of controlled polymerization techniques, more sophisticated block copolymer architectures are now available. Investigation of the micellization behavior of such... 

Some particularities of the extraction of ions from an aqueous organic phase, and of the phase catalyzed polyetherification will be summarized. These will represent the fundamentals of our work on the synthesis of some novel classes of functional polymers and sequential copolymers. Examples will be provided for the synthesis of functional polymers containing only cyclic imino ethers or both cyclic imino ethers as well as their own cationic initiator attached to the same polymer backbone ABA triblock copolymers and (AB)n alternating block copolymers and a novel class of main chain thermotropic liquid crystalline polymers containing functional chain ends, i.e., polyethers. 

The adsorption behavior of AB- or ABA-type block copolymers in which block A is polyelectrolytic and block B hydrophobic is very interesting. As expected, these polymers serve as dispersants, micelle-forming agents and surface-active agents. 

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