ABA-type block copolymers

Preparation and thermal crosslinking reactions of oc, -vinylbenzyl terminated polysulfone-b-polydimethylsiloxane, ABA type block copolymers have been discussed 282,313) However, relatively little characterization was reported. Molecular weights of polysulfone and PDMS segments in the copolymers were varied between 800-8,000 and 500-11,000 g/mole, respectively. After thermal curing, the networks obtained showed two phase morphologies as indicated by the detection of two glass transition temperatures (—123 °C and +200 °C) corresponding to PDMS and polysulfone phases, respectively. No mechanical characterization data were provided. 

Fig. 25 ABA-type block copolymer formed by reaction of maleimide-functionalised PEG with cysteine-flanked [(AG)3EG]n p-sheet element. Reproduced with permission from Smeenk et al. [69]. Copyright Wiley-VCH... 

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... 

As previously described, the polymers obtained by 7 and 8 further serve as mono- and difunctional photoiniferters, respectively. If the poly(St)s 42 and 43 are used for the polymerization of MMA as a second monomer, AB- and ABA-type block copolymers, respectively, would be synthesized, as shown in Eqs. (49) and (50) ... 

Di- or trifunctional initiators have also been developed to design the polymer structures including ABA-type block copolymers, and star polymers and star block copolymers. 

In general, A and B subchains in an AB- or an ABA-type block copolymer have different solubilities or affinities for a solvent or other polymers. Therefore, it is expected that a block copolymer is surface-active when dissolved in a suitable solvent or mixed in polymer melts108. This property of block copolymers is now utilized to stabilize or flocculate colloidal dispersions. Blocks A, which are insoluble in a given solvent, are anchored in an insoluble polymer particle, and blocks B, which are soluble in the solvent, form a surface layer around the particle. 

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. 

Since soluble multifunctional initiators are more readily available in cationic polymerization than in the anionic counterpart, ABA type linear triblock copolymers have been almost exclusively prepared using difunctional initiation followed by sequential monomer addition. The preparation and properties of ABA type block copolymer thermoplastic elastomers (TPEs), where the middle segment is PIB, have been reviewed recently [47]. 

Because of the commercial interest in amphiphilic block copolymers the properties of the amphiphilic linear polymer-dendrimer have been most extensively studied. An illustration of the various phases that can be observed is shown in Fig. 6 for the case of an ABA type block copolymer, where A is the hydrophobic dendrimer and B the hydrophilic PEO [119]. The exact location of the... 

The polymerization of norbornene, Eq. (19), is stopped by cooling the reaction mixture to room temperature. The active polymer 11 can be stored for long periods of time. Heating 11 to temperatures above 65 °C in the presence of monomer causes renewed chain propagation. The subsequent addition of different cyclic olefins, such as endo- and exo-dicyclopentadiene, benzonorbomadiene and 6-methylbenzonorbornadiene resulted in the formation of well-defined AB- and ABA-type block copolymers, Eq. (21) [38]. Triblock copolymers 13 with narrow molecular weight distributions (polydispersity = 1.14) were prepared. Thus, the living character enables the preparation of new uniform block copolymers of predictable composition, microstructure and molecular weight. 

A large variety of AB- and ABA-type block copolymers have been prepared from vinyl ethers via sequential living cationic polymerizations. For example, as shown in Scheme 2 (A), isobutyl vinyl ether (IBVE) is first polymerized with the HI/ZnI2 or HCl/ZnCl2 initiating system, and from the resulting living polymer, the second monomer, 2-acetoxyethyl vinyl... 

Block copolymers that consist of hydrophilic and hydrophobic segments are typical amphiphilic polymers, a variety of which have been synthesized by living cationic polymerization. Figure 9 schematically illustrates the structures of some of these amphiphilic polymers thus far obtained though the examples therein are based on poly(vinyl ether) segments, any other appropriate segments may be incorporated. As seen in the illustrations, macromolecular amphiphiles are not necessarily linear AB- and ABA-type block copolymers but may be graft, multiarmed, and network polymers, where the basic components are amphiphilic block copolymers. 

The formation of ABA-type block copolymers is expected, due to the well known termination mechanism of growing poly(styrene) by radical-radical combination. This procedure, therefore, can be used to prepare AB- or ABA-type... 

Thermodynamic treatments of block copolymer morphology, such as those of Helfand (1,2,3) and Meier (4), which are based on anionically polymerized, monodisperse segment AB and ABA type block copolymers, are not applicable in the present case. The large number of short segments in each molecule, the possibility of crystallinity in one or both phases, and the likelihood of metastable morphologies work together to preclude the analytical approaches presently available. 

It is postulated that the morphology of ABA type block copolymers with 20 % or more polyol is that of a continuous rubber network extending through a nylon phase, the deformation behaviour and ultimate strength being determined by the rubber phase. Improvement of impact strength is due to shear flow. 

ABA-type block copolymers B-12 with a hard PMMA as the outer segment (A) and a soft poly(nBA) as the inner segment (B) are expected as all-acrylic thermoplastic elastomers. Examples of B-12 have been prepared with copper and nickel catalysts via bifunctional initiation.359-364 Unfortunately, the copolymers by R—Br/Ni-2 via the macroinitiator method were reported to be inferior as thermoplastic elastomers to those by living anionic polymerizations. A possible reason is the presence of short PMMA seg-... 

The ABA-type block copolymers B-86 to B-88 were synthesized via termination of telechelic living poly-(THF) with sodium 2-bromoisopropionate followed by the copper-catalyzed radical polymerizations.387 A similar method has also been utilized for the synthesis of 4-arm star block polymers (arm B-82), where the transformation is done with /3-bromoacyl chloride and the hydroxyl terminal of poly(THF).388 The BAB-type block copolymers where polystyrene is the midsegment were prepared by copper-catalyzed radical polymerization of styrene from bifunctional initiators, followed by the transformation of the halogen terminal into a cationic species with silver perchlorate the resulting cation was for living cationic polymerization of THF.389 A similar transformation with Ph2I+PF6- was carried out for halogen-capped polystyrene and poly(/>methoxystyrene), and the resultant cationic species subsequently initiated cationic polymerization of cyclohexene oxide to produce... 

Poly(l,4-butadiene) segments prepared by the ruthenium-mediated ROMP of 1,5-cyclooctadiene can be incorporated into the ABA-type block copolymers with styrene (B-106) and MMA (B-107).397 The synthetic method is based on the copper-catalyzed radical polymerizations of styrene and MMA from the telechelic poly(butadiene) obtained by a bifunctional chain-transfer agent such as bis(allyl chloride) or bis-(2-bromopropionate) during the ROMP process. A more direct route to similar block copolymers is based on the use of a ruthenium carbene complex with a C—Br bond such as Ru-13 as described above.67 The complex induced simultaneous or tandem block copolymerizations of MMA and 1,5-cyclooctadiene to give B-108, which can be hydrogenated into B-109, in one pot, catalyzed by the ruthenium residue from Ru-13. 

Polysiloxane-based ABA-type block copolymers B-l29 can be prepared from commercially available poly(dimethylsiloxane) as a stating material followed by functionalization and the subsequent copper-catalyzed radical polymerization of styrene.409... 

Matyjaszewski et al. showed, however, that when a Cu(I)Br catalyst was used in conjunction with bromine-based initiators for a similar system (St/tBA), the polymerizations were well controlled and chain extension in either direction was possible [194]. This is different from the nitroxide system where chain extension of a pSt macroinitiator with acrylate monomers resulted in low blocking efficiency [71 ]. AB and ABA type block copolymers were prepared using monofunctional and difunctional ptBA or pSt macroinitiators. Deprotection of the ferf-butyl esters in these AB block copolymers afforded amphiphilic block copolymers capable of being used as surfactants for emulsion polymerizations [194,195]. 

Living bifunctional poly-l,3-dioxepane (polyDXP) was used as initiator for the polymerization of 1,2-dimethoxyethylene (DME) to prepare ABA-type block copolymers 121) ... 

The domain structure and crystalline texture of AB and ABA type block copolymers of ethylene oxide (EO) and isoprene (Is) are studied, and the effects of the casting solvents and the fractional compositions of each block segment are determined. The domain structures of EO-Is copolymers are essentially identical to those of EO-Is-EO copolymers, but they strongly depend on the fractional compositions and the casting solvents. The role of casting solvent in the different domain formation mechanisms is interpreted in terms of an interrelation of two binodal surfaces that represent the critical concentration for crystallization of the EO segment and the critical concentration for micelle formation of the incompatible EO and Is segments. 

Preparation of AB and ABA Type Block Copolymers of Ethylene Oxide and Isoprene. The block copolymers were synthesized by Szwarc s anionic polymerization technique (11) with cumyl potassium and sodium naphthalene as the initiators for EO-Is diblock copolymers and EO-Is-EO triblock copolymers, respectively, and the ethylene oxide and isoprene contents varied from 0 to 100%. The detailed procedure is being published (12). 

Table I. Characterization of AB and ABA Type Block Copolymers of Ethylene Oxide and Synthesized Isoprene...

Finally, a phase inversion occurs and the spherical EO domains of ca. 400 A are dispersed in the amorphous Is matrix (Figure 2e). The spherical EO domains are crystallized by wide angle x-ray diffraction (cf. diblock copolymers in Figure 4) and DSC thermograms of thin film specimens. The size of the dispersed EO domains was ca. 400 A in diameter for EO-Is-EO 5, ca. 750 A for EO-Is-EO 6, and ca. 400 A for EO-Is-EO 7. These values did not necessarily agree with the theoretical values for amorphous AB and ABA type block copolymers (3, 4, 5, 6), probably also because the EO segment is crystallizable. 

The domain formation mechanism and structure of these copolymers were similar to those of amorphous AB and ABA type block copolymers of styrene and isoprene when benzene, a nonselective solvent, was used as the casting solvent except that one phase was crystallizable and the domain structures were distorted with resultant irregularities in size, shape, and mutual arrangement of domains. The irregularities are attributable to the development of crystalline texture. This is probably valid also for a selective solvent that is good for the crystallizable EO segment but poor for the noncrystallizable Is segment (see below). 

A polymeric micelle is a macromolecular assembly that forms from block copolymers or graft copolymers, and has a spherical inner core and an outer shell (1). As shown in Fig. 1 in which an AB type block copolymer is used, a micellar structure forms if one segment of the block copolymer can provide enough interchain cohesive interactions in a solvent. Most studies of polymeric micelles both in basic and applied aspects have been done with AB or ABA type block copolymers... 

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