ABA triblock copolymer

Although ROMP can exhibit living polymerization characteristics, this is not the case for monomers such as cyclooctene (COE) or cyclooctadiene (COD). These growing polymer chains possess reactive double bonds chain transfer (CT) between polymers inhibits a living kinetic profile, and the MWD is statistically driven toward a value of 2. CT can occur intra- or intermolecularly, or with an externally added CTA [26]. 

Nanostructures with Domain Sizes Exceeding lOOnm 

Bowden and coworkers [44] synthesized high MW molecular brush BCs by grafting from a main chain that was constructed via ROMP with complex 1. 

The polymers were then annealed at 110 C for 96 h, after which the PLA domains were removed by treatment with NaOH. SEM analysis revealed that the material possessed disordered arrays of large holes that were continuous throughout the material. 

Sveinbjornsson et al. [49] investigated the effects of molecular brush BC MW on the PC properties of self-assembled films. Molecular brush BCs were constructed through the sequential polymerization of PLA and PSt macromonomers. 

---

Poly(dimethyl siloxanc) with vinyl or hydrosilanc (Si-H) chain ends have been converted to ATRP initiator ends e.g. Scheme 9.62) by hydrosilylalion, Bis-functional dimethyl siloxane polymers prepared in this way were used in polymerizations of S, MA, tsobornyl acrylate and BA to form ABA triblock copolymers. 

ABA triblock copolymers (as well as multiblock copolymers) cannot be made by sequential addition of monomer A, monomer B, and again monomer A, except in the few cases in which the electron affinity of both monomers is almost the same 27,28, 118>. There are however two ways to circumvent this difficulty ... 

Song, C. X., and Feng, X. D., Synthesis of ABA triblock copolymers of e-caprolactone and DL-lactide, Macromolecules. 

Brzezinska KR, Deming TJ (2001) Synthesis of ABA triblock copolymers via acyclic diene metathesis polymerization and living polymerization of alpha-amino acid-N-carboxyanhy-drides. Macromolecules 34 4348 354... 

Elastomeric Polydiene ABA Triblock Copolymers with Crystalline End Blocks... 

ABA triblock copolymers of the styrene-diene type are well known, and owe their unique properties to their heterophase morphology. This arises from the incompatibility between the polystyrene A blocks and the polydiene B blocks, leading to the formation of a dispersion of very small polystyrene domains within the polydiene matrix. This type of elastic network, held together by the polystyrene "junctions", results in thermoplastic elastomer properties. 

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. 

ABA triblock copolymers, where A was PBd and B either PS or PMMA were prepared by the combination of ROMP and ATRP techniques [122], The PBd middle blocks were obtained through the ROMP of cyclooctadi-ene in the presence of l,4-chloro-2-butene or cis-2-butene-l,4-diol bis(2-bromo)propionate using a Ru complex as the catalyst. The end allyl chloride or 2-bromopropionyl ester groups were subsequently used for the ATRP of either styrene or MMA using CuX/bpy (X = Cl or Br) as the catalytic system (Scheme 50). Quantitative yields but rather broad molecular weight distributions (Mw/Mn higher than 1.4) were obtained. 

Fig. 4 Mean-field phase diagrams for melts of a AB diblock copolymer r = 0 and b symmetric ABA triblock copolymer (r = 0.5) plotted in terms of segregation /N and composition /a calculated with SCFT. From [32]. Copyright 2000 American Institute of Physics...

Fig. 57 Schematic comparison of chain conformations of the midblock for ABC and ABA triblocks. ABC triblock terpolymers (a) have bridge conformations only, whereas ABA triblock copolymers (b) have bridge and loop conformations. From [159]. Copyright 2002 Wiley...

The differences observed between AB di- and ABA triblock copolymers could be explained because two A blocks must escape from the micellar core in the case of ABA triblock chains. 

ABA triblock copolymers, sometimes referred to as dumbbell shaped copolymers when B is a linear macromolecule. 

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. 

Under carefully selected reaction conditions, the polyetherification of an u)-phenol oligomer with an a, o)-di(electrophilic) oligomer produces unexpectedly pure ABA triblock copolymers(11). while the polyetherification of an a, u)-di(electrophilic) oligomer with an a, u)-di (nucleophilic) oligomer represents a new method for the synthesis of perfectly alternating (AB) block copolvmers(9-12.22). 

An example for the synthesis of poly(2,6-dimethyl-l,4-phenylene oxide) - aromatic poly(ether-sulfone) - poly(2,6-dimethyl-1,4-pheny-lene oxide) ABA triblock copolymer is presented in Scheme 6. Quantitative etherification of the two polymer chain ends has been accomplished under mild reaction conditions detailed elsewhere(11). Figure 4 presents the 200 MHz Ir-NMR spectra of the co-(2,6-dimethyl-phenol) poly(2,6-dimethyl-l,4-phenylene oxide), of the 01, w-di(chloroally) aromatic polyether sulfone and of the obtained ABA triblock copolymers as convincing evidence for the quantitative reaction of the parent pol3rmers chain ends. Additional evidence for the very clean synthetic procedure comes from the gel permeation chromatograms of the two starting oligomers and of the obtained ABA triblock copolymer presented in Figure 5. 

Table 4. Tensile properties of the poly(CL-[7-DXO-[7-CL) ABA triblock copolymers (see Scheme 39) ...

To sum up, the living character of the aluminum alkoxide mediated ROP of lactones has permitted the synthesis of novel ABA triblock copolymers, the composition and molecular weight of which can purposely be tuned up for displaying excellent elastomeric properties. Interestingly, the inherent biodegradability of each partner, PCL and PDXO, would open up new applications for these novel thermoplastic elastomers. 

Living anionic polymers and block copolymers can be linked by coupling reactions. A living AB block copolymer can be linked by 1,6-dibromohexane to yield an ABA triblock copolymer... 

Jang et al. (2004) observed that ABA triblock copolymers composed of a docosyl chain, a rigid aromatic segment, and a flexible PEO dendrimer assemble into a hexagonal columnar or body-centered cubic structure in the solid state for the first- and second-generation dendrons, respectively (Fig. 11.27). 

Figure 11.27 Schematic representation of the hexagonal columnar structure of an ABA triblock copolymer with a G1 dendron and the body-centered cubic stmcture of an ABA triblock with a G2 dendron.

Given the morphological complexity of AB diblock and ABA triblock copolymers, it might be expected that the phase behaviour of ABC triblocks would be even more rich, and indeed this has been confirmed by recent experiments from a number of groups. From a practical viewpoint, ABC triblocks can also act as compatibilizers in blends of A and C homopolymers (Auschra and Stadler 1993). In addition to the composition of the copolymer, an important driving force for structure formation in these polymers is the relative strength of incompatibilities between the components, and this has been explored by synthesis of chemically distinct materials. 

Fig. Z41 Phase diagram for a symmetric ABA triblock copolymer (x = 0.25) (Mayes and Olvera de la Cruz 1989).

Fig. 3.28 Phase diagram determined from computer simulations of an ABA triblock copolymer with Nr — 10 in a selective solvent for the B hlock and insoluble end blocks (Nguyen-Misra and Mattice 1995a).

The cylindrical morphology occurs upon the phase separation of nonsym-metric diblock copolymers as well as ABA triblock copolymers. In thin films, the... 

The membrane formed by the self-assemby of an amphiphilic ABA triblock copolymers was used by Meier and coworkers to reconstitute membrane proteins in artificial... 

Nanotubes can be filled with other chemical species, including water molecules. This has been achieved using ABA triblock copolymers to create soft-walled polymer... 

Figure 16.2 Illustration of an ABA triblock copolymer, a water-containing nanotube made from this copolymer, and a TEM image of the polymer nanotube. From Grumelard [962]). Copyright 2004, Royal Society of Chemistry.

---