Polystyrene-Based Triblock Copolymers

Reynhout et al. [52] recently published a completely solid phase method for the synthesis of peptide-based triblock copolymers. Amine-functionalized polystyrene was first coupled to an aldehyde-modified resin to give a polystyrene functionalized secondary amine on the resin, which could then be coupled to the next amino acid (Fig. 11). Then the sequence Gly - Ala - Asn -Pro - Asn - Ala - Ala - Gly (GANPNAAG)—a known -hairpin folding peptide found in the CS protein of the Malaria parasite plasmodium falciparum— was synthesized, using standard Fmoc peptide chemistry. After removing the final Fmoc group from the peptide, a carboxylic acid-functionalized polystyrene was coupled to the terminal amine. 

The discovery of living cationic polymerization has provided methods and technology for the synthesis of useful block copolymers, especially those based on elastomeric polyisobutylene (Kennedy and Puskas, 2004). It is noteworthy that isobutylene can only be polymerized by a cationic mechanism. One of the most useful thermoplastic elastomers prepared by cationic polymerization is the polystyrene-f -polyisobutylene-(>-polystyrene (SIBS) triblock copolymer. This polymer imbibed with anti-inflammatory dmgs was one of the first polymers used to coat metal stents as a treatment for blocked arteries (Sipos et al., 2005). The SIBS polymers possess an oxidatively stable, elastomeric polyisobutylene center block and exhibit the critical enabling properties for this application including processing, vascular compatibility, and biostability (Faust, 2012). As illustrated below, SIBS polymers can be prepared by sequential monomer addition using a difunctional initiator with titanium tetrachloride in a mixed solvent (methylene chloride/methylcyclohexane) at low temperature (-70 to -90°C) in the presence of a proton trap (2,6-dt-f-butylpyridine). To prevent formation of coupled products formed by intermolecular alkylation, the polymerization is terminated prior to complete consumption of styrene. These SIBS polymers exhibit tensile properties essentially the same as those of... 

Since the earliest successful synthesis of block copolymers in seminal investigation of living anionic polymerization by Szwarc [ 1,2], diene-based block copolymers, such as polystyrene- tocA -polybutadiene (SB diblock), polystyrene-fetocA -polyisoprene (SI diblock), polystyrene-fetocfc-polybutadiene-( tocfc-polystyrene (SBS triblock), and polystyrene- /oc -polyisoprene-I tocA -polystyrene (SIS triblock) copolymers have been widely used in the thermoplastic elastomer industry due to their unique physical properties not attainable with the constituent homopolymers alone. 

Antony, P., Puskas, J.E., and Kontopoulou, M. The Rheological and Mechanical Properties of Blends Based on Polystyrene-Polyisobutylene-Polystyrene Triblock Copolymer and Polystyrene. Proceedings of MODEST, International Symposium on Polymer Modification, Degradation and Stabilization, Budapest, Hungary, 2002. 

Figure 8.12 TEM photographs of triblock copolymers dispersed in a DGEBA-diamine epoxy network. The triblock copolymer is polystyrene-b-polybuta-diene-b-poly(methyl methacrylate), and the epoxy hardener is (a) -methylene bis [3-chloro-2,6 diethylaniline], MCDEA, and (b) 4,4 -diamino diphenyl sulfone, DDS. In the case of the epoxy system based on MCDEA, the PMMA block is miscible up to the end of the epoxy reaction. In the case of the epoxy system based on DDS, the PMMA block phase-separates during reaction. (From LMM Library.)... 

The principles of nomenclature for copolymers are based on their structure and are given in Table 1.2, where A and B represent the names of repeating units. For example, a statistical copolymer of ethylene and propylene would be called poly(ethylene-stat-propylene), and a triblock copolymer of styrene (A) sand isoprene (B) would be called polystyrene-Z)/oeA -polyisoprene- /ocA -polystyrene. In some cases it is necessary to introduce square brackets in the nomenclature to clarify the notation. Let us see an example An alternating copolymer of styrene and maleic anhydride would be called poly[styrene-a/ (maleic anhydride)]. 

Studies on the morphology and on the melt rheological, tensile, and impact properties were carried out on ternary blend of iPP with two of the following polymers low and high density polyethylene, styrene-b-ethylene butylene-b-styrene triblock copolymer, polystyrene, and acrylonitrile-butadiene-styrene terpolymer (30-33). The results are interpreted for the effect of each individual component by comparing the ternary blends with the respective iPP-based binary blends as the reference systems. 

Triblock and random polyampholytes based on DMAEM-MMA-MAA were examined for their phase separation behaviour [52]. Triblock polyampholytes have a much broader phase separation region than the random ones. The specific structure of PMAA-fc-PlM4VPCl with the excess of cationic or anionic blocks at the lEP is close to the structure of non-stoichiometric IPC. It is suggested that its nucleus consists of intraionic IPC surrounded by cationic blocks protecting it from precipitation [53]. ABC triblock copolymers of polystyrene-b/ock-poly(2-(or 4)vinylpyridine)-fc/ock-poly(methacrylic acid) were synthesized by living anionic polymerization [53 a]. Interpolymer complexation of the polyvinylpyri-dine and poly(methacrylic acid) blocks in the micellar solution was studied in relation to pH in solution by potentiometric, conductimetric and turbidimetric titration and in bulk by FTIR spectroscopy. 

FIGURE 13.8 Schematic illustration of phase separation in a thermoplastic elastomer based on a styrenic triblock copolymer such as SBS. The isolated spherical domains containing the polystyrene end blocks form the hard phase, which acts as both intermolecular tie point ("physical crosslinks ) and filler. The continuous phase from the polybutadiene midblock imparts the elastomeric characteristics to this polymer. 

The main advantage of such triblock copolymers is that they can be moulded and recycled simply by heating the material above the glass transition temperature of polystyrene, unlike classical vulcanised rubbers, which cannot be reused without degradation as they are chemically cross-linked. Such SBS-based materials are called thermoplastic elastomers. However, for various reasons, including cost, the commercial use of such polymers is rather limited compared with the use of natural and classical synthetic rubbers. 

ATRP has been used to prepare AB diblock, ABA triblock, and most recently ABC triblock copolymers (17). To date, the techniqne has been used to create block copolymers based on polystyrene and varions polyacrylates (16). However, it is possible to synthesize a so-called macroinitiator by other polymerization mechanisms (anionic, cationic, etc), and nse this in the ATRP of vinyl monomers. Examples, such as the anionic polymerization of PEG macroinitiators for ATRP synthesis of polyethylene oxide/polystyrene block copol5uners, are discussed in Reference 16. 

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