Multi-block copolymer

Solution polymerisation can yield random, di-block, tri-block or multi-block copolymers. It is important to note that the tri-block, or multi-block copolymers, belong to that class of material termed thermoplastic elastomers and it is only the random copolymer types that are considered here. 

G.G. Malliaras, J.K. Herrema, J. Wildeman, R.H. Wieringa, R.E. Gill, S.S. Lampoura, and G. Hadziioannou, Tuning of the photo- and electroluminescence in multi-block copolymers of poly[(silanylene)-thiophene]s via exciton confinement, Adv. Mater., 5 721-723, 1993. 

Qu Y, Payne SC, Apkarian RP, ConticeUo VP. Self-assembly of a pol3rpeptide multi-block copolymer modeled on draghne silk proteins. J Am Chem Soc 2000 122 5014-5015. 

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... 

All graft copolymers are composed of a backbone and side chains which are chemically different from the backbone. Block copolymers are composed of one or more segments of poly(M1) (A) and one or more segments of poly(M2) (B). Depending on the number of blocks the products are either A—B (di-block copolymers), A—B—A (tri-block copolymers) or -(-A—B -n (multi-block copolymers). 

SCBs play an important role in the formation of other block copolymers. For example, the relatively less nucleophilic poly(ethylene oxide) oxyanion cannot initiate the polymerization of styrene, which needs a more nucleophilic alkyllithium initiator. To enable the synthesis of multi-block copolymers from various combinations of monomers by anionic mechanisms, it is important to modify the reactivity of the growing anionic chain end of each polymer so as to attack the co-monomer. There have only been a few reports on the polymerization of styrene initiated by an oxyanion (see <2001MM4384> and references cited). Thus, there exists a need for a transitional species that is capable of converting oxyanions into carbanions. In 2000, Kawakami and co-workers came up with the concept of the carbanion pump , in which the ring-strain energy of the SCB is harnessed to convert an oxyanion into a carbanion (Scheme 13) <2000MI527>. 

Na, K., Lee, K. H., Lee, D. H., and Bae, Y. H. (2006), Biodegradable thermo-sensitive nanoparticles from poly(L-lactic acid)/poly(ethylene glycol) alternating multi-block copolymer for potential anti-cancer drug carrier, Eur. J. Pharm. Sci., 27(2-3), 115— 122. 

The use of polysilanes as photoinitiators of radical polymerization was one of the hrst means whereby they were incorporated within block copolymer structures [38 0], albeit in an uncontrolled fashion. However the resulting block copolymer structures were poorly defined and interest in them principally lay in their application as compatibilisers for polystyrene (PS) and polymethylphenylsilane blends PMPS. The earliest synthetic strategies for relatively well-defined copolymers based on polysilanes exploited the condensation of the chain ends of polysilanes prepared by Wurtz-type syntheses with those of a second prepolymer that was to constitute the other component block. Typically, a mixture of AB and ABA block copolymers in which the A block was polystyrene (PS) and the B block was polymethylphenylsilane (PMPS) was prepared by reaction of anionically active chains ends of polystyrene (e.g. polystyryl lithium) with Si-X (X=Br, Cl) chain ends of a,co-dihalo-polymethylphenylsilane an example of which is shown in Fig. 2 [43,44,45]. Similar strategies were subsequently used to prepare an AB/ABA copolymer mixture in which the A block was poly(methyl methacrylate) (PMMA) [46] and also a multi- block copolymer of PMPS and polyisoprene (PI) [47]. 

A particularly interesting block copolymer made by the coupling approach was a multi-block copolymer of PMPS and poly (ethylene oxide) (PEO). This was prepared by reacting the Si— X chain ends of PMPS with the hydroxyl chain ends of well-defined commercial sample of poly(ethylene glycol) [47] (Fig. 3). Allhough... 

Fig. 3 Synthesis of a polymethylphenylsilane-poly(ethylene oxide) multi-block copolymer...

The first self-assembling block copolymers were PS-fe-PMPS- -PS synthesised by Matyjaszewski and Moller. They observed micellar aggregates by ATM after casting dilute dioxane solutions (a solvent selective for the PS block) of the copolymer. The observed micelles were taken to have internal PMPS cores and were measured at 25-30nm in diameter [73], The hrst self-assembling amphiphilic polysilane block copolymers to be investigated was the PMPS-PEO multi-block copolymer with normal distribution PMPS blocks and uniform low polydispersity PEO blocks. After dialysis aqueous dispersions of this copolymer formed micellar as well as vesicular structures [78, 79] as shown in Eig. 19. 

Polystyrene (PS) in its atactic and syndiotactic forms is a brittle thermoplastic, even in an orientated state [4]. To improve the toughness of aPS, impact modification has been practised for a long time, either by polymerizing the styrene in the presence of a polybutadiene rubber leading to high-impact polystyrene, commonly called HIPS, or by blending the polystyrene with multi-block copolymers, mainly of the styrene-butadiene-styrene (S-B-S) type. 

The structure of a multi block copolymer does not seem suitable for the purpose. On the other hand, the structure of graft copolymers with short polysiloxane branches is considered to be the exact structure for the purpose, since one may maintain film forming properties, flexibility of slloxane chain (high permeation rate), and high selectivity. The flexibility of the slloxane chain can be maintained in side chains of graft copolymers and film forming properties and selectivity of permeation can be provided by backbone For this purpose, polystyrene was chosen as the backbone material... 

As ionic polymerizations with stringent reaction conditions are more difficult to carry out than normal free-radical processes, the latter are invariably preferred where both free-radical and ionic initiations give a similar product. For example, commercial polystyrenes are all free-radical products, though styrene polymerization can be initiated with free radicals as well as with appropriate anions or cations. However, to make research grade polystyrenes with exceptionally narrow molecular-weight distributions and di-block or multi-block copolymers of styrene and other monomers, ionic processes are necessarily employed. 

Ahn CH, Chae SY, Bae YH, Kim SW. Synthesis of biodegradable multi-block copolymers of poly(L-lysine) and poly(ethy-lene glycol) as a non-viral gene carrier. J Contr Rel 2004 97 567-574. 

PEN Penco, M., Donetti, R., Mendichi, R., and Ferrutti, P., New poly(ester-caibonate) multi-block copolymers based on polyflactic-glycolic acid) and polyfe-caprolactone) segments, Macromol. Chem. Phys., 199, 1737, 1998. 

Chung, H.J., Lee, Y. Park, T.G. 2008, Thermo-sensitive and biodegradable hydrogels based on stereocomplexed Pluronic multi-block copolymers for controlled protein delivery . Journal of Controlled Release, vol. 127, no. 1, pp. 22-30. 

Park T, Zimmerman SC (2006) A supramolecular multi-block copolymer with a high propensity for alternation. J Am Chem Soc 128(43) 13986-13987... 

In the swollen block copolymeric materials, the insoluble block of the polymer aggregates into microphase separated domains that in turn are dispersed in well-defined, spatially periodic supramolecu-lar structures.(18) While the morphologies of these structures are sometimes indistinguishable from textures assumed by thermotropic (and lyotropic) MLCs, these block copolymer phases are frequently more solid-like in their physical properties. Multiple interconnections between microphase separated domains tend to establish static, 3-dimensional, translationally ordered topologies in the materials. These aspects of the mesogenic block copolymers can be emphasized if, for example, multi-block copolymers — chains with several distinct monomer compositions along its contour length — are swollen,(19) Reference 20 provides a current entry to the literature of mesomorphic block copolymers. 

A major goal of the research in proton exchange membrane fuel cells is the development of high-temperature membranes that may operate at 120 °C and low humidity. One route are multi-block copolymers with completely disulfonated PAES and naphthalene polyimide oligomers [107]. 

PAES multi-block copolymers from blocks of different hydrophobic properties have been synthesized. The hydrophobicity can be adjusted by an oligomeric sulfonation [108]. Naphthalene-based moieties were used as hydrophobic units. Membranes from these copolymers showed a relatively low oxidative stability because of higher water uptake and swelling. 

For a direct hydrazine fuel cell, anion conductive aromatic multi-block copolymers, PAES with quaternized ammonio-substituted fluorene groups have been described [118]. The ammonio groups can be selectively introduced onto the fluorene-containing units. 

Multi-block copolymers from a hydrophilic disul-fonated poly(arylene ether sulfone) and a hydrophobic fluorine-terminated poly(arylene ether benzoni-trile) have been synthesized [45,46],... 

G-, tri and multi-block Copolymers, or Segmented Copolymers. In a diblock copolymer, a chain with certain chemical composition (a homopolymer or a copolymer chain) is linked by the chemical bond to the chain... 

In addition, block copolymers can be produced by adding a second monomer once the first one has completely reacted. Tri- and multi-block copolymers can be prepared by subsequent additions of different monomers. Thus, styrene-butadiene-styrene tri-block copolymers produced by anionic polymerization are used as thermoplastic elastomers. Also, star and hyperbranched polymers can be obtained through this technique by simply using suitable initiation systems [17]. 

Block copolymers. The multi-component systems are intramolecular, with each component occupying a certain length of chain sequences, as shown in Fig. 2.8a. They can be diblock, triblock or even multi-block copolymers. Upon the change of composition, the microphase separation in block copolymers can fabricate various geometries of regularly packed microdomain patterns with nano-scale resolution, as will be introduced in Sect. 9.3. 

Multi-block copolymers can form a greater variety of ordered phase structures than diblock copolymers, via self-assembly. Some of them have been widely applied as the matrix materials, such as styrene-butadiene-styrene (SBS) thermal elastomers, acrylonitrile-butadiene-styrene (ABS) copolymers and polyurethanes. 

Table 12.1 Physical and gas transport properties of Pebax and Polyactive multi-block copolymers [24,64-67]...

Table 12.2 Thermal properties of PEO-PBT multi-block copolymers [Reprinted with permission 67] ...

Figure 12.3 Thermai gravimetric analyses of PEO-PBT multi-block copolymers. Reprinted with permission from Advanced Functional Materials, Tailor-made polymeric membranes based on segmented block copolymers for CO2 separation, by A. Car, C. Stropnik, W.

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