Block copolymers by anionic polymerization

The possibility of employing block copolymers as materials that might possess desirable properties was originally considered by Mark In the first period the effort in preparing block copolymers was directed to radical polymerization and it was only in 1956 that Szwarc obtained well-defined block copolymers by anionic polymerization . In block copolymers, the incompatibility between polymeric chains becomes an advantage a phase separation of the blocks occurs leading to the formation of microdomains which are responsible for the ecific properties of block copolymers. For instance, the presence in a molecule of an elastomeric block linked by its ends to thermoplastic blocks generates a polymer in which reversible physical multifunctional cross-links allow the behaviour of conventional vulcanized elastomers at room temperature, but the material remains easily moldable at elevated temperature just as normal thermoplastic resins ° ... 

Many different approaches have been used to synthesize star-block copolymers including anionic, cationic, radical, and condensation polymerization techniques, and even combinations of them [9]. The majority of the molecules produced thus far were prepared by anionic polymerization procedures. The dominant way of preparing star-block copolymers by anionic polymerization is the coupling of preformed diblock or triblock living copolymer chains with a suitable compound to produce the central linking point. In this way divinylbenzene (DVB) was first used in order for a central core to be created [ 10]. This was achieved by adding a predetermined amount of the divinyl compound to a solution of living diblock chains (Scheme 1). 

DPV-capped PIB chains in a mixture yield the same macroanion (Scheme 2) and lead to the same block copolymer by anionic polymerization. 

Fig. 4 Approaches to the synthesis of polysilane block copolymers by living polymerization techniques (a) via anionic polymerization of masked disilenes (b) via anionic ring-opening polymerization of cyclotetrasilanes...

Block Copolymerization. A polymerization with long chain lives can be used to make block copolsrmers (qv). An important commercial example is styrene/butadiene blocks produced by anionic polymerization (qv). A solution polymerization is done in a batch reactor, starting with one of the two monomers. That monomer is reacted to completion and the second monomer is added while the catalytic sites on the chains remain active. This produces a block copolymer of the AB form. Early addition of the second monomer produces a tapered block. If the second monomer is reacted to completion and replaced by the first monomer, an ABA triblock is obtained. This process is not easily converted to continuous operation because polsrmerizations inside tubes rarely approach the piston-flow environment that is needed to react one monomer to completion before adding the second monomer. Designs using static mixers (also known as motionless mixers) are a possibility. 

As indicated above, an extraordinary range of compositions can be synthesized in a relatively simple way that will encompass surfactant properties that depend on composition and use temperature. Virtually no other polymer system can span this range of interfacial or surfactant properties. The polymerization mechanisms and procedures used are described earlier in this chapter and though most often anionic-step polymerization is used, it is possible to prepare the block copolymers by cationic polymerization. 

Remark. It has to be stressed that in the syntheses of block copolymers by anionic means, the first monomer (A) to be polymerized should not exhibit an electro-affinity higher than that of the second monomer (B) (see Section 8.6). 

Thermoplastic Elastomers. These represent a whole class of synthetic elastomers, developed siace the 1960s, that ate permanently and reversibly thermoplastic, but behave as cross-linked networks at ambient temperature. One of the first was the triblock copolymer of the polystyrene—polybutadiene—polystyrene type (SheU s Kraton) prepared by anionic polymerization with organoHthium initiator. The stmcture and morphology is shown schematically in Figure 3. The incompatibiHty of the polystyrene and polybutadiene blocks leads to a dispersion of the spherical polystyrene domains (ca 20—30 nm) in the mbbery matrix of polybutadiene. Since each polybutadiene chain is anchored at both ends to a polystyrene domain, a network results. However, at elevated temperatures where the polystyrene softens, the elastomer can be molded like any thermoplastic, yet behaves much like a vulcanized mbber on cooling (see Elastomers, synthetic-thermoplastic elastomers). 

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. 

Polystyrene homopolymer produced by free radical initiators is highly amorphous (Tg = 100°C). The general purpose rubber (SBR), a block copolymer with 75% butadiene, is produced by anionic polymerization. 

Though living anionic polymerization is the most widely used technique for synthesizing many commercially available TPEs based on styrenic block copolymers, living carbocationic polymerization has also been developed in recent years for such purposes [10,11], Polyisobutylene (PlB)-based TPEs, one of the most recently developed classes, are synthesized by living carbocationic polymerization with sequential monomer addition and consists of two basic steps [10] as follows ... 

An interesting procedure has been proposed for the synthesis of amylose-b-PS block copolymers through the combination of anionic and enzymatic polymerization [131]. PS end-functionalized with primary amine or dimethylsilyl, -SiMe2H groups were prepared by anionic polymerization techniques, as shown in Scheme 56. The PS chains represented by the curved lines in Scheme 56 were further functionalized with maltoheptaose oligomer either through reductive amination (Scheme 57) or hydrosilyla-tion reactions (Scheme 58). In the first case sodium cyanoborohydride was used to couple the saccharide moiety with the PS primary amine group. 

Double hydrophilic star-block (PEO-fo-PAA)3 copolymers were prepared by a combination of anionic and ATRP of EO and fBuA [150]. Three-arm PEO stars, with terminal - OH groups were prepared by anionic polymerization, using l,l,l-tris(hydroxymethyl)ethane, activated with DPMK as a trifunctional initiator. The hydroxyl functions were subsequently transformed to three bromo-ester groups, which were utilized to initiate the polymerization of f-butyl acrylate by ATRP in the presence of CuBr/PMDETA. Subsequent hydrolysis of the f-butyl groups yielded the desired products (Scheme 74). 

The precipitated silica (J. Crosfield Sons) was heated in vacuo at 120° for 24h. before use. Two grades of surface areas 186 and 227 m g l (BET,N2), were used during this project. Random copolymers, poly(methyl methacrylates) and polystyrene PS I were prepared by radical polymerization block polymers and the other polystyrenes were made by anionic polymerization with either sodium naphthalene or sodium a methylstyrene tetramer as initiator. The polymer compositions and molecular weights are given in Table I. 

It has been shown recently (10) that such block structures could be tailored precisely by the general method summarized hereabove. It is indeed possible to convert the hydroxyl end-group of a vinyl polymer PA (f.i. polystyrene, or polybutadiene obtained by anionic polymerization terminated with ethylene oxide),into an aluminum alcoholate structure since it is well known that CL polymerizes in a perfectly "living" manner by ring-opening insertion into the Al-0 bond (11), the following reaction sequence provides a direct access to the desired copolymers, with an accurate control of the molecular parameters of the two blocks ... 

Application of amphiphilic block copolymers for nanoparticle formation has been developed by several research groups. R. Schrock et al. prepared nanoparticles in segregated block copolymers in the sohd state [39] A. Eisenberg et al. used ionomer block copolymers and prepared semiconductor particles (PdS, CdS) [40] M. Moller et al. studied gold colloidals in thin films of block copolymers [41]. M. Antonietti et al. studied noble metal nanoparticle stabilized in block copolymer micelles for the purpose of catalysis [36]. Initial studies were focused on the use of poly(styrene)-folock-poly(4-vinylpyridine) (PS-b-P4VP) copolymers prepared by anionic polymerization and its application for noble metal colloid formation and stabilization in solvents such as toluene, THF or cyclohexane (Fig. 6.4) [42]. 

Block copolymers of (R,S)-(3-butyrolactone and eCL have been synthesized by combining the anionic ROP of the first monomer with the coordinative ROP of the second one (Scheme 15) [71]. The first step consisted of the synthesis of hydroxy-terminated atactic P(3BL by anionic polymerization initiated by the alkali-metal salt of a hydroxycarboxylic acid complexed with a crown ether. The hydroxyl end group of P(3BL could then be reacted with AlEt3 to form a macroinitiator for the eCL ROP. 

The range of monomers that can be incorporated into block copolymers by the living anionic route includes not only the carbon-carbon double-bond monomers susceptible to anionic polymerization but also certain cyclic monomers, such as ethylene oxide, propylene sulfide, lactams, lactones, and cyclic siloxanes (Chap. 7). Thus one can synthesize block copolymers involving each of the two types of monomers. Some of these combinations require an appropriate adjustment of the propagating center prior to the addition of the cyclic monomer. For example, carbanions from monomers such as styrene or methyl methacrylate are not sufficiently nucleophilic to polymerize lactones. The block copolymer with a lactone can be synthesized if one adds a small amount of ethylene oxide to the living polystyryl system to convert propagating centers to alkoxide ions prior to adding the lactone monomer. 

The block copolymer is prepared by anionic polymerization. As in Example 3-19, the greatest care must be taken to exclude air and moisture. 

Some Block Copolymers Obtained by Anionic Polymerization Techniques... 

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