Techniques for Formation of Block Copolymers

These polyazoesters can be used to produce block copolymers in a two-step procedure. In the first step a fraction of the azo group is decomposed in presence of a first monomer. Thus, an azo group-containing prepolymer is produced. Decomposition of the rest of the azo groups in the presence of a second monomer results in the formation of block copolymers. Bulk, solution, and precipitation polymerization can be used as polymerization techniques. The efficiency in the preparation of the azo group-containing-prepolymer can be as high as 0,8 if bulk polymerization is carried out in such a way that short blocks are obtained. In the second step, typical values for the efficiency are 0.3-0.4. 

Non-living polymerization techniques can be combined with CRP methods to produce block copolymers. The first example of transforming a hydroxy functionality into an ATRP initiator was demonstrated by Gaynor and Matyjaszewski [223], who converted a polysulfone,prepared through the condensation polymerization of 4,4-difluorosulfone with an excess of bisphenol A, to an ATRP initiator by reaction with 2-bromopropionyl bromide for subsequent controlled polymerization reactions (cf. Scheme 26). The transformation proved to be quantitative and the macroinitiator (Mn=4030,Mw/Mn=1.5) was used for formation of triblock copolymers with St (Mn=10,700,Mw/Mn=l.l) or nBA (Mn=15,300,Mw/Mn=1.2) as shown in Fig. 31 [223]. DSC analysis provided evidence of the presence of two distinct blocks with Tg=153-159 °C (polysulfone) and 104 °C (pSt) or -41 °C (pBA). 

An interesting extension of electroinitiated polymerization techniques is the formation of block copolymers. For example, it has been found that after polymerization of styrene and tetrahydrofuran in a divided cell, further reaction of the two homopolymers provided an A-B-A block copolymer (Scheme 9) (where A is tetrahydrofuran and B is styrene). 

Block copolymers can also be synthesized by controlled radical polymerization [162]. This technique is very interesting for probable industrial applications, because of its low sensitivity against impurities and the mild reaction conditions. In principle, all methods of the controlled radical polymerization, as they are described in the chapter above, can be utilized more or less successfully for the formation of block copolymers. 

Experiments were run with natural and synthetic rubber by using an internal mixer, roll mills, and an extruder. As the gelation is a mechano-chemical process, independent of the elastomer structure, the reaction was applied to plastomers masticated while in the viscoelastic state [188]. The reaction takes place with poly(methyl methacrylate), poly(vinyl acetate), and polyethylene. No evidence for the cross-linking of polystyrene was observed. The addition of aluminum isopropoxide aids the formation of block copolymer. This technique was applied to the system polyethylene-poly(vinyl acetate). 

A cursory review of the literature reveals that the ELC technique with micellar mobile phases has proven to be very beneficial in the characterization of micellar systems (184-186,190-192,227,228). For example, microcolumn exclusion LC has been applied to the determination of the CMC value of surfactants (or micellar-forming proteins), determination of the kinetic rate and equilibrium association constants for surfactant (or protein) micellization (184,192), determination of the size or size distribution of micelles (especially those formed from block copolymers or milk casein) (185,186,191,192,225) as well as for estimation of the time required for formation of micelles (or micelle-forming macromolecules) (186) among others. The size and stability of reversed micelles has also been evaluated using ELC (195). 

Alternatively, SPBs can be prepared by the dissolution of block copolymers in suitable solvents. " This leads to the formation of micelles with a hydrophobic core and a hydrophilic shell, which show a similar structure to SPB. This technique has been extensively investigated in the literature because the self-assembly of block copolymers is an interesting subject in itself. It should be noted, however, that the micelles have considerable core polydispersity, which can be direaly assessed using small-angle neutron scattering (SANS). The finite breadth of the size distribution renders the analysis of the various properties, for example, the flow behavior, more difScult. Moreover, the interface between the hydrophobic core and the hydrophilic shell cannot be very sharp because the micelles result from self-assembly. 

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