Sequential block copolymerization

Due to the lack of vinyl monomers giving rise to crystalline segment by cationic polymerization, amorphous/crystalline block copolymers have not been prepared by living cationic sequential block copolymerization. Although site-transformation has been utilized extensively for the synthesis of block copolymers, only a few PIB/crystalline block copolymers such as poly(L-lactide-fc-IB-fc-L-lactide) [92], poly(IB-fr- -caprolactone( -CL)) [93] diblock and poly( -CL-fr-IB-fr- -CL) [94] triblock copolymers with relatively short PIB block segment (Mn< 10,000 g/mol) were reported. This is most likely due to difficulties in quantitative end-functionalization of high molecular weight PIB. 

Carboxyl groups may be introduced into block copolymers via direct polymerizations of free-acid monomers or protection—deprotection procedures. Block copolymers of styrene and nitrophenyl methacrylate (B-40) are used for the latter method, where the activated ester pendant is effectively converted into methacrylic acid or acrylamide under mild conditions.166 A homogeneous aqueous system with copper catalysts gives block copolymers with benzoate groups (B-41) via sequential block copolymerization of the two water-soluble monomers.247... 

The fiber-optic TR-FTIR technique has also been used to investigate the role of additives such as proton traps and electron pair donors (EDs) in carbocationic polymerisation. The role of additives is not clear and is actively debated. " In the sequential block copolymerization of IB and Sty it was shown that that in order to obtain efficient crossover from the living IB, the use of additives (both electron pair donors such as N,N-dimethylacetamide (DMA) and proton traps like 2,6-di-tert-butylpyridine (DrBP), or diphenylethylene is necessary. TR-FTlR monitoring revealed that when DMA was added from the beginning of the IB polymerization phase, the band characteristic of the C=C stretch in IB at 1655 cm appeared as a split signal, as shown in Figure 12a. 

Living cationic sequential block copolymerization is one of the simplest and most convenient methods to provide well-defined block copolymers. The successful synthesis of block copolymers via sequential monomer addition relies on the rational selection of polymerization conditions, such as Lewis acid, solvent, additives, and temperature, and on the selection of the appropriate order of monomer addition. For a successful living cationic sequential block copolymerization, the rate of crossover to a second monomer ( ) must be faster than or at least equal to that of the homopolymerization of a second monomer (i p). In other words, efficient crossover could be achieved when the two monomers have similar reactivities or when crossover occurs from the more reactive to the less reactive monomer. When crossover is from the less reactive monomer to the more reactive one a mixture of block copolymer and homopolymer is invariably formed because of the unfavorable Rcr/Rp ratio. The nucleophilicity parameter (N) reported by Mayr s group might be used as the relative scale of monomer reactivity [171]. 

When the reactivity of the two monomers is similar and steric factors are absent sequential block copolymerization can be used successfully. Alkyl vinyl ethers have similar reactivity and therefore a large variety of AB or BA type diblock copolymers could be prepared by sequential block copolymerization. A recent review is available [172]. Typical examples are shown [173] in Figure 26.1. 

Figure 26.1 Typical examples of AB or BA type diblock copolymers prepared by sequential block copolymerization.

Sequential block copolymerization of IB with more reactive monomers such as aMeSt, p-MeSt, IBVE, or methyl vinyl ether (MeVE) as a second monomer invariably leads to a mixture of block copolymer and PIB homopolymer. To overcome the difficulty in the crossover step, a general methodology has been developed for the synthesis of block copolymers when the second monomer is more reactive than the first one. It involves the intermediate capping reaction with non(homo)polymerizable monomers such as diarylethylenes and 2-substituted furans. 

Owing to the lack of vinyl monomers giving rise to crystalline segment by cationic polymerization, amorphous/crystaUine block copolymers have not been prepared by living cationic sequential block copolymerization. Although site transformation has been utilized extensively for the synthesis of block copolymers, only a few... 

The preparation of novel glassy(A)-b-rubbery(B)-l)-crystalline(C) linear triblock copolymers have been reported where A block is PaMeSt, B block is rubbery PIB, and C block is crystalline PPVL. The synthesis was accomplished by living cationic sequential block copolymerization to yield living poly(aMeSt-l)-IB) followed by site transformation to polymerize PVL [243]. In the first synthetic step, the GPC traces of poly(aMeSt-b-IB) copolymers with (w-methoxycarbonyl functional group exhibited bimodal distribution in both refractive index and UV traces, and the small hump at higher elution volume was attributed to PaMeSt homopolymer. This product was fractionated repeatedly using hexanes/ethyl acetate to remove homo PaMeSt and the pure poly(aMeSt-b-IB) macroinitiator was then utilized to initiate AROP of PVL to give rise to poly(aMeSt-b-IB-b-PVL) copolymer. 

Poly(L-lactide)- -poly(e-caprolactone) (PLLA-fe-PCL) diblock copolymers were synthesized by controlled/ living sequential block copolymerization as initiated by aluminum trialkoxides in toluene solution. These procedures were reported in detail previously [50,98]. Table 11.6 lists the molecular weight characterization data obtained by size exclusion chromatography (SEC) and by NMR. The diblock nomenclature we have used denotes the PLEA block as L and the PCL block as C, the subscripts indicate the approximate composition in wt% and the superscripts the approximate number average molecular weight of the entire block copolymer in kg/mol. 

The preparation of novel glassy(A)-b-mbbery(B)-b-crystalline (C) linear triblock copolymers has been reported where A block is PaMeSt, B block is mbbery PIB, and C block is crystalline poly (pivalolactone) (PPVE). The synthesis was accomplished by living cationic sequential block copolymerization to yield living... 

This success led to the precision synthesis of amphiphilic star block copolymers of VEs with hydroxy or carboxy groups, which were prepared in a similar way (Figure 14). Living block copolymers with ester-containing VEs, obtained by sequential block copolymerization, were used in the Unking reaction, instead of homopolymers. Deprotection of the ester... 

The sequential block copolymerization of la with styrene efficiently proceeded to afford an objective AB diblock copolymer, poly(la)- 7/ocfe-polystyrene, with well-defined structures. This success further confirms the living nature of the anionic polymerization of la. Similarly, a well-defined BA diblock copolymer, polystyrene-l7/ocfe-poly(la), was synthesized by reversing the sequence of monomer addition, namely styrene followed by la. Thus, the possible crossover copolymerization indicates that the electrophilicities of la and styrene as well as the nucleophilicities of both living polymers are very similar. 

Sterically bulkier 2,6-diisopropylphenyl (16c), 2,6-di(tert-butyl)-4-methylphenyl (16d), and 2,6-di(tert-butyl)-4-methoxyphenyl esters (16e) were more effective to protect the carboxylic acid of 16. These ester-protected styrene monomers, 16c-16e, underwent living anionic polymerization without any side reaction in THF even at -78 °C. The resulting living polymers were stable for 24 h under such conditions. Polymers with predictable molecular weights and narrow molecular weight distributions (A4w/A4n< 11) were quantitatively obtained. The success of postpolymerization and the sequential block copolymerization with tert-butyl methacrylate (tBMA) further supports the living nature of the polymerization of 16c-16e. Thus, tert-butyl and sterically bulkier phenyl esters satisfactorily protect the carboxylic acid function of 16 to enable the living anionic polymerization of their ester-protected monomers. 

The living nature of the anionic polymerization of 28a and 28b was also demonstrated by the success of postpolymerization and the sequential block copolymerization of 28a or 28b... 

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