Triblock sequences, living polymerization

To control the sequence length of HEMA-STY triblock copolymer, Okano et al. [81] later adopted the anionic living-polymerization method. Junction... 

As illustrated in Fig. 24, the addition of ethylene during the living polymerization of propylene resulted in rapid increases in both yield and Mn of the polymers. After the rapid increases which required several minutes, yield and lVln increased by a slower rate, identical with that of the propylene homopolymerization. The propylene content in the resulting polymers attained a minimum value several minutes after the addition of ethylene. These results indicate that the second stage of the polymerization with ethylene was complete within several minutes to afford a diblock copolymer, followed by the third stage of propylene homopolymerization leading to the formation of a triblock copolymer. The 13C NMR spectra of the diblock copolymers showed that the second block was composed of an ethylene-propylene random copolymer sequence. 

True block copolymers containing long blocks of each homopolymer in a diblock, triblock, or multiblock sequence are formed by simultaneous polymerization of the two monomers when n > 1 and r2 8> 1. However, block copolymers are prepared more effectively by either sequential monomer addition in living polymerizations, or by coupling two or more telechelic homopolymers subsequent to their homopolymerization. Alternatively, if the two monomers do not polymerize by the same mechanism, a block copolymer can still be formed by sequential monomer addition if the active site of the first block is transformed to a reactive center capable of initiating polymerization of the second monomer. 

Moreover, ABA triblock copolymers with PIB as the middle sequence were synthesized by cationic-to-ATRP transformation. In this case, a few units of St were added to living difanctional PIB after the IB had reacted. The isolated PIBs could act as bifnnctional macroinitiators for ATRP of St, MA, MMA, and isobornyl acrylate (iBoA). Likewise, Chen employed 1-chloro-l-phenyl ethyl-telechelic PIB obtained by cationic living polymerization for the preparation of triblock copolymers of IB with p-acetoxystyrene (PAcOSt-I -PIB-l7-PAcOSt) or St (PSt-l -PIB-l7-PSt) (Scheme 52). 

Sequential addition of different monomer charges to a living anionic polymerization system is useful for producing well-defined block copolymers. Thermoplastic elastomers of the triblock type are the most important commercial application. For example, a styrene-isoprene-styrene triblock copolymer is synthesized by the sequence... 

The establishment of the living radical polymerization of NIPAM encouraged not only many polymer chemists but also polymer physicists to prepare functionalized NIPAM polymers with various controlled sequences and/or shapes, as discussed in this chapter. In the following parts, block, random, or graft copolymers will be simply designated by the acronym A-B for a diblock copolymer, A-B-A for an ABA-type triblock copolymer, A-B-C for an ABC-type triblock copolymer, A-co-B for a random copolymer, A-g-PNIPAM for grafting of NIPAM segments onto the polymer A. For example, PNIPAM-PEO stands for a diblock copolymer of PNIPAM and PEO. 

In comparison to binary block copolymers relatively little work on ternary block copolymers has so far been published. There are more independent variables in ternary block copolymers as compared to binary block copolymers. While in the latter only one independent composition variable and one interaction parameter exist, in ternary systems there are two independent composition variables and three interaction parameters. This leads to a richer phase diagram. In addition, the block sequence also can be changed, which introduces another tool to influence the morphology [165]. As mentioned before in the case of diblock copolymers, systematic studies of triblock copolymers became possible with the development of sequential polymerization techniques with living anionic polymerization being still the most important one. 

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