Diblock sequences, living polymerization

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. 

In the formation of block copolymers by sequential addition of monomers it generally does not matter which monomer is polymerized first, and diblock or multiblock copolymers of narrow MWD and of any desired sequence length are readily prepared. Termination is usually effected by reaction of the living ends with aldehydes ketones can be used for terminating titanacyclobutane ends, while unsaturated ethers are used for terminating ruthenium carbene complexes. 

Living radical polymerization (atom transfer radical pol5mierization) has been developed which allows for the controlled polymerization of acrylonitrile and comonomers to produce well defined linear homopolymer, statistical copolymers, block copolymers, and gradient copolymers (214-217). Well-defined diblock copolymers with a polystyrene and an acrylonitrile-styrene (or isoprene) copolymer sequence have been prepared (218,219). The stereospecific acrylonitrile polymers are made by solid-state urea clathrate polymerization (220) and organometallic compounds of alkali and alkaline-earth metals initiated polymerization (221). 

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. 

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. 

Lastly, mention will be made of two further amide block copolymer syntheses not mentioned above. The first employs living polymer anions from styrene, isoprene or methyl methacrylate to initiate the polymerization of isocyanates to diblock polymers containing nylon 1 sequences. In this approach, selective polymerization can be achieved at the unhindered isocyanate group of diisocyanates such as tolylene 2,4-diisocyanate to give products with pendant NCO groups (32) which are crosslinkable with diols to give tough resins. 

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