Copolymers tapered

Copolymers produced by living polymerization processes differ from those produced by conventional polymerization in one important aspect. Living polymerization processes produce gradient or tapered copolymers. Such copolymers are known from anionic living polymerization. 

The phase diagram of the tapered copolymer melt computed within the two shell approximation [42] is shown in Fig. 14. The distribution function of A monomer along the tapered copolymer chains is... 

Block copolymer synthesis from living polymerization is typically carried out in batch or semi-batch processes. In the simplest case, one monomer is added, and polymerization is carried out to complete conversion, then the process is repeated with a second monomer. In batch copolymerizations, simultaneous polymerization of two or more monomers is often complicated by the different reactivities of the two monomers. This preferential monomer consumption can create a composition drift during chain growth and therefore a tapered copolymer composition. 

Gradient copolymers, also known as tapered copolymers, describe a chain topology that is generally unique to living polymerizations [6,8]. They are intermediates between statistical and block copolymers, as shown in Fig.4. 

In this notation A and B can represent different stereo configurations, or microstructures of the same mer. Likewise, either A and/or B may be random or tapered copolymer blocks rather than homopolymeric blocks. A tapered A-B block is one that begins with a section of pure A mers and gradually incorporates B mers until at the end it becomes a section of pure B mers. The A-B-C designation represents the incorporation of a third monomer segment and would most probably comprise a three-phase system. An A-B-A block polymer based on styrene and butadiene would mean in this notation a polystyrene block-polybutadiene block-polystyrene block structure, that is, S-B-S. 

Some A-B block polymers have also been developed for use in manufacturing mechanical goods by conventional vulcanization. These are usually materials in which one of the blocks is polystyrene and the other block is a tapered copolymer of styrene and butadiene. They are used in blends with other conventional rubbers, such as SBR and natural rubber, in which they improve the processability of the final product. 

The random living cationic polymerization of 3-pinene with styrene and pMeSt was also inspected [44]. With the 3-pinene/styrene mixtures, a faster consumption of the former monomer occurred, yielding tapered copolymers with Mn 5 000 and MWD of 1.4 at 100/65 per cent consumption of 3-pinene/styrene. In contrast, with the 3-pinene/j 7MeSt mixtures, both monomers were consumed at nearly the same rate, yielding statistical copolymers with Mn 5 000 and MWD of 1.5 at both monomer conversions higher than 90 per cent. 

The blocks of each of the polymers may be comprised of (1) homopolymer blocks, (2) statistical polymer blocks, (3) random copolymer blocks, (4) tapered copolymer blocks, (5) gradient copolymer blocks, and (6) other forms of block copolymers. Any copolymer topology that allows immiscible segments present in the system to phase separate inherently or upon annealing, either alone or in the presence of a solvent, is a snitable precursor material. 

When the monomers are added simultaneously with the initiator in the first step solvent, the copolymer formed is rather severely tapered from pure polyisoprene to pure polystyrene, because of the difference in reactivity of the isoprene and styrene monomers. Such a tapered copolymer is equivalent to a block copolymer with block segments rich in isoprene and block segments rich in styrene. The structure of the entire polymer after second step polymerization looking just at segment junctions in or near the interphase region is consequently ... 

Figure 2. Segmental density distribution of block copolymers with styrene-isoprene copolymer random and tapered terminal blocks segments. The upper figure is for a random styrene-isoprene copolymer end-block with uniform composition. The lower figure is that of a severely tapered copolymer end block. The A domain is viewed as equivalent to a block copolymer which is itself microphase-separated with its own interphase region. 

Graded or tapered copolymers exhibit compositional gradients along the chain. One end of the chain is enriched of A, the other of B. In this sense, a block copolymer is the extreme of a graded polymer. 

The u-butyllithium-initiated polymerizations of myrcene proceed in a living manner in benzene (5-30°C) as well as in tetrahydrofuran (THF —30-15°C). Quantitative conversions can be obtained within 2 h (benzene, 30°C) or less than 1 h (THF, 15°C). The polymers have MWs in the range of 5-30 kg/mol, and PDl values are 1.4-1.6 (benzene) and 1.1-1.5 (THF). The polymyrcenes prepared in benzene consist of 85-89% 1,4 units and 11-15% 3,4 units, similar to those obtained by radical polymerization (see above). Increasing either polymerization temperature or initiator concentration causes an increase of the fraction of 3,4 units. On the other hand, polymyrcenes prepared in THF exhibit 40-50% 1,4 units, 39 14% 3,4 units, and 10-18% 1,2 units. Also here, the amount of 1,4 units is found to decrease with increasing polymerization temperature or initiator concentration. The copolymerization of myrcene and styrene results in the formation of block-like or tapered copolymers. The initial copolymers formed in benzene are rich in myrcene, and styrene is preferably incorporated at later stages of the reaction the situation is reversed when the copolymerization is performed in THF [38]. 

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