Main chains, block copolymers

A reversed sequence of the same procedure has also been followed [151] by using ABME in conjunction with EMP+PF and CHO in order to obtain, under UV irradiation, poly(CHO) containing an azo linkage in the main chain. As the photolysis of the az group at 350 nm irradiation is not achievable due to the higher absorption coefficient of the residual benzoin methyl ether end groups, poly(CHO) is thermally decomposed in the presence of styrene, to give mainly three-block copolymers (Scheme 46). 

In their structure the copolymers may contain the monomeric units randomly, and their overall composition is determined by the composition of the initial feed mixture of monomers (see Section 2.3). Alternating copolymers (a//-copolymers) also are known, where the monomers alternate regularly along the chain. Other types include block polymers where a linear arrangement of groups of one type of monomers is present, graft polymers that have side chain blocks connected to a polymer main chain, per-copolymers where ordered sequences of more than two units are present, etc. 

FIGURE 2. Trends of the isotropic-nematic (open symbols) and nematic-smectic C (full symbols) transition temperatures of the polyester block in block copolymers 1 (A JL), 2 (O, ), and 3 ( J ) and of the isotropic-nematic (V, copolymers 1 S, copolymers 2), nematic-smectic A (ffl, copolymers 2) and isotropic-smectic A (O, copolymers 3) of the side-chain block as functions of the main-chain block content. 

Photochromic diarylethenes can be incorporated into polymers by following free-radical polymerization [75], polycondensation [76], Fridel-Crafts alkylation [77], or oxidative polymerization [78], The side-chain and main-chain homopolymers, copolymers, block copolymers, and jr-conjugated polymers synthesized by the above-mentioned methods have shown good photochromic properties in films and solution phase owing to the higher content of diarylethene chromophores in the polymer system compared to that in simple guest-host systems [79-81], Selected examples are illustrated in Figure 9.8. 

Properties have been determined for a series of block copolymers based on poly[3,3-bis(ethoxymethyl)oxetane] and poly [3,3-bis(methoxymethyl)oxetane]- (9-tetrahydrofuran. The block copolymers had properties suggestive of a thermoplastic elastomer (308). POX was a good main chain for a weU-developed smectic Hquid crystalline state when cyano- or fluorine-substituted biphenyls were used as mesogenic groups attached through a four-methylene spacer (309,310). Other side-chain Hquid crystalline polyoxetanes were observed with a spacer-separated azo moiety (311) and with laterally attached mesogenic groups (312). 

The main experimental techniques used to study the failure processes at the scale of a chain have involved the use of deuterated polymers, particularly copolymers, at the interface and the measurement of the amounts of the deuterated copolymers at each of the fracture surfaces. The presence and quantity of the deuterated copolymer has typically been measured using forward recoil ion scattering (FRES) or secondary ion mass spectroscopy (SIMS). The technique was originally used in a study of the effects of placing polystyrene-polymethyl methacrylate (PS-PMMA) block copolymers of total molecular weight of 200,000 Da at an interface between polyphenylene ether (PPE or PPO) and PMMA copolymers [1]. The PS block is miscible in the PPE. The use of copolymers where just the PS block was deuterated and copolymers where just the PMMA block was deuterated showed that, when the interface was fractured, the copolymer molecules all broke close to their junction points The basic idea of this technique is shown in Fig, I. 

A graft copolymer is a polymer that is comprised of molecules with one or more species of blocks connected as side chains to the backbone, having constitutional or configurational features different from those in the main chain. The simplest case of a graft copolymer can be represented as follows ... 

In detail, the structure of a macroinitiator with active sites in the main chain is classified into two types that derive different types of block copolymers, as shown in Fig. 1. 

A block copolymer composed of liquid crystalline polymer (LCP) segments or that composed of segments having an LCP unit in their main chain or side chain was synthesized [67,68]. The latter showed partial compatibility and second-phase separation even when in a melt liquid crystalline state. 

In one of their notable examples, the hydroboration polymerization of low molecular weight allyl-telechelic polyisobutylene with tripylborane (trip = 2,4,6-triisopropylphenyl) was found to yield air-stable organoboron segmented block copolymers. These boron main-chain polymers (8) (Fig. 8), unlike the general ones, were stable to air. The stability was due to the steric hindrance of the bulky tripyl groups preventing oxygen attack of the borons.28... 

A copolymer in which chains or branches of one polymer are grafted on to the main chain of the other polymer. See Block Copolymer and Heveaplus. 

Thus, the problem on the growth of a block copolymer chain in the course of the interphase radical copolymerization may be formulated in terms of a stochastic process with two regular states corresponding to two types of terminal units (i.e. active centers) of a macroradical. The fact of independent formation of its blocks means in terms of a stochastic process the independence of times ta of the uninterrupted residence in every a-th stay of any realization of this process. Stochastic processes possessing such a property have been scrutinized in the Renewal Theory [75]. On the basis of the main ideas of this theory, the set of kinetic equations describing the interphase copolymerization have been derived [74],... 

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

They also synthesized polymeric iniferters containing the disulfide moiety in the main chain [149,150]. As shown in Eq. (30),polyphosphonamide,which was prepared by the polycondensation reaction of phenyl phosphoric dichloride with piperadine, was allowed to react with carbon disulfide in the presence of triethylamine, followed by oxidative coupling to yield the polymeric iniferter 32. These polymeric iniferters were used for the synthesis of block copolymers with St or MMA, with the composition and block lengths controlled by the ratio of the concentration of the polymeric iniferter to the monomer or by conversion. The block copolymers of polyphosphonamide with poly(St) or poly(MMA) were found to have improved flame resistance characteristics. 

Recently, Kroeze et al. prepared polymeric iniferter 34 including poly(BD) segments in the main chain [152]. They successfully synthesized poly(BD)-block-poly(SAN), which was characterized by gel permeation chromatography, elemental analysis, thermogravimetric analysis, NMR, dynamic mechanical thermal analysis, and transmission electron microscopy. By varying the polymerization time and iniferter concentration, the composition and the sequence length were controlled. The analysis confirmed the chain microphase separation in the multiblock copolymers. 

Later, Kallitsis and coworkers reported similar PPP copolymers, with oligophenylene blocks separated by nonconjugated aliphatic chains. Copolymers 496 [595], 497 and 498 [596] contained the oligophenylene blocks in a main chain or as pendant substituents,... 

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