Copolymers, block method

Living polymerization processes immediately lend themselves to block copolymer synthesis and the advent of techniques for living radical polymerization has lead to a massive upsurge in the availability of block copolymers. Block copolymer synthesis forms a significant part of most reviews on living polymerization processes. This section focuses on NMP,106 A TRP,265,270 and RAFT.- 07 Each of these methods has been adapted to block copolymer synthesis and a substantial part of the literature on each technique relates to block synthesis. Four processes for block copolymer synthesis can be distinguished. 

Poly(arylene oxide) copolymers were prepared by simultaneous and sequential oxidation of 1 1 mixtures of 2, 6-dimethylphenol (DMP), 2-methyl-6-phenylphenol (MPP), and 2,6-diphenylphenol (DPP), and methods were developed for determination of their structure. DMP and DPP yielded either random copolymers or block copolymers with crystallizable DMP and DPP blocks, depending on the order of oxidation and reaction conditions. Four types of copolymers were produced from MPP and DPP random copolymers, block copolymers with crystallizable DPP blocks, short block copolymers with DPP segments too short to permit crystallization, and mixed block copolymers containing DPP blocks and randomized MPP-DPP segments. Redistribution is so facile in the DMP-MPP system that only random copolymers were obtained, even on oxidation of a mixture of the two homopolymers. 

Therefore the distinction between the different methods of synthesis which will be discussed in this paper will be based exclusively on their reaction mechanism and not, as usually in similar reviews, in function on the nature of the end copolymers (block or graft copolymers) (82, 83, 97, 116). 

The growing B arms have anionic sites at their outer ends thus providing the possibility of reacting with electrophilic compounds or other monomers towards the preparation of end-functionalized stars or star-block copolymers. This method can be carried out in inert atmosphere, avoiding the use of the highly demanding and time consuming vacuum technique. It was first reported by Okay and Funke [11] and by Eschwey and Burchard [12] and developed by Rempp and collaborators [13-16]. Scheme 3 illustrates the DVB method. 

Block Copolymers. Block and graft copolymers have generally similar effects of collecting at interfaces and stabilizing dispersions of one homopolymer in another. Most graft copolymers are made at present by free radical methods whereas most commercial block copolymers are synthesized by ionic or stepgrowth processes. As a result, the detailed architecture of block copolymers is more accurately known and controlled. 

Block Copolymers. Several methods have already been used for the synthesis of block copolymers. The most conventional method, that is, the addition of a second monomer to a living polymer, does not produce the same spectacular results as in anionic polymerization. Chain transfer to polymer limits the utility of this method. A recent example was afforded by Penczek et al. (136). The addition of the 1,3-dioxolane to the living bifunctional poly(l,3-dioxepane) leads to the formation of a block copolymer, but before the second monomer polymerizes completely, the transacetalization process (transfer to polymer) leads to the conversion of the internal homoblock to a more or less (depending on time) statistical copolymer. Thus, competition of homopropagation and transacetalization is not in favor of formation of the block copolymers with pure homoblocks, at least when the second block, being built on the already existing homoblock, is formed more slowly than the parent homoblock that is reshuffled by transacetalization. 

This polymer was synthesized via NMRP (Nitroxide Mediated Radical Polymerization) (Benoit et al. 1999) by sequential polymerization of 2VP and a mixture of NIPAAm and DMIAAm. Using the macroinitiator method, the preparation of well-defined linear block copolymers consisting of a homo polymer block P2VP (pH-sensitive) and a random copolymer block of PNlPAAm (temperamre sensitive) with DMIAAm (photo crosslinker) was possible. 

Copolymers, Block Polymers, and Graft Polymers. When more than one monomer is involved, monomer-based names are more complex. Some common polymers have been given names based on an apparent structure, as with polyfethylene terephthalate) A better system has been approved by the lUPAC. With this method, the arrangement of the monomeric units is introduced through use of an italicized connective placed between the names of the monomers. For monomer names represented by A, B, and C, the various types of arrangements are shown in Table 1. 

Fig. 5 Top scanning electron microscope view of an ordered mesoporous Ti02 (anatase) film produced by a block-copolymer templating method [25]...

Polymers that cannot be described by the repetition of a single CRU or comprise units not all connected identically in a directional sense can also be named on a structure basis. These include copolymers, block and graft polymers, and star polymers. They are given names of the type poly(A/B/C...) , where A, B, C, etc. are the names of the component constitutional units, the number of which are minimized. The constitutional units may include regular or irregular blocks as well as atoms or atomic groupings, and each is named by the method described above or by the rules of organic nomenclature. 

Xu et al. (1999a) prepared compatibilized blends of PS and the Zn salt of sulfonated PS by addition of poly(styrene-b-4-vinylpyridine) diblock copolymer. Characterization methods included SEM, DSC, SAXS, and FTIR. The effect of block copolymer level was studied. Evidence was found for Zn-mediated cross-linking between sulfonate groups and pyridine nitrogen. 

Block Copolymers. Several methods such as ultrasonics (100), radiation (101), and chemical techniques (102,103), including the use of polymer ions, polymer radicals, and organometallic initiators, are available to prepare Block Copolymers of acrylonitrile. Acrylonitrile can be used as either the first-or the second-phase monomer. Depending on the mechanism of termination, a diblock of the AB type and a triblock of the ABA type can be formed by disproportionation or transfer for the former, and recombination for the latter. Some of the comonomers are styrene, methyl acrylate, vinyl chloride, methyl methacrylate, vinyl acetate, acrylic acid, and re-butyl isocyanate. An overview and survey of alternating and block copolymers can be found in Reference 104. 

The differences in the IR and Raman spectra of random copolymers, block copolymers, and polymer mixtures, A and B , will be covered in a moment. It should be appreciated that it is difficult to distinguish between polymer mixtures of the form A - - B and block copolymers defined as A -B from either IR or Raman spectra, because the chemical bonding between species A and B is only one of many bonds within a polymer chain. Column chromatographic separation followed by IR or Raman spectral identification or a GPC-IR method [16] is needed to determine whether a sample is a block copolymer or a mixture. In the case of a random copolymer in which component B is very small, B is mixed into the A A chain sequence in the form -A -B-A ,-. While the IR and Raman spectra of the A sequences may stay essentially the same as the A homopolymer, the spectra of B in an... 

Lomonte and Tirpak [1 ] have developed a method for the determination of the percentage of ethylene incorporated in ethylene-propylene block copolymers. Standardisation is done from mixtures of the homopolymers. Both standards and samples are scanned at 180 °C in a spring-loaded demountable cell. The standardisation is confirmed by the analysis of copolymers of known ethylene content prepared with " C-labelled ethylene. By comparison of the infrared results from the analyses performed at 180 °C and also at room temperature, the presence of the ethylene homopolymer can be detected. These workers derived an equation for the quantitative estimation of the percentage of ethylene present as copolymer blocks. 

The method seems to be of wide application [36a] and was extended to block polymers with poly(thio-phene) (PT) sequences, (PS,-PT) and to poly(metha-crylate) blocks (PMMA) associated with PPP [36b]. The fact that these copolymer blocks have a micellar structure led to new organized polymers, with unexpected morphologies in the solid state, as for instance the regular honey-comb morphology obtained with PS,-PPP block copolymers [36]. 

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