Comonomer sequence distribution control

The broad range of control of solution polymer structure and macrostructure of styrene-butadiene rubbers that is only possible using lithium catalysts was discussed in Section 2. We have seen how the microstructure of the butadiene units in the chain and comonomer sequence distribution can be controlled with the addition of polar modifiers and/or variations in pol5onerization process variables. Additionally, the unique control of macrostructure features and the new possibilities offered by reactive functional groups were discussed as part of the molecular engineering capabilities of solution anionic polymerizations. 

A mixture of two monomers that can be homopo-lymerized by a metal catalyst can be copolymerized as in conventional radical systems. In fact, various pairs of methacrylates, acrylates, and styrenes have been copolymerized by the metal catalysts in random or statistical fashion, and the copolymerizations appear to also have the characteristics of a living process. The monomer reactivity ratio and sequence distributions of the comonomer units, as discussed already, seem very similar to those in the conventional free radical systems, although the detailed analysis should be awaited as described above. Apart from the mechanistic study (section II.F.3), the metal-catalyzed systems afford random or statistical copolymers of controlled molecular weights and sharp MWDs, where, because of the living nature, there are almost no differences in composition distribution in each copolymer chain in a single sample, in sharp contrast to conventional random copolymers, in which there is a considerable compositional distribution from chain to chain. Figure 26 shows the random copolymers thus prepared by the metal-catalyzed living radical polymerizations. 

These equations show that the composition of the copolymer formed from a specific comonomer mixture is controlled by the monomer reactivity ratios for the copolymerization. Additionally, they control the sequence distribution of the different repeat units in the copolymer. If ta > 1 then "> A prefers to add monomer A (i.e., it prefers to homopropagate) and extended sequences of A-type repeat units are introduced, whereas if ta < 1 A prefers to add monomer B, i.e., to cross-propagate. In a similar way, ra describes the behaviour of monomer B. The effects of some specific combinations of ta and re values upon copolymer composition and repeat unit sequence distribution are considered in the next section. 

The sheer size and value of the polyethylene industry ensure that there is continued research, progress, and development in catalysis, for their potential commercial impact. Although this whole subject is not within the scope of this chapter, we mention a couple of aspects of the progress, which offer the potential to impact this industry. In 1995, DuPont introduced work, carried out with them at the University of North Carolina—via the largest patent applicafion ever in the USA. They disclosed what are described as post-metallocene catalysts. These are transition and late transition metal complexes with di-imine ligands, which form part of the DuPont Versipol technology. Such catalysts create highly branched to exceptionally linear ethylene homopolymers and linear alpha-olefins. Late transition metals offer not only the potential for the incorporation of polar comonomers, which until now has only been possible in LDPE reactors, but also their controlled sequence distribution, compared to the random composition of free radical LDPE copolymers. Such copolymers account for over 1 million tons per annum [20]. Versipol has so far only been cross-licensed and used commercially by DuPont Dow Elastomers (a former joint venture, now dissolved) in an EPDM plant. 

Olefin copolymerization and reactor blend formation are important processes to tailor polyolefins. Copolymer properties depend upon the sequence distribution of the comonomers, which is controlled by means of catalyst as well as process technology. Today most copolymers are produced either in solution processes or in solvent-free gas phase polymerization. Recent breakthroughs in catalyst development are stimulating production of a novel range of copolymers, especially of ethylene copolymers. In the past, special catalysts were designed to produce three classes of ethylene copolymers with different comonomer content ... 

Most step copolymerizations are taken to high extents of reaction in order to produce copolymers with suitably high molar masses (Sections 2.2.4 and 2.2.5). A consequence of this is that the overall compositions of the copolymers obtained correspond to those of the comonomer mixtures used to prepare them. However, it must be borne in mind that the sequence distribution of the different repeat units along the copolymer chains is an important factor controlling the properties of a copolymer and that the distribution is affected by differences in monomer reactivity. 

Monomer reactivity ratios are important quantities since for a given instantaneous comonomer composition, they control the overall composition of the copolymer formed at that instant and also the sequence distribution of the different repeat units in the copolymer. From Equation (2.86), they are the ratios of the homopropagation to the cross-propagation rate constants for the active centres derived from each respective monomer. Thus if a> 1 then prefers to add monomer A (i.e. it prefers to homopolymerize), whereas if rA[Pg.120]

Consideration of the relationship between the effects of radiation on homopolymers and copolymers raises the question of the variation from homopolymer behaviour with sequence length. Every copolymer has a distribution of sequence lengths for each comonomer. At what minimum sequence length does methyl methacrylate not show the high scission of PMMA The future will probably see the development of processes for making polymers with controlled mini-block sequences to maximize a number of properties such as scission yield, adhesion, flexural strength, Tg.. 

The polymerization of cyclotrisiloxane with mixed siloxane units was expected to allow for a synthesis of siloxane-siloxane copolymers with a uniform distribution of units. These copolymers are not accessible by the kinetically controlled copolymerization of two cyclic trisiloxane comonomers, which leads to a microsequential order of siloxane units and a gradient arrangement of monomer units along the chain 16J7), So far the sequencing in poly siloxane, obtained from cyclotrisiloxane having two kinds of siloxane units, have been studied only for the polymerization in the presence of anionic initiators (16,18,19). The use of cationic routes for a controlled synthesis of copolymers with a uniform distribution of units is, therefore, of interest. 

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