Microstructure of copolymers

Some very peculiar features have been discovered in the microstructures of copolymers. Thus, Hanna et al. (1993) showed that a random copolymer of two aromatic monomers has chains in which random but similar sequences of the two monomers on distinct chains find each other and come into register to form a... 

Statistical characteristics of the second type define the microstructure of copolymer chains. The best known characteristics in this category are the fractions P [/k) (probabilities) of sequences Uk involving k monomeric units. The simplest among them are the dyads U2, the complete set of which, for example, for a binary copolymer is composed of four pairs of monomeric units M2M, M2M2. The number of the types of k-ad in chains of m-component copolymers grows exponentially as mk so that with practical purposes in mind it is generally enough to restrict the consideration to sequences Uk] with moderate values of k. Their calculation turns out to be rather useful... 

Furthermore, studies of the microstructure of copolymers formed by the low-temperature copolymerisation of cis-1 -(2 H)-propene (or trans isomer) and perdeuteropropene in the presence of soluble vanadium-based Ziegler-Natta catalysts showed syndiospecific propagation to involve a monomer insertion of the cis type [27]. 

Study of microstructure of copolymers based on monomer yields... 

Microstructure of copolymers typically refers to the proportion and the arrangement of the monomer units in the polymeric backbone. In their structure the copolymers may contain the monomeric units arranged randomly, they may alternate regularly, may form large blocks of one type of monomer, or may appear as side chain blocks connected to a polymer main chain (see Section 1.1). This distribution also depends on the relative amounts of each monomer present in the copolymer [7]. Analytical pyrolysis, particularly Py-GC-MS, has been used successfully for the analysis of microstructure of copolymers (see e.g. [8]). Pyrolysis generates small fragments that represent sections of the polymer and can make distinctions between random and block copolymers fairly straightfonward. 

Kochervinskii and Murasheva [115] studied the microstructure of copolymers of vinylidene fluoride and tetrafluoroethylene of 71 29 composition using NMR. They showed that there is 5 mol% of diads in the tetrafluoroethylene blocks and 2.5 mol% of head-to-head defects in the VDF blocks. 

In the case of copolymerization using catalysts that do not rearrange the siloxane chain and under non-equilibrium conditions a theoretical model can be developed to describe the microstructure. This same microstructure can be analysed by NMR of proton or silicon-29 nuclei. The concordance between theory and experiment enables confirmation of the correctness of either the theoretical mathematical model or the assignment of NMR signals. Quantitative information on the microstructure of copolymers as a function of time or at any conversion or for any comonomer composition can be obtained. For copolymerization under less precise conditions, the microstructure can be derived from the siloxane copolymer itself by direct observation of the magnetic resonance signals of the silicon-29 nuclei. 

The kinetics of copolymerization and the microstructure of copolymers can be markedly influenced by the addition of Lewis acids. In particular, Lewis acids are effective in enhancing the tendency towards alternation in copolymerization of donor-acceptor monomer pairs and can give dramatic enhancements in the rate of copolymerization and much higher molecular weights than are observed for similar conditions without the Lewis acid. Copolymerizations where the electron deficient monomer is an acrylic monomer e.g. AN, MA, MMA) and the electron rich monomer is S or a diene have been the most widely studied." Strictly alternating copolymers of MMA and S can be prepared in the presence of, for example, dictliylaluminum scsquichloridc. In the absence of Lewis acids, there is only a small tendency for alternation in MAA-S copolymerization terminal model reactivity ratios are ca 0.51 and 0.49 - Section 7.3.1.2.3. Lewis acids used include EtAlCT, Et.AlCL ElALCL, ZnCT, TiCU, BCl- LiC104 and SnCL. 

A study of the microstructure of copolymers made use of a simple solution procedure for the copolymerization of methyl acrylate (MA) with A-vinylcarba-zole (NVK). The ratio of the feed composition ranged from (0.1429 moles of MA to 1.00 of NVK) to (7 moles of 1 of NVK)—a 49-fold range in composition. The experiments were carried out to high conversions [41]. Preparation 2-8 is based on this work. The reactivity ratios calculated from this series were approximately... 

Both proton and carbon NMR spectroscopy are very frequendy used to determine the average a cumulative axnpositioi of (emulsion) co- and terpolymers, and also the microstructure of copolymers (see Section 11.2.4). Except for solid-stale NMR, die only requirmient for the applicability of regular NMR to any system is that all polymer chains are soluble in (or at least swollen by) a suitable deuterated solvent (or solvent mixture). Fct most polymer sanples this is feaable, unless the polymers are too heavily crosslinked or chemically loo heterogeneous. Kooiig has given an excellent review on NMR of polymers [49]. 

Black Ramhez AL, Ogle JW, Schmitt AL, Lenhardt JM, Cashion MP, Mahanthappa MK, Craig SL (2011) Microstructure of copolymers formed by the reagentless, mechanochemical remodeling of homopolymers via pulsed ultrasound. ACS Macro Lett 1 23-27... 

Montaudo, M.S. and Montaudo, G., Further Studies on the Composition and Microstructure of Copolymers by Statistical Modeling of Their Mass Spectra, Macromolecules, 25, 4264 (1992). 

The microstructures of copolymers prepared by chemical modification are obviously amenable to study using exactly the same techniques that are applied to copolymers prepared by more normal routes. Here, part of the value of such studies is that they can impart information concerning the mechanism of the chemical modification reactions. Some examples of polymer modification are discussed in this section. 

The shape of dispersed phase particles is determined by the flow field and heat gradients that affect polymer orientation. For instance, the microstructure of copolymers of PE and PP is similar to the skin-core textures described for PE [228]. The orientation of the dispersed phase can affect the mechanical properties of the system. Spherical domains are more conunonly formed in systems where phase separation occurs while the polymers are liquid. The SEM appears to reveal spherical dispersed phase particles (Fig. 5.48), although tilting can show they are actually... 

Often mass data alone do not permit exact structural assignment, as the same mass can result from different microstructures of copolymers... 

Laser Raman spectroscopy has been proposed as a useful technique for probing the microstructure of copolymers. Good correlations were found between the concentrations of some isolated, dyad, triad and tetrad comonomer sequences in vinyl chloride/vinylidene chloride copolymers and certain scattering intensities [99]. The positions and intensities of particular absorption bands have also been correlated with chain microstructure in an infrared study of ethylene/vinyl chloride copolymers, previously characterised by C-NMR analysis [100]. More recently, FTIR spectra have been analysed for monad, dyad and triad monomer sequence-distribution dependencies in random styrene/acrylonitrile copolymers [101]. Changes in peak intensities from normalised spectra were correlated with microstructure probabilities assignments were given if there existed a linear relationship between peak intensity and the number fraction of a microstructure. 

IR spectra have been shown to be very useful for examining the compositional microstructure of copolymers [19-22]. However, whilst the measurement of the properties of the monomer constituents in a copolymer using characteristic IR absorption bands is comparatively easy (and has been used widely for various copolymer systems) by comparison with the NMR technique, the analysis of the sequence distribution of monomer units which comprise a copolymer is often difficult and complex [18]. Therefore, there have only been a small number of reports of the use of IR spectroscopy for sequence microstructure analysis for a limited number of copolymer systems [23-25]. 

Another technique which is extremely useful in deducing the microstructure of copolymers, cleavage by acid or fusion with solid alkahes followed by identification and determination of the cleavage products by GC (See Sections 3.7.1, 3.7.6 and 3.13.1). 

Copolymerization of acrylamide with MAA in the presence of poly(alkylammonium chloride) as a template was described by Liu and co-workers. Microstructure of copolymer obtained by template polymerization, containing also phenoxy acrylate, was examined by NMR spectroscopy. Copolymerization was carried out in aqueous solutions with different pH. Dependence of hydrogen bonds between acrylamide blocks and MAA blocks on pH value of the solution was discussed in detail. 

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