Copolymer composition and sequencing

For the analytical spectroscopist working in a quality assurance laboratory or a research technical-support group, the need to provide cost-effective, robust, reproducible and simply operated quantitative methods of compositional and microstructure analysis can be a prime task and important challenge. Vibrational spectroscopy techniques have proved themselves among the most powerful for the routine quantitative analysis of polymers, with the wide variety of sampling procedures usually offering a method (at least semi-quantitative) for even the most intractable of materials. 

The determination of the vinyl acetate (VA) content in ethylene-vinyl acetate (EVA) copolymers serves as a good illustration of the traditional key-band application of Beer s law. Commercial EVA products are available 

The IR determination of the content and sequence distribution of ethylene units in propylene/ethylene (P/E) copolymers has received considerable attention over many years [e.g. 89-97]. Many of these studies have concerned the rocking-mode absorption band for -(CH2)n-, the position of which depends on the value of n, i.e. the sequence length of methylene units. Absorption bands 

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. 

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The problem of predicting copolymer composition and sequence in the case of chain copolymerizations is determined by a set of differential equations that describe the rates at which both monomers, Ma and MB, enter the copolymer chain by attack of the growing active center. This requires a kinetic model of the copolymerization process. The simplest one is based on the assumption that the reactivity of a growing chain depends only on its active terminal unit. Therefore when the two monomers MA and MB are copolymerized, there are four possible propagation reactions (Table 2.17). 

Obviously, what we would really like to do is not just have a feel for tendencies, useful as this is, but also calculate copolymer composition and sequence distributions, things that can also be measured by spectroscopic methods. We will start by using kinetics to obtain an equation for the instantaneous copolymer composition (it changes as the copolymerization proceeds). Later we will use statistical methods to describe and calculate sequence distributions. In deriving the copolymer equation, we only have to consider the propagation step and apply our old friend, the steady-state assumption, to the radical species present in the polymerization, and... 

To predict the course of a copolymerization we need to be able to express the composition of a copolymer in terms of the concentrations of the monomers in the reaction mixture and some ready measure of the relative reactivities of these monomers. The utility of such a model can be tested by comparing experimental and estimated compositions of copolymers formed from given monomer concentrations. Asa general rule in science, the preferred model is the simplest one which fits the facts. For chain-growth copolymerizations, this turns out to be the simple copolymer model, which was the earliest useful theory in this connection [1,21. All other relations which have been proposed include more parameters than the simple copolymer model. We focus here on the simple copolymer theory because the basic concepts of copolymerization are most easily understood in this framework and because it is consistent with most copolymer composition and sequence distribution data. 

Some of the most significant applications of modem MS to synthetic polymers are (a) chemical stmcture and end-group analysis, (b) direct measurement of molar mass and molar mass distribution, (c) copolymer composition and sequence distribution, and (d) detection and identification of impurities and additives in polymeric materials. 

Two general methods have been developed for the determination of copolymer composition and sequence, namely a method which uses a combination of mass spectral intensities and the chain statistics approach. " In the approach based on a combination of MS intensities one computes the average copolymer composition (molar fraction of A units in the copolymer), Ca-... 

Theoretical spectra corresponding to a specific copolymer composition and sequence can be generated by assuming that peak intensities reflect relative oligomers abundances. 

NMR has received much attention as a method for copolymer analysis. Both copolymer composition and sequence can be determined. and C-NMR are both used, but often C-NMR is preferred, since the signals possess better resolution. 

As discussed in Chapter 2, mass spectral peak intensities can be used to determine copolymer composition and sequence and various methods have... 

While copolymer composition and sequence distribution are functions only of the reactivity ratios, the same is not true for polymerization rate. The overall rate of monomer consumption is given by ... 

As noted in Chapter 1, many synthetic copolymers have a distribution of mer sequences and mer compositions, as well as a distribution of molecular weight. The precise description of such a system requires many measures of the mer distribution. Detailed examples of copolymer composition and sequence will be considered throughout this text. 

Radical Addition to C=C Bonds. Radical addition to C=C bonds are of importance for free-radical polymerization as this reaction forms the propagation step, and thus influences the reaction rate and molecular weight distribution in both conventional and controlled free-radical polymerization, and the copolymer composition and sequence distribution in free-radical copolymerization. Numerous studies have examined the applicability of high level theoretical methods for stud5dng radical addition to C=C bonds in small radical systems (32,33,37,93,94). The most recent study (37) included W1 barriers and enthalpies, and geometries and frequencies at the CCSD(T)/6-31 lG(d,p) level of theory, and is the highest level study to date. The main conclusions from this study, and (where still relevant) the previous lower level studies, are outlined below. 

Fig. 9.12. Average copolymer composition and sequence length as functions of chain length for batch copolymerization. Kinetic data kd - 6.25 X 10 s ki - 3.75 x 10 m (kmol s) kpM — 1000 m (kmol s ) hpAB — 100 m (kmol s)...

IR spectroscopy is one of the most important methods for the determination of both copolymer composition and sequence distribution. The requirements to IR bands used for these purposes are practically opposite and are therefore discussed separately. 

Primary structure of homopolymers, elucidation of chain stereo-chemistry and conformation, elucidation of polymerization mechanisms, determination of rotational barriers, determination of copolymer composition and sequence. 

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