Copolymers monomer sequence distribution

Anionic polymerizations initiated with alkyllithium compounds enable us to prepare homopolymers as well as copolymers from diene and vinylaromatic monomers. These polymerization systems are unique in that they have precise control over such polymer properties as composition, microstructure, molecular weight, molecular weight distribution, choice of functional end groups and even copolymer monomer sequence distribution. Attempts have been made in this paper to survey these salient features with respect to their chemistry and commercial applications. 

C-13 NMR of copolymers provides information regarding composition of a copolymer, monomer sequence distribution and end chain propagation mechanism (22-24). In the present studies, TCA-ST copolymers were studied at different mole compositions in order to see the effect of mole composition on the splitting pattern. An alkyl ester group of a copolymer displayed certain influence on the chemical shift of the carbonyl and quaternary carbon resonances. Quarternary carbons in ST-ST-ST triad always resonate at higher field than that of TC-TC-TC triad. 

Although reactivity ratios indicate that VP is the more reactive monomer, reaction conditions such as solvent polarity, initiator type, percent conversion, and molecular weight of the growing radical can alter these ratios (138). Therefore, depending on polymerization conditions, copolymers produced by one manufacturer may not be identical to those of another, especially if the end use appHcation of the resin is sensitive to monomer sequence distribution and MWD. 

Any understanding of the kinetics of copolymerization and the structure of copolymers requires a knowledge of the dependence of the initiation, propagation and termination reactions on the chain composition, the nature of the monomers and radicals, and the polymerization medium. This section is principally concerned with propagation and the effects of monomer reactivity on composition and monomer sequence distribution. The influence of solvent and complcxing agents on copolymerization is dealt with in more detail in Section 8.3.1. 

Tire simplest model for describing binary copolyinerization of two monomers, Ma and Mr, is the terminal model. The model has been applied to a vast number of systems and, in most cases, appears to give an adequate description of the overall copolymer composition at least for low conversions. The limitations of the terminal model generally only become obvious when attempting to describe the monomer sequence distribution or the polymerization kinetics. Even though the terminal model does not always provide an accurate description of the copolymerization process, it remains useful for making qualitative predictions, as a starting point for parameter estimation and it is simple to apply. 

In traditional treatments of copolymerizaiion kinetics, the values of the ratios sA and % are implicitly set equal to unity (Section 7.3.1.2.2). Since they contain no terms from cross propagation, these parameters have no direct influence on either the overall copolymer composition or the monomer sequence distribution they only influence the rate of polymerization. 

Methods for evaluation of reactivity ratios comprise a significant proportion of the literature on copolymerization. There are two basic types of information that can be analyzed to yield reactivity ratios. These are (a) copolymer composition/convcrsion data (Section 7.3.3.1) and (b) the monomer sequence distribution (Section 7.3.3.2). The methods used to analyze these data are summarized in the following sections. 

NMR spectroscopy has made possible the characterization of copolymers in terms of their monomer sequence distribution. The area has been reviewed by Randall,100 Bovey,139 Tonelli,101 Hatada140 and others. Information on monomer sequence distribution is substantially more powerful than simple composition data with respect to model discrimination,25,49 Although many authors have used the distribution of triad fractions to confirm the adequacy or otherwise of various models, only a few25 58,141 have used dyad or triad fractions to calculate reactivity ratios directly. 

The solvent in a bulk copolymerization comprises the monomers. The nature of the solvent will necessarily change with conversion from monomers to a mixture of monomers and polymers, and, in most cases, the ratio of monomers in the feed will also vary with conversion. For S-AN copolymerization, since the reactivity ratios are different in toluene and in acetonitrile, we should anticipate that the reactivity ratios are different in bulk copolymerizations when the monomer mix is either mostly AN or mostly S. This calls into question the usual method of measuring reactivity ratios by examining the copolymer composition for various monomer feed compositions at very low monomer conversion. We can note that reactivity ratios can be estimated for a single monomer feed composition by analyzing the monomer sequence distribution. Analysis of the dependence of reactivity ratios determined in this manner of monomer feed ratio should therefore provide evidence for solvent effects. These considerations should not be ignored in solution polymerization either. 

Harwood112 proposed that the solvent need not directly affect monomer reactivity, rather it may influence the way the polymer chain is solvated. Evidence for the proposal was the finding for certain copolymerizations, while the terminal model reactivity ratios appear solvent dependent, copolymers of the same overall composition had the same monomer sequence distribution. This was explained in... 

There are also expressions that describe the terminal model in terms of monomer sequence distributions in the copolymer [Burke et al., 1994a,b, 1995 Cheng, 1995, 2000], The Mj centered triad fractions are given by... 

The implicit penultimate model was proposed for copolymerizations where the terminal model described the copolymer composition and monomer sequence distribution, but not the propagation rate and rate constant. There is no penultimate effect on the monomer reactivity ratios, which corresponds to... 

The phenomenal growth in commercial production of polymers by anionic polymerization can be attributed to the unprecedented control the process provides over the polymer properties. This control is most extensive in organolithium initiated polymerizations and includes polymer composition, microstructure, molecular weight, molecular weight distribution, choice of functional end groups and even monomer sequence distribution in copolymers. Furthermore, a judicious choice of process conditions affords termination and transfer free polymerization which leads to very efficient methods of block polymer synthesis. 

Monomer Sequence Distribution in the Copolymer by C NMR. The latex samples which were precipitated in the mixed solvent (ispropanol/hexane = 45/55 and dried in a vacuum oven at 45-50°C, were used for the C NMR study. The polymer samples were dissolved (5-7% weight volume) in deuterated pyridine (PYR d%) with tetramethylsilane (TMS) as a zero external reference. The NMR measurement and the theoretical calculation were carried out in CNRS, Laboratoire des Matriaux Organiques, in Lyon, France using C NMR 30 MH (Bruker). 

The data shown in Tables HI and TV show that the 13C nmr spectra of vinyl chloride-vinylidene chloride copolymers have a redundancy of structural relationships. By analyzing a range of compositions, this system has been found to yield a reasonable description of both monomer composition and monomer sequence distribution. The data also show that this copolymer is a good example of a system best described by first order Markovian statistics as compared to Bernoullian statistics. 

The most important structural features of amorphous SAN copolymers are the weight fraction (h an) of acrylonitrile and the molecular weight distribution (MWD). These features control the solid-state properties and fabrication performance. Also important are the type and level of conjugated chromopores and the monomer sequence distribution. These features control the visual appearance of the SAN copolymer. The chromophores may introduce unwanted yellowness. A nonuniform sequence distribution may cause unwanted haze from phase separation. 

Sargent M, Koenig JL and Maeker NL (1991) FT-IR analysis of the monomer sequence distribution of styrene-acrylonitrile copolymers. Appl Spectrosc 45 1726-32. 

The microstructure of acrylamide-sodium acrylate copolymers was determined by NMR (36). The monomer sequence distribution was found to conform to Bernouillian statistics and the reactivity ratios of both monomers were close to unity. These results which differ from those obtained for copolymers prepared in solution or emulsion (37) confirmed a polymerization process by nucleation and interparticular collisions. 

The properties of a copolymer depend on its composition, monomer sequence and stereochemical structure. Although compositional analysis can be achieved by several methods other than NMR spectroscopy, quantitative data on monomer sequence distribution can only be obtained from NMR spectroscopy. I3C NMR chemical shifts of C=0 carbons of PMMA are sensitive to pentad to heptad stereochemical sequences. The C=0 carbon signals for the copolymers of methacrylates are also sensitive to triad comonomer sequence. Thus it should be difficult to assign both tactic and comonomer sequence signals, especially in the case of copolymers with low stereoregularity. 

NOESY has also been used to elucidate the chain conformation of poly(styrene-a/ -MMA).220,221 2D INADEQUATE has been applied to studies of monomer sequence distribution in ethylene-propylene copolymer.223 Additivity rules for the 13C chemical shifts of ethylene-propylene copolymer were devised for configurational sequences as well as substituent effects.226... 

Ellerbe.J.S., Cox,R.C., Lane,L.H. Monomer sequence distribution in ethylene-propylene copolymers by computer analysis of infrared spectra. Anal. Chem. 40, 370-379 (1968). 

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