TRIAD NMR

You can now test the models. So far we have only talked about Bemoullian statistics, where the stereochemistry of the addition of a monomer does not depend upon the configuration at the end of the chain. Just to make sure you understand, from NMR triad information—i.e., (mm), (mr) and (rr) data—we can readily calculate the conditional probabilities P(r/m), P(m/m), P(m/r) and P(r/r) as shown in Equations 7-32. Bemouillian statistics are followed if Equations 7-33 are obeyed. 

At present, no evidence exists for long runs of either meso or racemic placements in the Wurtz dechlorination reaction used in the synthesis of polysilylenes. Deviation from a random stereochemical polymerization has been suggested by West et al. (3) in the case of poly(phenylmethylsilylene) (PPMS). An analysis of Si NMR triads indicates that addition of monomer units with the same stereochemistry is preferred, which results in an increase in the intensity of syndiotactic and isotactic triad structures compared with a random stereochemical polymerization. 

Figure 6.5 Calibration curve for IR absorbance ratio A998/A973 versus NMR triad isotacticity. ( ) = high temperature annealed, (o) = hot pressed only, (A) = ambient temperature annealed. Reproduced with permission from D.R. Burfield and RS.T. hoi. Journal of Applied Polymer Science, 1988, 36, 2, 279.

In a direct comparison of results obtained by this method with those obtained with Luongo s [19] widely used calibration, it is apparent that the agreement is rather poor if the percent atactic values are compared with NMR triad readings. If Luongo s atactic and isotactic samples are assigned values of mm = 0.35 and mm = 1.00, respectively, then a good agreement is obtained ... 

Table 3. Hill et aL [65] NMR triad distribution for SAN copolymerization in toluene at...

The copolymer of styrene and butyl acrylate was polymerized in the batch mode and the NMR triad data obtained [27], The analysis shown in Tables 7.16 and 7.17 clearly indicates the existence of compositional heterogeneity for these copolymers. 

Analysis of conversion to 63% of styrene (S)/butyl acrylate (A) for the NMR triad data [25]... 

The lower standard deviations for the calculated NMR triad intensities are indicative of the presence of compositional heterogeneity. 

Kuroda and co-workers [34] fractionated ethylene-1-butene copolymers by successively extracting it with diisopropyl ether at 20 "C (Fraction 1), w-hexane at 20 "C [Fraction 2), n-hexane at its boiling point (Fraction 3), and cyclohexane at its boiling point (Fraction 4). The residual polymer (cyclohexane insolubles) is designated Fraction 5. The whole (unfractionated) polymer is called W. The C-NMR triad distribution data reported by Kuroda and co-workers are summarised in Table 7.2. 

Figure 8.5 Calibration curve for IR absorbance ratio Ajjg/Ajyj versus NMR triad isotacticity.

To investigate the triads by NMR, the resonances associated with the chain substituent are examined, since structures [XV] -[XVII] show that it is these that experience different environments in the various triads. If dyad information is sufficient, the resonances of the methylenes in the chain backbone are measured. Structures [XIII] and [XIV] show that these serve as probes of the environment in dyads. 

The nmr spectmm of PVAc iu carbon tetrachloride solution at 110°C shows absorptions at 4.86 5 (pentad) of the methine proton 1.78 5 (triad) of the methylene group and 1.98 5, 1.96 5, and 1.94 5, which are the resonances of the acetate methyls iu isotactic, heterotactic, and syndiotactic triads, respectively. Poly(vinyl acetate) produced by normal free-radical polymerization is completely atactic and noncrystalline. The nmr spectra of ethylene vinyl acetate copolymers have also been obtained (33). The ir spectra of the copolymers of vinyl acetate differ from that of the homopolymer depending on the identity of the comonomers and their proportion. 

Tacticity is most often determined by NMR analysis and usually by looking at the signals associated with the -CXY- group (refer Figure 4.3). The analysis then provides the triad concentrations (mm, mr and rr) and the value of m or P(m) is given by cq. 10. 

The arrangement of monomer units in copolymer chains is determined by the monomer reactivity ratios which can be influenced by the reaction medium and various additives. The average sequence distribution to the triad level can often be measured by NMR (Section 7.3.3.2) and in special cases by other techniques.100 101 Longer sequences are usually difficult to determine experimentally, however, by assuming a model (terminal, penultimate, etc.) they can be predicted.7 102 Where sequence distributions can be accurately determined Lhey provide, in principle, a powerful method for determining monomer reactivity ratios. 

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. 

NMR/Fractionation. The combination of NHR and fractionation is an excellent approach to study polymer mixtures.(17) If the components of the mixture are completely fractionated, then IIMR characterization is straightforward. If the components of the mixture are similar in structure such that separation is partial, then the computerized approach described above can be used to study the NMR/fractionation data and to deconvolute the consonants in each fraction. Thus far, three computer programs have been written to study MMR/fractionation data, two for polymer tacticity (MIXCO.CSX, MIXCO.C4X), and one for copolymer triad sequences (MIXCO.TRIADX). 

The data analyzed in this work were reported by Hill et al. ( ) for the copolymerization of styrene with acrylonitrile. They are shown in Table III in the form of triad fractions measured by C-NMR for copolymers produced at various feed compositions. One reason for choosing this particular dataset is that the authors did indicate the error structure of their measurement. 

The characteristics of C NMR spectra for all copolymers were similar. The triad distributions for all copolymo" from NMR monomer insertion are shown in Table 2. Based on the triad distribution of ethylene/l-hex aae copolymers in Table 2, we found that microstructurc of copolymer obtainrai fiom aluminoxane system was slightly different in monomer incorporation, but found significantly when borated system was applied. We suspected that this difference was arising fiom the diffaences in bimetallic complex active species between [aluminoxane] [catalyst] and [Borate] [catalyst] which had the electronic and gMmetric effects fiom the sterric effect of larger molecule of borate compare to the aluminoxane on the behaviors of comonomer insertion in our systems. 

The nucleophilic substitution on poly(vinyl chloroformate) with phenol under phase transfer catalysis conditions has been studied. The 13c-NMR spectra of partly modified polymers have been examined in detail in the region of the tertiary carbon atoms of the main chain. The results have shown that the substitution reaction proceeds without degradation of the polymer and selectively with the chloroformate functions belonging to the different triads, isotactic sequences being the most reactive ones. 

The series of E-V copolymers obtained by partial reduction of PVC with (n-Bu)3SnH were found (2) to have the same chain length as the starting PVC ( 1000 repeat units). Their microstructures were determined by 13C NMR analysis (2) as indicated in Figure 1. The results of this analysis are presented in Table I in terms of comonomer diad and triad probabilities. 

We have chosen the PVC diad and triad compounds 2,4-dichloropentane (DCP) and 2,4,6-trichloroheptane(TCH) as subjects for our attempt to obtain quantitative kinetic data characterizing their (n-Bu)3SnH reduction in the hope that they will serve as useful models tor the reduction of PVC to E-V copolymers. Unlike the polymers (PVC and E-V), DCP and TCH are low molecular weight liquids whose high resolution 13C NMR spectra can be recorded from their concentrated solutions in a matter of minutes. Thus, it is possible to monitor their (n-Bu)3SnH reduction directly in the NMR tube and follow the kinetics of their dechlorination. 

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