NMR tacticity

A general methodology has been developed for the treatment of NMR data of polymer mixtures. The methodology is based on reaction probability models and computer optimization methods, resulting in a family of computer programs called MIXCO. The use of MIXCO programs enabled three components to be resolved from NMR tacticity data of fractionated polybutylene. 

The use of the HIXCO methodology for the NMR tacticity data of whole (unfractionated) polymers is straightforward.(16)... 

The use of statistical models to interpret (and to rationalize) NMR tacticity and sequence data is well established (97,98). In this volume the enantiomorphic-site model has been used by Segre et al. in their studies of polypropylene at high fields (55). A two-site model has been en loyed by Shimozawa et al. to observe the effects of internal donors m propylene polymerization (56). Other models for polyolefins have been reported in the literature, e.g., the multi-site model (99), the dual catalytic-site/chain-end model (100), the perturbed model (101), the consecutive two-site model (102), the four-component model (103), and the chain end model (104). 

Ke5Words cluster index, comonomer distribution, monomer dispersity, NMR, H NMR, NMR, tacticity, sequence length, copol3uner. 

Keywords NMR-high resolution, NMR-solid state, C NMR, tacticity, regio-irregularity, spin-lattice relaxation, necessary relationship, homopolymer, chain mobility, chain packing, molecular conformation. 

The stereoregularity and regioregularity of propylene sequences in EP copolymers can be analytically evaluated by NMR. Tacticity is obtained as the percentage of mm triads from the signals of the central tertiary CH carbon in the propylene triad (PPP) sequences (Tpp), using Equation 12.1... 

Four possible calibration curves of IR absorption ratio versus NMR tacticity were investigated for PP films hot pressed at 120-200 "C absorption at 10.02 pm referenced to bands 10.28 pm or 8.57 pm, and the absorption at 11.89 pm with the same reference bands i.e., ratios A10.02/A10.28, A10.02/A8.57, All.89/Al0.28 and A11.89/A8.57, respectively. Burfield and Lo [21] use the 10.28 pm band as reference to provide linear calibration curves for both ratios. Calibrations based on the 8.57 pm reference are curves that provide rather low sensitivity in the important high isotacticity (mm >0.8) region. Burfield and Lo [21] consider the 10.28 pm band as the most appropriate reference and consequently data for this band are described, i.e., ratios A10.02/A10.28 (Figure 6.5) and A11.89/A10.28 (Figure 6.6). A further advantage of calibrations involving the 10.28 pm reference is the possibility of representation as the simple linear equation ... 

Figure 10. NMR tacticity index as a function of polymerizotion temperoture bulk homopolymerization process - cocotalyst mDEAC...

Figure 12. Relation between the reference G5 stiffness modulus and the NMR tacticity index - bulk homopolymerization process - cocatalyst mOEAC...

The tacticity of PP has been measured by FTIR using the peak at 1000 cm which is sensitive to isotactic helices and ratioing to the internal standard peak 1170 cm. A linear correlation was made with NMR tacticity measurements [21]. 

For both copolymers and stereoregular polymers, experimental methods for characterizing the products often involve spectroscopy. We shall see that nuclear magnetic resonance (NMR) spectra are particularly well suited for the study of tacticity. This method is also used for the analysis of copolymers. 

Table 2.14. Suggested tactics for solving structures using NMR...

Millan (98) studied the effect of tacticity on the ionic dehydrochlorination and chlorination of PVC. For the dehydrochlorination reaction, both the reaction rate and the polyence sequence distribution depend markedly on the syndiotactic content. Chlorination appeared to be easier through heterotactic parts than through syndiotactic sequences as shown by C-NMR. 

The application of NMR spectroscopy to tacticity determination of synthetic polymers was pioneered by Bovey and Tiers.9 NMR spectroscopy is the most used method and often the only technique available for directly assessing tacticity of polymer chains. "2 7 8 0JI The chemical shift of a given nucleus in or attached to the chain may be sensitive to the configuration of centers three or more monomer units removed. Other forms of spectroscopy (e.g. TR spectroscopy l2 lJ) are useful with some polymers and various physical properties (e.g. the Kerr effect14) may also be correlated with tacticity. 

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 mechanism of B polymerization is summarized in Scheme 4,9. 1,2-, and cis- and trews-1,4-butadiene units may be discriminated by IR, Raman, or H or nC MMR speclroseopy.1 92 94 PB comprises predominantly 1,4-rra//.v-units. A typical composition formed by radical polymerization is 57.3 23.7 19.0 for trans-1,4- c7a -1,4- 1,2-. While the ratio of 1,2- to 1,4-units shows only a small temperature dependence, the effect on the cis-trans ratio appears substantial. Sato et al9J have determined dyad sequences by solution, 3C NMR and found that the distribution of isomeric structures and tacticity is adequately described by Bernoullian statistics. Kawahara et al.94 determined the microslructure (ratio // measurements directly on PB latexes and obtained similar data to that obtained by solution I3C NMR. They94 also characterized crosslinked PB. 

The program will be demonstrated with poly(vinyl alcohol) for tacticity analysis and with copolymer vinylidene chloride isobutylene for monomer sequence analysis. Peak assignments in C-13 spectra were obtained independently by two-dimensional NMR techniques. In some cases, assignments have been extended to longer sequences and confirmed via simulation of the experimental data. Experimental and "best-fit" simulated spectra will be compared. 

An excellent way to treat such data is to use reaction probability models.(1,2) In the NMR analysis of tacticity, it is frequently possible to distinguish whether the configuration is chain-end controlled or catalytic-site controlled during polymerization. Various statistical models have been proposed. The chain-end controlled models include Bemoullian (B), and first- and second-order Markovian (Ml and M2) statistics.(1) The simplest catalytic-site controlled model is the enantiomorphic site (E) model.(3) The relationship between the chain-end and catalytic-site controlled models and possible hybrid models have been delineated in a recent article.(4)... 

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). 

In this case, the intensities corresponding to the various tacticity sequences are directly analyzed. The information content, however, is somewhat lower than in the case where the NMR data of pairs of polymer fractions are analyzed simultaneously. Nevertheless, useful information is available. 

Of course, water molecules are not NMR silent and NMRI engineering has, indeed, advanced at a remarkable pace to provide extraordinary technical capabilities. These capabilities now enable studies of systems beyond those in the biomedical arena, systems that are, in many respects, far more technically challenging. This has led to the development of innovative and fascinating strategies and tactics to deal with NMRI-unfriendly samples and conditions. 

The results for arylsilanes are not fully understood, but the spectra may reflect partial tacticity in these polymers. Further work is needed studies of model compounds with known relative configuration would be particularly helpful. Silicon-29 NMR of polysilane copolymers also shows great promise, especially for distinguishing block-like from fully random copolymers. 

The H-NMR spectrum of 2 in CDCI3 (Figure 1) exhibits broad unresolved resonances in the aromatic region similar to those found in the monomer. Broad signals with lack of resolution are consistent with magnetic non-equivalence of the methyl group protons resulting from a mixture of triad tacticities. 

---