Tacticity determination

The nature of the alkyl group from the esterifying alcohol, the molecular weight, and the tacticity determine the physical and chemical properties of methacrylate ester polymers. 

The classical representation of a homopolymer chain, in which the end groups are disregarded and only one monomer residue is considered, allows no possibility for structural variation. However, possibilities for stercoscqucnce isomerism arise as soon as the monomer residue is considered in relation to its neighbors and the substituents X and Y are different. The chains have tacticity (Section 4,2.1). Experimental methods for tacticity determination are summarized in 4.2.2 and the tacticity of some common polymers is considered in 4.2.3. 

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. 

If a solvent effect is present, one would expect an influence of the initial solvent/monomer ratio although weak. This was indeed found by Goeldi and Elias (8). The effect of monomer conversion is apparently too small to be detected by the present accuracy of tacticity determinations (8, 23). 

Tacticity determinations by the first method have been carried out using enantiomers of the following derivatives of norbom-2-ene 1-methyl-302,308, exo-5-methyl-309,310,... 

Elastomeric poly acetaldehyde is amorphous and atactic. High resolution NMR studies were carried out in order to determine the actual tacticity of the polymer (4, 5, 23). Poly acetaldehyde consists mainly of heterotactic sequences isotactic sequences are also present in the polymer. The ratio of heterotactic to isotactic varies, but it is usually between 2-3 1. A detailed description of the tacticity determination by high resolution NMR will be given in a subsequent paper by Goodman and Brandrup (23). 

Like many scientific fields, polymer chemistry has benefited from the introduction of NMR spectroscopy, and it is now widely used in areas such as tacticity determination, analysis of end-groups and irregular linkages, comonomer sequence determination in copolymers, and chain dynamics studies. This information is of primary importance for polymer characterization, but it also provides profound and stimulating knowledge about the reaction mechanism by which the polymer of interest is formed. 

Tacticity determination of vinyl polymers by NMR has been one of our main concerns in connection with stereoregular polymerization. Major developments in the stereochemical assignment of NMR spectra of vinyl polymers and... 

We have been concerned with the precision and accuracy of NMR data of polymers since we first started NMR studies on polymers.1-4 Using continuous-wave (CW) spectrometers, the effects of measurement conditions including temperature, sample concentration and radiofrequency (rf) field strength, were examined using several polymer and copolymer samples. Since our research group have been deeply involved in stereospecific polymerization of methacrylates, one of the main concerns about NMR measurement was the precision of tacticity determination by NMR. The errors in determining the tacticity of poly(methyl methacrylate) (PMMA) and those in the results of polymerization of methyl methacrylate (MMA) by radical and anionic initiators were examined and found to be satisfactorily small.4 Although there... 

Reliability of tacticity determination and reproducibility of the polymerization of methyl methacrylate... 

Table 11. Effect of pulse repetition time on relative peak intensity and tacticity determination for radically prepared PMMA by 500-MHz 1H NMR. ...

Non-Bernoulian behaviour of tacticity was also reported for poly(vinyl chloride) obtained at 5°C and — 30°C.164 After careful assessment of the precision and accuracy in tacticity determination, triad and tetrad fractions were found to be well-explained by the first-order Markovian statistics. 

Tacticity determination of vinyl polymer by 1H NMR spectroscopy was first achieved for PMMA by Nishioka169 and Bovey.170 13C NMR chemical shifts of carbons in PMMA show higher sensitivities to longer stereosequences.163 171 173 The carbonyl carbons in many polymethacrylates exhibit a splitting pattern similar to that in PMMA. However, a recent report on 13C NMR spectra of polymers of various chlorophenyl methacrylates indicates that the number and positions of the chloro substituent strongly affect the spectral patterns, and the carbonyl carbon signal of poly(2,4,6-trichlorophenyl methacrylate) does not show a splitting due to tacticity.174... 

Derivation of polymer also serves as a useful method for tacticity determination. Poly(triphenylmethyl methacrylate) is easily converted to PMMA by hydrolysis and the subsequent methylation with diazomethene. The polymers obtained by anionic polymerization not only in toluene but also in tetrahydrofuran are highly isotactic.189 Even the radical polymerization of the monomer gives an isotactic polymer. 

Analysis and Characterization. Gas chromatography was performed with a Hewlett-Packard model 5880A equipped with capillary columns and a flame-ionization detector. Gel permeation chromatography was performed with a Hewlett-Packard HPLG model 1090A with a refractive index detector. Tacticity determinations were conducted with an F-19 nuclear magnetic resonance (NMR) spectrophotometer (Varian XL-100) at 94.12 MHz for triad analyses (13). Inherent viscosity measurements were performed in 2-butanone at 25.0 °C. 

Tacticity determinations by the first method have been carried out using enantiomers of the following derivatives of norbom-2-ene 1-methyl- (Hamilton 1983, 1984a), exo-5-methyl- (Ivin 1979b, 1980b), 5,5-dimethyl- (Ho, H.T. 1982a,b), endo,exo-5,6-dimethyl- (O Dell 1994 Sunaga 1994), 1,7,7-trimethyl-... 

The methyl ester protons give rise to a single signal, which is independent of tacticity (not shown in Figure 3-9). Therefore, the methyl ester proton signal cannot be used for tacticity determinations. The chemical shift is not affected by the configuration of the main chain since they are too far removed from an asymmetric center. 

In all of these systems, propagation involves radical species, and therefore chemoselectivities (reactivity ratios and transfer coefficients), regioselectivities (head-to-head units), and stereoselectivities (tacticity) determined for CRPs are similar to those in conventional RP. 

Based on this finding, they proposed a method for determining the tacticity of amorphous samples of PVC. Because some samples cannot be easily transformed into an amorphous state, Schneider and co-workers [68] devised an IR method of tacticity determination which is independent of sample crystallinity. From the temperature dependence of IR spectra of PVC samples prepared by different methods, the intensity of the band at 14.40 pm (proportional to the number of isotactic diads in the sample), as well as that of the tacticity-independent C-H stretching band, was found to be independent of sample crystallinity. These lines were applied to the tacticity determination in PVC, measnred in potassium bromide pellets. The numerical tacticity value was obtained from the known values of absorbance coefficients of S h and S h type C-Cl stretching bands in solution, and from the shape of the spectrum. 

The diad tacticity determined from the relative peak intensities of the diastereomeric tetramers in the pyrograms was consistent with that obtained by proton NMR, suggesting that no appreciable thermal isomerisation occurred during pyrolysis. The thermal degradation mechanisms to yield the diastereomeric tetramers from PMMA without isomerisation open up the possibility of estimating the triad tacticityol PMMA from the distribution of diastereoisomeric pentamers. 

The above method of tacticity determination depends upon there being no regioselectivity, i.e. no bias towards HT or HH/TT addition in the polymerisation of the racemate, and in fact this is the case with S-substituted norbomene derivatives. 

Figure 1 presents the dyad tacticity determined from 300 MH NMR (5) versus the Tg measured by DSC for the poly(alkyl a chforoacrylates) studied. All values are corrected to "infinite" molecular weight as follows. The molecular weight dependence of Tg for a linear polymer is given by the Flory-Fox relationship... 

The better definition of spectra from crystalline polymers can give an indication of polymer tacticity since atactic polymers are generally non-crystalline and there are certain polymers such as poly(methyl methacrylate) and polypropylene in which specific absorption bands can be assigned to the presence of molecules with particular types of tacticity. Some idea of the tacticity for samples of these polymers can, therefore, be obtained from measurements of the strength of the absorption of the relevant bands, but in general this method is not as accurate as n.m.r. for tacticity determination. 

To develop better stereocontrol agents, it is crucial to understand stereoselection at a fundamental mechanistic level and appreciate the subtle but crucial differences between tacticity determination in ionic and coordination versus radical processes. As previously outlined, the propagation step of Hving radical polymerization is identical to that of free-radical polymerization and so the mechanism of tacticity determination in these processes is also identical. In radical polymerization, the planar (sp hybridized) carbon radical of the polymer terminus is prochiral, and its stereochemistry is determined during subsequent monomer addition. The addition of monomer occurs exclusively to the anti-h.ce of the radical, as the syn-face is shielded by the steric bulk of the penultimate group (Scheme 2). Hence, the prochiral polymer terminus can adopt either pro-meso or pro-racemo arrangements defined by the relative orientation of the terminal and penultimate side-chains with respect to the macromolecular C—C backbone. 

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