Syndiotactic stereosequences

Fig. 4.17 illustrates the potential of carbon-13 NMR to detect the presence of isotactic (a), syndiotactic (b), and atatactic (c) vinyl polymers with polypropylene as sample [521], The spectrum of atactic polypropylene (Fig. 4.17(c)) displays the signals of all possible stereosequences including iso- and syndiotactic ones. Using the empirical increment systems for alkane carbon shift prediction [85, 201, 202] and including y effects of Zy = — 5 ppm specifically obtained by analysis of stereoisomeric polypropylene partial sequences between 3,5-dimethylheptane and 3,5,7,9,11,13,15-heptamethylheptadecane as a heptad model, the methyl carbon-13 shifts of all 36 possible heptads can be calculated... 

Spin-lattice relaxation times of carbon-13 in different polypropylene stereosequences differ slightly while nuclear Overhauser enhancements are almost identical (1.8-2.0) [533] isotactic sequences display larger Tx values than the syndiotactic stereoisomers. Other vinyl polymers behave correspondingly [534]. Carbon-13 spin-lattice relaxation times further indicate that dynamic properties in solution depend on configurational sequences longer than pentads. The ratio 7J(CH) 7J(CH2) varies between 1.6 to 1.9 thus, relaxation can be influenced by anisotropic motions of chain segments or by unusual distributions of correlation times [181],... 

Although several techniques have been used to characterize stereosequence distribution, we suggest that the percent crystallinity and temperature of melting measurements are more generally applicable than any other technique presently available. Bovey and co-workers (7) showed how NMR measurements can be used to determine the triad distribution in polymers such as polymethyl methacrylate in which there is sufficient difference between the NMR spectra corresponding to syndiotactic, isotactic, and heterotactic triads to allow quantitative measurements to be made. This type of measurement unfortunately is restricted to few systems and would lead to a unique description of the stereostructure of the chain only when a model involving one or two probability parameters is applicable (See Appendix I). 

On the other hand, the temperature of melting and crystallinity measurement can describe only those stereosequences that crystallize. In propylene oxide polymers which we have studied, for example, isotactic sequences are the only ones that crystallize. Therefore, in these polymers we can determine only the stereosequence length of isotactic units and the average length of the uncrystallizable units. The syndiotactic and atactic sequences in the uncrystallizable blocks are not distinguishable from the crystallinity and melting temperature measurements. 

Ewen and Razavi [52] have shown that stereoselective Cs-symmetric metallocenes (18-20) with their enantiotopic vacancies form syndiotactic polymers. The rrrr stereosequences indicate enantiomorphic site control with chain migratory insertion errors arise from site isomerization without insertion and occasional reversal in diastereoface selectivity. 

A monosubstituted vinyl monomer yields polymers having three regioregular arrangements of configuration (Fig. 5), described by triad stereosequences. The isotactic structure has all R groups on the same side of the backbone (mm) the syndiotactic structure has R groups on alternate sides of the polymer s backbone (rr) and the heterotactic or atactic structure has R groups randomly oriented on either side of the polymer s backbone. 

H and 13C NMR relaxation studies in concentrated chloroform solution307 308 and 13C relaxation studies of cross-linked gels in benzene- 309 were carried out on PMMA. The dependence of the segmental mobility on stereosequence was not affected by the presence of covalent cross-links in the chain. The isotactic sequences are more mobile than the syndiotactic sequences.309... 

Polymerization of CFE in urea at -80"C has a mild stereoregulating influence compared to bulk polymerization at 60 °C, although the spectra in Figure 2 show that both polymers are substantially atactic. Three major triad resonances can be discerned more readily in spectrum (b), and these are assigned to mm, mr, and rr stereosequences as shown simply by analogy with the poly(l,2-difluoroethene) assignments (14,15). At present we have no basis to make definitive assignments, but those indicated are at least consistent with the known syndiotactic bias exerted by urea on the polymerization of complexed vinyl monomers (10). 

The major splitting of the isoregic resonance into the three peaks is due to triad stereosequences, as noted above. Assuming that polymerization at low temperature in urea favors syndiotactic propagation, we assign these peaks to isotactic, heterotactic, and syndiotactic sequences in order of increasing field as... 

The isotactic (mm), heterotactic (mr), and syndiotactic (rr) triad stereosequence probabilities for (1) commercial PVF, (2) PVF derived from PVCF, (3) PVF derived from PCFE, (4) urea PVF and (5) PVF epimerized at 50°C 08). Note that the observed heterotactic probability is mr + rm. 

In some cases, a site isomerization (i.e. a chain migration from a site to the other without monomer insertion) can occur. When this mechanism influences the stereosequence distribution (as is the case in syndiotactic polymerizations), the statistical model should contain a probability parameter to describe this event. 

On the other hand, stereocomplexes and solvatophobic bonds are specific to macromolecular multimerization. Certain polymers with mutually complementary stereostructures form what are called stereocomplexes whose stoichiometry is dependent on the stereosequence length. Examples of such stereocomplexing are provided by the pairs poly(7-benzyl-D-glutamate) with poly(7-benzyl-L-glutamate) and isotactic with syndiotactic poly(methyl methacrylates) (PMMA). The specific enthalpy of stereocomplex formation with it- and st-PMMA is, for example, a linear function of the syndiotactic diad mass fraction, with a maximum at Wst = 0.58 (Figure 6-11). If, however. 

Moreover, the hypothesis that only syndiotactic sequences in a fully planar conformation would crystallize is inconsistent with the distribution of stereosequences in PAN samples, which is essentially that expected in the case of a Bernoulli type statistics with equal m and r diads concentrations [184-186]. 

The 220-MHz H NMR spectra of three polypropylene (PP) samples are presented in Fig. 20.8 [5-7]. Note the apparent greater resolution of the spectra in (a) and (c) recorded for the stereoregular samples (isotactic and syndiotactic) than for atactic PP in (b). The impression of degraded resolution in the spectrum for atactic PP is a consequence of the overlapping of many slightly different resonance frequencies or chemical shifts corresponding to the various triad and tetrad stereosequences present in the atactic sample. Only the rr (rrr) and mm (mmm) triads (tetrads) are present in the stereoregular syndiotactic and isotactic samples, respectively. 

Taking the apparently straightforward example of sequential isotactic (... mmmmmm...) and syndiotactic (... rrrrrr...) blocks, in the ideal case (Figure 8.2a), any mixed stereosequences (e.g., mmmr, mmrr, mrrr) would be the sign of a. .. mmmmrrrr... juncture. Real polypropylenes. 

Stereosequences in PLAs are routinely determined by the examination of nuclear magnetic resonance (NMR) spectra, particularly with regard to the NMR methine resonance and the proton decoupled NMR methine resonance. Figure 25.4 shows the possible PLA tetrad stereosequences (/ = isotactic dyad = syndiotactic dyad). The original ( C and H) NMR peak assignments for tactic PLAs were put forward at the tetrad level of sensitivity by Kricheldorf et al. based... 

FIGURE 25.4 The possible stereosequences in PLA at the tetrad level (i = isotactic dyad = syndiotactic dyad). 

In the NMR spectra of PLA, the observed resonances can be assigned to stereosequence distribution in the polymer and reflect its history including the stereochemistry of the feed composition, polymerization kinetics, and extent of transesterification and racemization [57-60]. The assignments are designated as various combinations of z isotactic pairwise relationship (RR and SS) and s syndiotactic pairwise relationship (RS and SR). In the NMR spectra, the combinations RR and SS are indistinguishable and have identical chemical shifts, as would RS and SR [57, 61]. 

The NMR spectrum exhibits resonances of particular polymers that possess stereosequence sensitivity. In the case of PLA, NMR spectra can distinguish the diads —LD— (or —DL—) and —LL— (or —DD—). But, the similar diads, —DD— and —LL—, or —LD— and —DL—, do not show different chemical shifts. In the stereosequence of PLA, the —DD— and —LL— produce an isotactic pairwise relationship, while —LD— and —DL— have the structure in a syndiotactic pairwise relationship. The observations from NMR have shown difficulties, such as overlaying of chemical shifts, insufficient resolution and probability of stereosequence formation due to polymer chains remaining in a huge macromolecule. For instance, for the stereosequence sensitivity of length n, there are possible combinations of pairwise... 

Figure 4.7 Comparison of the (a) Kricheldorf et al. (1996) and (b) Zell et al. (2002) tetrad stereosequence assignments of the methine carbon and proton in PLA synthesized using meso-lactide. The lines between peaks in the and NMR spectra indicate connectivity observed in the heteronuclear correlation NMR spectra (/ designates isotactic, s designates syndiotactic).

Table 4 Presentation of the relevant C NMR normalized spectroscopic stereosequence distributions (%) for syndiotactic polypropylene samples produced with 5/MAO at different polymerization temperatures...

Table 12 Presentation of syndiotactic polypropylene temperatures NMR spectroscopic samples produced with stereosequence distributirais (%) fra-5/MAO at different polymerizatirai ...

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