Rm triads

When it is necessary to specify the internal stereochemistry of the group, a prefix is required. In vinyl polymers there are meso (m) and racemic (r) diads and mm, mr, rr triads. The latter may be called isotactic, heterotactic and syndiotactic triads, respectively. Stereoregular vinyl polymers can be defined in terms of the regular sequences of diads thus an isotactic vinyl polymer consists entirely of m diads, i.e., it corresponds to the following succession of relative configuration -mmmmmm-, whereas a syndiotactic vinyl polymer consists entirely of r diads, corresponding to the sequence -rrrrrrr-. Similarly, a vinyl polymer consisting entirely of mr (= rm) triads is called a heterotactic polymer. 

On the contrary, the Incorporation below the plane starting from a "RR dlad Is difficult whereas the Incorporation Into a SS" dlad has a high probability (Figure 20). Therefore, whatever the configuration of the terminal dlad, r or m, steric hindrance occurs and favors the formation of a new meso dlad. Hence, we obtain predominantly mm triads from this terminal dlad m and rm triads from the terminal dlad r. 

If there is no regular arrangement of vertical lines in the adapted Fischer projection or alkyl substituents in the planar zigzag projection (Figure 3.2), then an atactic structure occurs. Atactic poly(a-olefin)s are characterized by the appearance in their chains of m diads and r diads in equal amounts. These diads constitute heterotactic mr triads and rm triads as well as isotactic mm triads and syndiotactic rr triads which also appear in equal amounts. 

It doesn t matter if you read the chain from left to right or right to left, the result is the same. The sequences shown on the bottom, however, read mr in one direction and rm in the other. But NMR doesn t read a chain in a particular fashion, to the spectrometer an rm triad is the same as an mr triad. 

NMR assignments for trimer 6 and trimer 7 are shown in Fig. 4.19. Thus the trimers are enchained in a 2,3-fashion just as for the dimers and only differ with regard to the enchainment of tlie third (or last) norbornene unit adjacent to the vinyl terminus. Trimer 6 consists of a rm triad of norbornene units, while trimer 7 is an mm triad. Presumably, the same is true for the two trimer peaks observed with m/z 338 (ethyl head group and a vinyl tail ) one consists of the rm triad of norbornenes and the other the mm. 

Every a-C atom at the centre of an rr triad arises as the result of at least two sequential r placements and every two sequential r placements produce one a-C atom. The probability Pjj that any a-C atom lies at the centre of an rr triad is therefore Pj. Similarly, the probability for an mm triad is (1 — Pj) and the probability Pn for an mr triad appears to be Pj(l — Pj). However, mr and rm triads give rise to the same frequency and are really the same, so that P j = 2Pj(l — Pj). The ratios of the intensities are in the ratios of the probabilities (which, of course, add up to 1) and are thus given by... 

Naturally, this catalyst is not perfect and stereoerrors are invariably present in the formed polymer. The principal stereoerrors arise from enantiofacial misinsertion at the more stereoselective site and site epimerization. Such mistakes give rise to mr and rm triads and m and r dyads in an otherwise consistent procession of mm and rr triads (Figure 2.17). Thus, perfect hemiisotactic polypropylene... 

SCHEME 4.3 Site epunerization in isotactic propylene polymerization generates single m errors (i.e., rm triads). 

Ordinarily, it is possible to distinguish between mm/rr and mr/rm triad stereosequences using standard NMR experiments, but distinction between the resonances of mm and rr triads can be made only if a spectrum from a stereoregular polymer of known relative configuration is available. Triple resonance 3D-NMR techniques combined with isotopic labelling have provided powerful tools for biomolecular structure determination which have tremendous potential applications in polymer chemistry [63-66]. 

The polymers generated by Ni(acac)2/MAO and other naked nickel-type catalysts are soluble in toluene and differ significantly from those derived from metallocene catalysts that are insoluble. Chemical shifts for the bridge carbon (C7) in the soluble polynorbornene were found between 33.8 and 34.3 ppm.Based on the assignment made by Al-Samak et for norbornene hydrotrimers (Figure 12), the chemical shift of 33.8 ppm belongs to mm and that of 34.2 ppm to mr and rm triads and an atactic polymer. Metallocene-based polynorbornenes show more mm triads and are more or less isotactic. Polynorbornenes synthesized by neutral nickel catalysts show in the C-NMR spectrum a peak at 35.0 ppm from C7 that indicates no substantial content of mm triads. 

Remark. Since only the relative configurations are considered, it is not necessary to differentiate mr and rm triads in the case of vinyl and related polymers. It is different for polymers exhibiting a true asymmetry for each monomer unit (see Section 3.3.2). 

For vinyl polymers it should be emphasized that mr and rm triads correspond to the same magnetic environment for He and thus resonate as a single signal. 

Ordinarily, it is possible to distinguish between mm/rr and mr/rm triad stereosequences using standard NMR experiments, however, distinction between the resonances of... 

When a new monomer unit is formed at the end of a growing polymer chain, the probability of generating m and r sequences are denoted by and P, respectively. We can consider polymer chain propagation following Bernoullian statistics, where the probability of producing a new m or r structure at the end of the chain is not affected by the stereochemistry of the previous monomer. In this case, when a monomer is added to the chain, the relative fractions and NMR signal intensities from m and r dyads are determined by the addition probabilities Pm and P. The relative triad fractions, and, therefore, relative intensities of mm, rr and mr/rm triad s signals, can be calculated by eqs [58]-[63]. 

---