Mm triads

The new configurational sequences, mmmr, mmrr and mrrm will be produced from the isotactlc polymer by Isolated Inversions of configuration. The relative intensities for the mmmr, mmrr and mrrm pentads will be 2 2 1 which allows a positive Identification of the mrrm pentad. Correspondingly, the predominantly syndio-tactlc polymer will have mm triads Inserted into rr.r se-... 

In principle, each enchainment of a prochiral monomer such as propene generates a chiral carbon center. The relative orientation of two such centers can therefore give rise to meso-diads or racemo-diads. Three monomers consecutively enchained in meso-fashion give rise to a mm triad, and so forth (Scheme 8.37). 

Considering the possibility of formation of both transition states, we see unambiguously that Incorporation below the plane Is possible going from the RR" dlad, to a mm triad (Figure 19). 

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. 

Ameso-meso (mm) triad is a chain of three consecutive monomer links of S,S,S- or R,R,R configuration. 

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. 

A better selectivity than with 1,10-phenanthroline (90% MM-triads) was obtained with 5-nitro-l,10-phenanthroline [84] and, more recently, with 1,4-di isopropyl-1,4-diaza-l,3-butadiene 53 [85]. Model studies with the latter catalytic systems have... 

Fig. 14 3D- H, C, F correlation of poly(l-chloro-l-fluoroethylene) with slices at 5( F) = -98.2 (a), -99.4 (b), -100.7 (c) (d) schematic illustration of the 3D spectrum with relative positions of the shoes. Each shce in (a)-(c) displays two correlation signals representing the two CHj groups adjacent to a CF moiety. The polymer tacticity can be determined as follows the splitting of each signal reveals the stereochemistry of individual CFCI-CH2-CFCI diads, the two protons at a methylene carbon being chemically inequivalent in meso (m) and chemically equivalent in racemic (r) diads. Consequently, the fluorine atoms can be assigned to rr (a), rm (b) or mm triads (c). The possible tetrad structures can be determined by looking for identical C-H cross-peaks in different F slices e.g. the A cross-peaks in (a) do not occur in the other two sUces therefore, the methylene group A shows only correlations with F atoms in mm triads and must be centred in an mmm tetrad, etc. Reproduced, with permission, from ret 100. Copyright 1996 American Chemical Society.

Triad tacticities obtained from q-CH3, quaternary and C=0 carbon signals agreed well with one another.9 However, the mm triad fraction determined from the q-CH3 proton signal deviated from other values because of the overlapping of the a, a -azobisisobutyronitrile (AIBN) fragment signals. Correction for this contribution makes the mm value closer to the average (cf. Section 4.1). 

In order to determine whether a system obeys first-order Markovian statistics, a knowledge of pentad tacticity is required to a high degree of experimental accuracy. This is very difficult for the case of radically polymerized PMMA, since the fraction of mm triads is usually less than 5% of the total. 

Since complexes 92 and 93 are achiral, it is likely that they form tactic polymers via a chain-end control process (vide infra). To increase the isospecificity of complex 93 for polymerization, Schrock replaced the achiral alkoxide groups with a range of Q-symmetric diols to create asymmetric complexes 94—98. These complexes, especially rac-95, exert an exceptionally high degree of stereochemical control in the polymerization of 90 and 91, producing polymers that are >99% cis and have >99% isotactic (mm) triads. Even complex 98 that employs a conformationally flexible ligand produces highly isotactic, cis polymers. 

If the enantiomorphic site control is operative (top-half view), stereoerrors do not propagate, and the corresponding iso- and syndiotactic polymers are characterized by the presence of rrand mm triads, respectively. If chain-end control is operative (bottom-half view), stereoerrors propagate, and the corresponding iso- and syndiotactic polymers are characterized by the presence of Isolated r and m dlads, respectively. Reprinted from ref 115. Copyright 1992 American Chemical Society. 

Like the anionic polymerization of dienes, the anionic polymerization of alkyl methacrylates, especially MMA, is dependent on the counterion, solvent, and, to a certain extent, temperature [156, 173, 121]. In general, the stereochemistry of the anionic polymerization of alkyl methacrylates in toluene solution with lithium as the counterion is highly isotactic (68-99% mm) and the isotacticity increases with the steric requirements of the alkyl ester. For example 90% mm triads are obtained for t-butyl methacrylate at —70 °C in toluene [174], while 68% mm triads are observed for MMA [120]. Isospecificity for polymerizations in toluene... 

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