Rr triads

Several isospecific Ci-symmetry catalysts have also been described including (12-15). When activated with [Ph3C]+ [B(C6F5)4]-, (12) affords highly regioregular i-PP (mmmm = 95%) with the stereochemical defects predominantly being isolated rr triads, consistent with a self-correcting enantiomorphic site-control pathway. 2,73 The isospecificity was therefore explained by a mechanism... 

Variation in the substituents at the nitrogen donor atom has also been examined,126 and in one case isoselective polymerization of propylene was described (mmmm pentad = 56% using (31)).127 Syndioselective propylene polymerization with an rr triad content of 63% has been reported using (32)/MAO, although residual Me3Al must be removed from the MAO in order to suppress chain transfer to aluminum.128... 

In an attempt to combine the syndioselectivity of half-sandwich titanium catalysts with the living characteristics of anionic polymerization initiators, the use of half-sandwich calcium-based catalysts has been described.363 364 In neat styrene complex (152) affords 76% rr triad PS. However, polydispersities are still quite high (Mw/Mn > 2.2)... 

Cp 2Sm(jU-H)]2, (188), affords very high-molecular-weight PMMA with very low polydispersities (typically < 1.05).453-456 At — 95 °C the polymer formed is highly syndiotactic (95% rr triad). Isolation and X-ray analysis of (189), the 1 2 complex of (188) and MMA, provides strong support for the participation of a metal-enolate as the active site. (189) behaves in an identical manner to the hydride precursor, converting 100 equivalents MMA to polymer with Mn= 11,000 and Mw/Mn= 1.03.457 The successful structural characterization of (189) provides support for intermediates proposed earlier.458,459... 

Fig. 34 Plots of rr triad values as a function of calculated R2-H volume for propylene polymerization...

The measurement of polymer configuration was difficult and sometimes speculative until the early 1960 s when it was shown that proton NMR could be used, in several instances, to define clearly polymer stereochemical configuration. Bovey was able to identify the configurational structure of poly(methylmethacrylate) in terms of the configurational triads, mm, mr and rr, in a classic example (3). In the case of polypropylene, configurational information appeared available but was not unambiguously accessible because severe overlap complicated the identification of resonances from the mm, mr and rr triads (4). Several papers appeared on the subject of polypropylene tacticity but none totally resolved the problem (5). 

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. 

The formation of rr triads Is extremely Improbable, thus explaining the very low concentration measured for such triads In the polymers prepared. 

Table I Tacticities of samples obtained in DME and THF, using mono- and bifunctional initiators and Na+ and Cs+ as the counterions. I,H,S isotactic (ami), heterotactic (mr/rm) and syn-diotactic (rr) triads m,r meso and racemic dyads p 2mr/H persistence ratio ( p 1 for Bernoullian statistics).

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. 

Different types of brackets are used to make a distinction between distinguishable (n-ads) and indistinguishable [n-ads]. If your first reaction to this sentence is say what , don t panic, this is straightforward stuff. Check out the schematic depictions of mm and rr triads shown at the top of Figure 7-41. 

In the studies of mkrostructure of jK)ly(cyclohex ie oxide) obtained with (C6Hs)3C AsF initiator Blanchard a.o. observed the mm, mr and rr triads. The intensity values are equal to 0.15,0.47 and 0.39 reqrectively . The obtained polymers are amorphous, both cis- and trans-structures are present. 

Figure 1.6 Configurational sequences, (a) Spatial representations of meso (m) and racemic (r) configurations, (b) Planar projections of m and r configurations, (c) Isotactic (mm), heterotactic (mr), and syndiotactic (rr) triads.

Figure 31 Melting point of sPPs made with Cs-symmetric catalysts, versus the rr triad content. Note that samples with rr< 75% are amorphous in the second DSC heating scan, and require several days of annealing at room temperature to develop measurable crystallinity.

Fig. 20.2 shows the NMR spectra for the PVA in (A) the solution (B-D) gel and (E) solid states as measured by solution and solid-state NMR methods [16]. The NMR spectrum for the PVA aqueous solution obtained by the solution C NMR method (Fig. 20.2(A)) shows each of the signals for the CH and CH2 carbons split into multiple peaks due to its stereochemical configuration. The triply split peaks for the CH carbon are assigned to the mm, mr and rr triads from high frequency [17-19] and, furthermore, each of the triad peaks splits into pentad peaks. The split peaks for the CH2 carbon come from tetrad tacticity. In the solution NMR spectrum for the PVA gel shown in Fig. 20.2(B), the signal for the CH carbon splits into three peaks due to triad tacticity. The signals for the CH and CH2 carbon become broader as compared with those for the PVA solution. This is caused by the...

PVA samples with different tacticities, such as isotactic (iso-), atactic (at-) and syndiotactic (syn-) ones were used. The degree of polymerization, and the fractions of mm, mr and rr triads, are shown in Table 20.2, where m and r indicate the meso and racemic dyads, respectively. The CP/MAS NMR spectra for the three kinds of PVA gels with different tacticity (9% of polymer concentration) are shown in Fig. 20.7. As described in Section 20.4.1, the CH peaks are composed of both the three sharp peaks corresponding to the triad configurations (mm, mr and rr) and the three broad peaks (I, II and III at about 77, 71 and 65 ppm, respectively). 

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