Triad tacticity

The H-NMR spectrum of 2 in CDCI3 (Figure 1) exhibits broad unresolved resonances in the aromatic region similar to those found in the monomer. Broad signals with lack of resolution are consistent with magnetic non-equivalence of the methyl group protons resulting from a mixture of triad tacticities. 

C-NMR spectra were recorded on a Bruker DRX-300 (300 MHz). The 8-10% (w/v) solution of polymer in CDCI3 was used. The triad tacticity was determined from the peak intensities of the a-methyl carbon in the spectra. 

Poly(PO) formed with 17 as initiator under irradiation showed virtually the same NMR pattern as that for the polymer formed in the dark at 70 C, where the resonance due to the methyl group was very simple, indicating that the polymer consists of regular head-to-tail linkages. The diad and triad tactic-ities of the polymer, as determined by NMR [82], indicated the atactic struc-... 

Figure 2. Triad tacticity of poly (MMA-co-TrMA) obtained with BuLi in THF at —78°C.

Figure 7.1.3 H NMR-detected GPC trace of a sample of PMMA prepared with 1,1-diphenylhexyllithium in toluene at -78 °C (monitoring the methoxy proton resonance at 3.59 ppm), plus the triad tacticities of the eluting fractions...

Let us recall that triad tacticity describes isotactic (mm), syndiotactic (rr) and heterotactic stereotriads (mr) in the poly (a-olefin) chain ... 

Inoue et al. ( ) found that a porphyrin-Zn alkyl catalyst polymerized methyloxirane to form a polymer having syndio-rich tacticity. The relative population of the triad tacticities suggests that the stereochemistry of the placement of incoming monomer is controlled by the chirality of the terminal and penultimate units in the growing chain. There is no chirality around the Zn-porphyrin complex. Achiral zinc complex forms syndio-rich poly(methyloxirane), while chiral zinc complex, as stated above, forms isotactic-rich poly(methyloxirane). The situation is just the same as that for propylene polymerizations. Achiral vanadium catalyst produces syndiotactic polypropylene, while chiral titanium catalyst produces isotactic polypropylene. 

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). 

The terminal 13CH3 in the isopropyl group may show four signals due to triad tacticity at the chain end.103-105 As seen in the spectrum shown in Fig. 12b, only two peaks due to the terminal 13CH3 are observed at 20.69 and 21.76 ppm in... 

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...

In the CP/MAS NMR spectrum for the iso-PVA gel (Fig. 20.7(A)), peak I, together with three sharp peaks for the triad configurations, is obviously observed while peaks II and III are not so obvious. However, in the spectra for the at-PVA and syn-PVA gels shown in Figs. 20.7(B) and 20.7(C), respectively, peaks II and III are observed together with peak I. The signal areas for each of the configurations may reflect the fractions of the triad tacticity of the main chain of the PVA gel. 

In the spectrum, the SiCH signal is split into three singlets at 0.120, 0.125, and 0.131 ppm according to the triad tacticity, which were assigned to the isotactic, heterotactic, and syndiotactic triad, respectively. The calculated concentration of each triad starting from the optically active monomer with 60.8% ee assuming complete retention of Si stereochemistry in the reduction and in the polymerization via hydrosilylation is S H I = 1.0 2.0 3.3 (0.16 0.32 0.52). The actual concentration was 1.0 2.0 2.3 (0.19 0.37 0.44). 

Although the reaction of optically pure (S,5)-l,3-dimethyl-l,3-diphenyl-l,3-dis-iloxanediol with l,3-dihydro-l,3-dimethyl-l,3-diphenyl-l,3-disiloxane [(5,5) (S,R) (5,/ )] = 84 16 0] gave a poly(methylphenylsiloxane) of rather low molecular weight, its triad tacticity was found to be rich in syndiotacticity (5 /f / = 60 32 8). Some racemization seems to have occurred for the silane derivative with this catalyst. 

Figure 18-6. Variation of the triad tacticity with molar mass, that is, with the monomer/ initiator ratio, in the polymerization of a-methyl styrene with sodium naphthalene in tetrahydrofuran at 78° C. (From data of H.-G. Elias and V. S. Kamat.)...

Figure 18-7. Change in the triad tacticities with yield for the polymerization of 0.93 mol/liter methyl methacrylate with 0.03, (O), and 0.06, ( ), mol/liter butyl magnesium chloride as initiator in toluene at —5(fC. (From data of T. Migamoto, S. Tomoshige, and H. Inagaki.)...

Very recently, we have disclosed the stereoselective anionic polymerization of dibutylamino-substituted masked disilenes 1 [12]. The stereochemistry of the polymers is analyzed based on diad and triad sequences. Under the appropriate conditions, the polymerization produced a polymer rich in syndiotacticity up to 89 % in diad. Typical examples of the analysis are shown in Table 2. A high r content in the diad tacticity as well as a high rr fraction in the triad tacticity indicates clearly that the polysilane is rich in syndiotacticity. 

Run Initiator Polymn. Conditions Diad tacticity Triad tacticity ... 

Run Folynui. conditions Yield [%1 Mn(a](PD) Diad tactidly [b] Triad tacticity [b] ... 

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