Syndiotactic-rich

Also, divalent TiPh2 activated with [Al(Me)0]x appeared to be a catalyst for syndiospecific styrene polymerisation [71]. Even 5-TiCh or (Acac)3TiCl3, when activated with [Al(Me)0]x, could yield a mixture of isotactic and syndiotactic polystyrenes. Some other catalysts, such as rare-earth coordination catalysts, have been successfully used to obtain syndiotactic-rich polystyrenes [72],... 

Yang et al. examined rare earth coordination catalysts. The Nd(naph)3/ y4/(fBu)3 catalyst system was found to produce syndiotactic-rich polystyrene [8]. They proposed that the catalytically active species might be an ionic complex because the addition of CCI4 increased the catalytic activity. 

Anionic or neutral allylic samarium or neodymium species as catalysts polymerize styrene without addition of any co-catalysts (catalyst/styrene ratio = 1 1000). Random syndiotactic-rich material was obtained from tetra(allyl)lantha-nates, whereas the neutral tris(allyl)lanthanides or anionic t -bis(cyclopentadienyl)bis(allyl)lanthanates afforded... 

The lithium salt of the silylene-bridged diamide ligand L50 reacted with YbCls to form the ate diamide complex [Yb(L50)2(THF)2Li(THF)] (221) (Scheme 81). This complex showed moderate catalytic activity for the polymerization of MMA at room temperature. Syndiotactic-rich polymers with high molecular weights (M > 10" ) and relatively narrow molecular weight distributions (M /M = 1.45) were obtained (Scheme 12) [166]. 

Lyoo, W. S., Ha, W. S., Preparation of Syndiotacticity rich High Molecular Weight Polyfvinyl alcohol) Microfibrillar Fiber by Photoinitiated Bulk Polymerization and Saponification, J. Polym. Sci., Part A Polym. Chem. 1997, 35, 55 67. 

Monomers with bulky side groups do not form syndiotactic-rich polymers readily. Several attempts to prepare syndiotactic PPBA were made. The best results were achieved by polymerizing the monomer in toluene solution at -78° under U.V. irradiation, when a sample was obtained with 65% syndiotactic diads. 

M = Li, K, and Cs), in combination with either liCl or Et2Zn. Moreover, syndiotactic-rich, isotactic-rich, and atactic polymers could be obtained by turning the initiator system. The atactic and syndiotactic-rich polymers were both soluble in water, whereas the isotactic-rich polymers (m > 69%) were insoluble in water. The atactic polymers showed Tc-values of around 32 °C, similar to those of the polymers obtained by radical polymerization, while a Tc-value of 37 °C was observed in a syndiotactic-rich polymer (r = 83%). 

Dash means no data aPVA atactic polymer iPVA PVA rich in isotactic fiagments sPVA syndiotactic-rich polymer... 

FIGURE 14.15 NMR spectmm (CDCI3,50 128 MHz) of the phenyl C-1 carbon of syndiotactic-rich... 

Yamaura, K., Kuranuki, N., Suzuki, M., Tani-gami, T., and Matsuzawa, S. (1990). Properties of mixtures of silk fibroin/syndiotactic-rich poly( vinyl alcohol). Journal of Applied Polymer Science 41, 2409-2425. 

The syndiotactic-rich polymer, poly[3-(2,2,2-triphenylacetylamino)prop-ionic acid] (syndfo-poly[( -Ala-OH)-MA]), and the isotactic-rich polymer, (/so-poly[(j0-Ala-OH)-MA]), were examined to investigate the effect of polymer tacticity involving the side chain of the ligand upon CaCOs crystallization (Fig. 20). The diad contents for sy/7dm-poly[(y0-Ala-OH)-MA and zso-poly[(j0-Ala-OH)-MA] were determined as 77% and 70%, respectively, determined by the NMR spectra in solution of the ester-formed polymers. An extremely broad amide NH H-NMR signal for deproto-... 

Neutral and anionic samarium and neodymium species 54 and 55 with allyl ligands were reported to give syndiotactic-rich polystyrene [29]. Although the presence of lithium aUyl in equilibrium suggests an anionic polymerization mechanism, the formation of a syndiotactic-rich product indicates the participation of rare-earth metals in the polymerization. 

Isotactic-rich polymers showed Tg values between 115 and 122 °C, while atactic polymers presented slightly higher Tg at around 130 °C. On the other hand, the Tg s of syndiotactic-rich polymers were observed at 143-148 °C, and were apparently higher than those of isotactic and atactic polymers. The... 

A specific example is poly(vinyl chloride) (PVC), which is a monosubstituted vinyl polymer that has a syndiotactic-rich character and a conformation that can be either an extended all-trans structure or a folded syndiotactic structure. The vibrational modes of these conformational models obey different selection rules and have different dichroic properties that can be used to spectroscopically test these structures [7]. The folded syndiotactic model of PVC has the [p,0] classification that requires unique Raman lines (no coincident IR frequency) that are polarized. The extended syndiotactic model has the two unique classifications of [d,0] and [p,(r], which means that the unique Raman lines are depolarized, and the Raman lines that are polarized have perpendicular dichroism in the IR spectrum. In the Raman spectrum of PVC [8], polarized lines are observed at 363, 638, 694, 1172, 1335, 1430 and 1914 cm and IR bands are also observed at each of these frequencies. This result rejects the folded syndiotactic structure, because this structure requires the polarized lines to be unique. In addition, each of these frequencies is perpendicularly dichroic in the IR spectrum, a fact that supports the planar syndiotactic structure. 

Fig. 7.10. The proton resonance spectra of the fi-CHj units of syndiotactically rich (a) and isotactically rich (b) samples of PMMA at a high-field strength of 220 MHz. (Source Ref. [ 1 ].)...

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