Tacticity by NMR

Polymerization of MM in the presence of syndiotactic PMM was also reported. In this case, PMM of mostly isotactic structure was produced. Interaction between growing chains and the template is rather weak. The template influence is not as pronounced as in the case of isotactic template. Matsuzaki at al used deuterated syndiotactic PMM as template for polymerization of MM. The measurements of tacticity by NMR were... 

Tacticity is one of the most valuable parameters obtained from NMR spectra, and the stereochemical assignments of H and 13C NMR spectra of various vinyl polymers have been established.156 As discussed in Section 2.1, reliability of signal intensity is of prime importance in the determination of tacticity by NMR spectroscopy, since no other analytical means provide quantitative data on tacticity. 

Based on the estimation of tacticity by NMR, an isokinetic relationship was observed for various kinds of vinyl esters by Elias et al.85,86 and Nozakura et al.89. ... 

The first category includes methods used for the identification of any organic (simple or macromolecular) molecule, primarily spectroscopic ones the smdy of polymer tacticity by NMR spectroscopy described in Chapter 3 is an example. 

Tacticity is most often determined by NMR analysis and usually by looking at the signals associated with the -CXY- group (refer Figure 4.3). The analysis then provides the triad concentrations (mm, mr and rr) and the value of m or P(m) is given by cq. 10. 

Many radical polymerization systems were examined hy Challa at a/. A number of papers were devoted to polymerization of methyl methacrylate, MM, in the presence of poly(methyl methacrylate). The role of tacticity of template was stressed. Isotactic PMM forms stereocomplex with syndiotactic PMM. Polymerizations of MM in the presence of isotactic PMM were carried out and the product (template + daughter polymer) was analyzed by NMR. When MM is polymerized in the polar solvent in the presence... 

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-... 

In contrast to the crown ether 28, the cryptand K211 (33) is very beneficial to the control of MMA polymerization initiated by diphenyhnethyllithium in THF at —78 °C. As a result, PMMA with a very low polydispersity index (1.01) was obtained" . The effect of this ligand on the unimeric model, e.g. MiBLi, was studied by NMR spectroscopy. The equilibrium commonly observed in THF between tetramers and dimers is actually shifted towards a monomeric complexed species" . Both the E/Z molar ratio (90/10 instead of 0/100) and the chain tacticity (Table 5) are affected by the cryptand K211 (33) whatever the solvent used ". ... 

A wide range of trialkylaluminum with different bulkiness and Lewis acidity was investigated They influenced differently the PMMA tacticity. EtsAl and i-BusAl led to syndiotactic PMMA, whereas atactic PMMA was formed in the presence of f-BusAl. Ballard and coworkers successfully polymerized MMA in toluene at 0 °C in the presence of a mixture of f-BuLi and (2,6-di-f-butyl-4-methylphenoxy)diisobutylaluminum, that was prepared by reaction of triisobutylaluminum with 2,6-di-f-butyl-4-methylphenol. In order to shed light on the origin of the polymerization control, a mechanistic study was undertaken by NMR techniques. In the f-BuLi/EtsAl system, the actual polymerization initiator is a f-butyl carbanion. Actually, f-BuLi forms an ate complex with EtsAl. Later on, Muller and coworkers analyzed the unimeric model in the presence of EtsAl and MMA, and they confirmed the formation of a bimetallic ate complex It was accordingly proposed that the propagating species is an ate complex, which exhibits a decreased nucleophilicity compared to the uncomplexed species. 

The polymer composed of 2-cyclohexenyl-l,4 units can exhibit high Tg (176 °C) when the tacticity is controlled well [62]. Recently, [( -allyl)NiBr]2, in conjunction with MAO, was found to initiate stereo- and regiospecific polymerization of 1,3-cyclohexadiene [63]. Although the polymer has too poor solubility in organic solvents to be analyzed by NMR spectroscopy, the copolymers of 1,3-cyclohexadiene with 1,3-butadiene and norbornene prepared by the Ni catalyst show NMR spectra that indicate the presence of the 2-cyclohexene- 1,4-diyl unit formed via 1,4-polymerization of 1,3-cyclohexadiene (Eq. 11). X-ray diffraction analysis of the crystalline poly( 1,3-cyclohexadiene) as well as studies by molecular dynamics confirmed the cis-syndiotactic structure of the polymer [64]. 

Tacticity determination of vinyl polymers by NMR has been one of our main concerns in connection with stereoregular polymerization. Major developments in the stereochemical assignment of NMR spectra of vinyl polymers and... 

We have been concerned with the precision and accuracy of NMR data of polymers since we first started NMR studies on polymers.1-4 Using continuous-wave (CW) spectrometers, the effects of measurement conditions including temperature, sample concentration and radiofrequency (rf) field strength, were examined using several polymer and copolymer samples. Since our research group have been deeply involved in stereospecific polymerization of methacrylates, one of the main concerns about NMR measurement was the precision of tacticity determination by NMR. The errors in determining the tacticity of poly(methyl methacrylate) (PMMA) and those in the results of polymerization of methyl methacrylate (MMA) by radical and anionic initiators were examined and found to be satisfactorily small.4 Although there... 

The broad resonance of the a-methyl carbon observed in the CP/MAS/DD 13C NMR spectrum of atactic PMMA can be decomposed to three peaks corresponding to different tacticities by taking advantage of the differences in the rotational motions of the a-methyl carbons in different... 

The tacticity (pentad distribution) of the product polymer was analyzed by NMR (data not shown). The tacticity was virtually the same with and without the catalyst (T1 and 2,4,6-Me), meaning that the propagating species is a free radical. These results strongly indicate the existence of RT. 

Solution characterization by NMR proves to be an invaluable tool as these polymers possess H, and NMR active nuclei in the polymer backbone. In several cases, high resolution NMR has been used to investigate the tacticity of PFSs. " ... 

Stereochemistry (Tacticity) Vinyl polymers were the first to be classified stereochemically and studied by NMR. When the relative configuration of neighboring units among carbons in a polymer chain is considered, several stereochemically different diastereomers are possible. The first two diastereomers shown in Figure 16.18 are termed (i) isotactic and (ii) syndiotactic. 

Thus, observed tacticity values using either tetramer pair, A and A, or B and B, are in fairly good agreement with those by H-NMR. This fact suggests that any appreciable thermal isomerization does not contribute to the thermal degradation of PMMA to yield the tetramers since the associated radical transfers to yield the tetramers occur only at methyl or methylene carbons which are not asymmetric in the polymer chain. Moreover, it has been demonstrated that the diad tacticity of MMA sequences can be also precisely determined by basically the same method even in the copolymers of MMA and various acrylates and their crosslinked polymers, which are difficult to characterize by NMR. 

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