Tacticity, polymer

Detailed discussion of polymer tacticity can be found in texts by Randall,2 Bovey,1-3 Koenig,4-5 Tonelli6 and Hatada.7 In order to understand stereoisomerism in polymer chains formed from mono- or 1,1-disubstiluted monomers, consider four idealized chain structures ... 

NMR/Fractionation. The combination of NHR and fractionation is an excellent approach to study polymer mixtures.(17) If the components of the mixture are completely fractionated, then IIMR characterization is straightforward. If the components of the mixture are similar in structure such that separation is partial, then the computerized approach described above can be used to study the NMR/fractionation data and to deconvolute the consonants in each fraction. Thus far, three computer programs have been written to study MMR/fractionation data, two for polymer tacticity (MIXCO.CSX, MIXCO.C4X), and one for copolymer triad sequences (MIXCO.TRIADX). 

Polymer synthesis, advances in, 20 442 Polymer tacticity, 20 533 Polymer tubing, aeration through,... 

There are, however, things about the polymer structure which are not known for certain. We assume that the reaction occurs in a random manner along the polymer backbone but there is little evidence at all concerning this problem and a detailed analysis must await future research. In addition, we know very little about the effects of polymer tacticity on the reaction shown in Equation 21. This also remains to be studied. On the other hand, we are confident that this reaction does not lead to a novel crosslinking reaction sequence since these polymers are soluble in a number of different solvents (Table II). 

The dn/dc can show a dependence on polymer tacticity (41) and molecular weight, but these effects usually are minor relative to that of polymer composition (40). Also, the... 

Note 1 Terms referring to the tacticity of polymers (tactic, ditactic, tritactic, isotactic, cistactic, etc.) can also be applied with similar significance to chains, sequences, blocks, etc. 

In an extension of the work to chiral chemistry 244), the imidazolium cation of the ionic liquid was modified to carry a chiral substituent. The high cost of the chiral cation could be justified as the chiral ionic liquid, [MBMIM ] , can be reused. In the ATRP of soluble methyl acrylate, a small effect of the chiral ionic liquid on the polymer tacticity was observed. The use of a chiral ionic liquid as a solvent could lead to applications in other areas of catalysis. The synthesis of imidazolium-containing ionic liquids functionalized with chiral natural amino acids has already been reported 245), as have less expensive chiral ionic liquids(2- (5). 

Tabulate kinetic data or polymer tacticity data, depending on the experiment performed. 

Polymer tacticity These terms are first recorded in J. Polymer Sci. 20,251,1956. The words originate from Greek words that prefix tatto (put in order) a) iso- (the same), syndo- (every two and a- (not at all). 

Efficient and defect-free folding of the polymer will, at least, require a control of the regioselectivity of monomer incorporation and polymer tacticity. If, for example, the degree of tacticity control is poor, the stereochemical defects are irreversibly incorporated into the polymer, and the subsequent folding may fail or produce defective structures. By contrast, dynamic self-assembly may allow defects to be corrected and the hierarchical structure to be controlled or fine-tuned using external parameters (solvent, additives, temperature) prior to covalent fixation by polymerization. 

Determination of polymer tacticity. The type of tacticity can de determined by X-ray, but there are no generally satisfactory quantitative methods for determining the degree of tacticity. 

A complete quantitative description of the thermodynamics of polymer-polymer solutions also might need to include the effects of polymer tacticity. As demonstrated recently by Schurer et al. (50), changing the stereo configuration of poly (methyl methacrylate) from isotactic to syn-diotactic causes it to become miscible with PVC. These results suggest the importance of the spatial articulation of interacting segments in the polymer. 

FIGURE 14.3 Nuclear spin symmetry relationships for main-chain radical la from PMMA, as a function of polymer tacticity. The two radicals shown have mirror plane ( meso ) or C2 axis ( racemic ) symmetry elements establishing magnetic equivalence of each set of 3-methylene protons. 

Phosphine trapping and 31P NMR has also been used to monitor polymerizations of vinyl ethers and follow the polymer tacticity [208], However, less nucleophilic phosphines, such as tris(p-chlorophenyl)phos-phine, do not terminate the growing chains efficiently, and polymerization continues slowly [66]. Nevertheless, trapping with less nucleophilic phosphines provides information on microstructure and also on kinetics in slow polymerizations. 

There are significant differences in rate and polymer tacticity, dependent on the nature of the transition metal compound and organo-metal compound (Tables 12 and 13). 

There has been a great deal of effort reported designed to make sense of how different metallocene complexes regulate polymer tacticity as a function of the symmetry properties of the catalyst. To begin to understand this, we must look at stereochemical possibilities of approach by propene to the metal. Figure 11-4 shows four such options, which may be classified as re-primary,, v/ -primary,. v/ -secondary, and re-secondary. The designations re and si refer to the faces of the propene C=C bond, and Figure 11-4 also details the difference between si... 

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.

Polymer Tacticity. Our initial results on the polymerization of several different p-substituted-a-methylstyrene monomers indicated that there was some relationship between polymer stereoregularity and both the type of initiator and substituent in these monomers ( ). However, our recent investigations with a much wider variety of monomers, catalysts and cocatalysts revealed that the classical approach to analyzing substituent effects in organic reactions, the use of the Hammett pa relationship, gave no simple and self-consistent relationship between tacticity and the a (or a ) constant for the para-substituent. These results are summarized in the data in Table I for the cationic polymerization of a-methylstyrene and a series of five p-substituted-a-methylstyrene monomers initiated with two different Friedel-Crafts catalysts, TiCl and SnCl, either alone or with a cocatalyst benzyl chloride (BC) or t-butyl chloride (TBC), in methylene chloride at -78°C. Where a cocatalyst was used, the initiator was presumably a carbonium ion formed by the following reaction ... 

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