Stereosequences number

We select repeat units at random, and if they are unreduced V units a check of whether or not the units adjacent to the selected unit are E or V is made. Having determined the triad structure (both comonomer and stereosequence) of the repeat unit selected for reduction, we divide the relative reactivity of this E-V triad by the sum of relative reactivities for all V centered E-V triads as listed in Table III to obtain the probability of reduction. A random number between 0.0 and 1.0 is generated, and if it is smaller than the probability of reduction of the selected E-V triad, we remove the chlorine from the central V unit which becomes an E unit. 

In the absence of degeneracy the number of possible stereosequences is equal to 2" (n being the number of stereogenic centers present in the sequence). In vinyl polymers, because of the equivalence of the two directions of the chain, many sequences differ only in the way they are observed (e.g., rrmr and rmrr) and must, therefore, be considered only once. The number of independent sequences, calculated by the Frisch, Mallows, and Bovey formula 10, 44) is shown in Table 2. In hemiisotactic polymers, where strict selection criteria exist, this number is drastically reduced (100). 

Stereosequences in polymers having the general formula — CHR—X—Y— [e.g., poly-a-amino acids, polymers derived from substituted oxiranes, thi-iranes, aziridines, lactones or lactams, and 1,4 polymers of 1- (or 4) monosub-stituted butadienes] are not affected by the degeneracy phenomena existing in vinyl polymers. Their number is indicated in the first column of Table 2. 

Methods are presented for calculating mean-square dipole moments, , of polypeptide chains, averaged over all configurations of the chain skeleton. They are applicable to chains of any number (x+ U of residues, the residues being In any specified sequence. Dipole moments of glycine peptides are calculated and compared with experimental determinations. The effects of stereosequence on the dipole moment are well reproduced by the calculations. Ip is taken to be 380 pm. 

Isoblock polypropylene is characterised by the appearance of stereosequences of m diads bridged by pairs of r diads [20,21], Such polypropylene, containing a small number of long blocks, is usually referred to as isotactic polypropylene. Note that the polymer chains in this case contain helix sequences of the same handedness (right or left) in the chain, which is due to the same relative configuration of tertiary carbon atoms in these sequences. Stereoblock polypropylene, which is also called block-isotactic polypropylene,... 

The estimation of chemical shifts by examining the spectra of model compounds is not always feasible, and the prediction models fail to distinguish between two or more stereosequences as they cannot always be distinguished on the basis of intensity alone. To overcome these limitations, large numbers of organic compounds have been analyzed by NMR and their chemical shifts have been used to determine a set of empirical correlations that are used to determine the structure based on the polymer s NMR spectrum. The carbon chemical shifts of hydrocarbon-based polymers such as polyethylenes can be predicted by empirical additivity rules such as ... 

Tacticity determination of vinyl polymer by 1H NMR spectroscopy was first achieved for PMMA by Nishioka169 and Bovey.170 13C NMR chemical shifts of carbons in PMMA show higher sensitivities to longer stereosequences.163 171 173 The carbonyl carbons in many polymethacrylates exhibit a splitting pattern similar to that in PMMA. However, a recent report on 13C NMR spectra of polymers of various chlorophenyl methacrylates indicates that the number and positions of the chloro substituent strongly affect the spectral patterns, and the carbonyl carbon signal of poly(2,4,6-trichlorophenyl methacrylate) does not show a splitting due to tacticity.174... 

Epimerization events were repeated until the chain reached equilibrium. The calculation was repeated ten times and the stereosequence concentrations evaluated after specified numbers of epimerization events were then averaged, printed and punched for use in spectra simulations. 

In reality, reverse addition occurs in less than 5% of the additions of V and H units, so that the probability of a triad containing two monomer units derived from reverse addition is very small. Furthermore, H units do not add to H units (4,5) at the end of a growing radical chain-end. These factors greatly reduce the number of possible triad structures in the polymer however, there are still 20 possible triad sequences to be considered. Some of the most prevalent structures are siumnarized in Figure 1. The number of possible structures is increased if we consider that with the HVH type of structures (HVH, HV H, H VH, HVH .), it is possible to have several stereosequences. 

The extensive, though smaller, splitting of resonances belonging to the same carbon type (CH, CH2, or CH3) observed in the C NMR spectra of atactic PP (see Figs. 20.9 and 20.10), must be produced by the presence of different stereosequences, because the numbers of a, /3, and y substituents possessed by each carbon type are independent of stereosequence. On the other hand, it is well known that the local conformations in vinyl polymers like atactic PP are sensitive to stereosequence [24]. The local magnetic field B, experienced by a carbon nucleus i must be dependent upon the local conformation in its vicinity, Thus,... 

A number of catalyst systems have been developed (2 fL>A which result In propylene oxide polymers of different stereosequence distribution. In the following, we review some of our work on the characterization of stereosequence length In propylene oxide polymers prepared with different catalysts, and more importantly, studies on the effect of the differences in stereose-sequence length on the crystallization behavior and mechanical properties of the polymers. 

A number of recent papers describe the use of high resolution NMR spectroscopy in identifying stereosequence distribution in polypropylene oxide (9-11,17-20). Using o -deuterated polypropylene oxide and 220 HMz NMR, Ogunl et al (19) have shown that the shift in methylene absorbtion can be used to identify the structures assigned to tail-to-tail linkages and to isotactic and syndiotactic triads. They showed that the crystalline fraction of polymer prepared with a dlethyl-zinc-water catalyst system did not contain a measurable amount of tail-to-tail linkages. 

However, it is clear from the NMR spectra of three PP samples presented in Figure 2.2 [13] that the multiple resonances appearing in the spectrum of atactic-PP (b), which are well known to be produced by different stereosequences [11], cannot be explained by the usual fi-, and y-substituent effects. Each methyl carbon in PP has one a-, two / -, and two y-substituents, each methine carbon has two a-, two / -, and four y-substituents, and each methylene carbon has two a-, four and two y-substituents independent of stereosequence. Some factor other than numbers and types of a-, P-, and y-substituents, and which depends on PP stereosequence, must be responsible for the multiplicity of... 

By use of the methods discussed here, and C-assignments have been made for a large number of vinyl polymers. The reader is referred to a number of excellent texts and reviews for detailed information [41-47,54,67,68]. It seems that the stereosequence distributions of most vinyl polymers can now be characterized by nmr without difficulty. Polystyrene, one of the first polymers to be studied by nmr, and its analogs, represent a significant exception to this generalization. For this reason, our present understanding of the nmr spectra of polystyrene will be discussed next. 

Nuclear magnetic resonance spectroscopy is also useful for characterizing the distributions of stereosequences in polyethers4 polyesters, and polyamides, although the method becomes difficult to apply when the asymmetric atoms are separated by a large number of atoms. Recent developments in this area include H- and c- methods for characterizing polylactides [23], a C-method for characterizing substituted poly(p-propiolactones) [50] as well as and... 

Bmch et in this same paper also performed extensive F ID- and 2D-NMR smdies on a commercial sample of PVF with a significant number of stmctures from inverse monomer additions. They were able to detect and assign many resonances from H-H and T-T stmcmres as well as fine stmcture from stereosequence effects. However, they noted that a number of experted peaks were not detected. This was most likely due to the combined effects of the much larger spectral window required to encompass all of the F resonances in this more complicated polymer stmcture, the poorer digital resolution in the 2D-NMR spectra, and the resulting cancellation of antiphase multiplet components of the 2D-NMR crosspeak patterns. 

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