Stereosequence

Many radical polymerizations have been examined from the point of view of establishing the stereosequence distribution. For most systems it is claimed that the tacticity is predictable within experimental error by Bemoullian statistics [i.e. by the single parameter P(m) - see 4.2.1],... 

A general purpose program has been developed for the analysis of NMR spectra of polymers. A database contains the peak assignments, stereosequence names for homopolymers or monomer sequence names for copolymers, and intensities are analyzed automatically in terms of Bernoullian or Markov statistical propagation models. A calculated spectrum is compared with the experimental spectrum until optimized probabilities, for addition of the next polymer unit, that are associated with the statistical model are produced. 

Stereosequence in homopolymers and monomer sequence in copolymers will influence the mechanical and physical properties of the polymer. 

Figure 1. 50.31 MHz I3C NMR spectra of PVC (a) and two partially reduced PVC s, E-V-84 (b) and E-V-21 (c). Please note the table of E-V microstructural designations in the upper right-hand corner of the Figure, where 0,1 = CH2, CHC1 carbons. Resonances correspond to underlined carbons. The assignment of different stereosequences is given in reference 2.

Simulation of the (n-Bu)3SnH reduction of PVC is carried out in a manner similar to that described for TCH. Instead of beginning with 100 TCH molecules we take a 1000 repeat unit PVC chain that has been Monte Carlo generated to reproduce the stereosequence composition of the experimental sample of PVC used in the reduction to E-V copolymers (2), ie. a Bernoullian PVC with P =0.45. At this point we have generated a PVC chain with a chain length and a stereochemical structure that matches our experimental starting sample of PVC. 

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 Figure 9 we have plotted the ratios of r/m W diads observed by 13C NMR in E-V copolymers obtained by the (n-Bu)3SnH reduction of PVC (2). They are compared to the r/m ratios resulting from our computer simulation of PVC reduction made possible by the observation of the kinetics of (n-Bu)3SnH reduction of DCP and TCH. The agreement is good, and provides us with a knowledge of E-V stereosequence as a function of comonomer composition. 

The physical properties (7-10) of our E-V copolymers are sensitive to their microstructures. Both solution (Kerr effect or electrical birefringence) and solid-state (crystallinity, glass-transitions, blend compatibility, etc.) properties depend on the detailed microstructures of E-V copolymers, such as comonomer and stereosequence distribution. I3C NMR analysis (2) of E-V copolymers yields microstructural information up to and including the comonomer triad level. However, properties such as crystallinity depend on E-V microstructure on a scale larger than comonomer triads. 

In summary, characterizations of stereosequences in polymers obtained by catalytic systems based on well-characterized metallocene complexes have produced a general acceptance of the chain migratory insertion mechanism and of models described in i-iii. 

Silyltitanation of 1,3-dienes with Cp2Ti(SiMe2Ph) selectively affords 4-silylated r 3-allyl-titanocenes, which can further react with carbonyl compounds, C02, or a proton source [26]. Hydrotitanation of acyclic and cyclic 1,3-dienes functionalized at C-2 with a silyloxy group has been achieved [27]. The complexes formed undergo highly stereoselective addition with aldehydes to produce, after basic work-up, anti diastereomeric (3-hydroxy enol silanes. These compounds have proved to be versatile building blocks for stereocontrolled polypropionate synthesis. Thus, the combination of allyltitanation and Mukayiama aldol or tandem aldol-Tishchenko reactions provides a short access to five- or six-carbon polypropionate stereosequences (Scheme 13.15) [28],... 

This strategy has recently been extended to optically active stereosequences, either by using a chiral protective group (carbamate) as an inductor, or by using (S)- or (R)-BINOL-TiCl2 as the catalyst for the Mukayiama reaction [29]. 

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). 

Necessaiy Relationships Among Stereosequences in Vinyl Polymers"... 

The symbols m, mm, and so on, are used for the frequency (or analytical concentration) of the corresponding stereosequences, normalized over all the sequences of the same length. 

Generation of Stereosequences in Atactic (Left) and Hemiisotactic (Right) Vinyl Polymers... 

Frisch, Mallows, and Bovey (44), and later Chujo, Kamei, and Nishioka (107) examined the same class of polymers and pointed out the existence of two series of three-center sequences that are intrinsically different, one centered on substituent A, 62, the other on B, 63 (Scheme 13). They were distinguished by different symbols, for example, by small and capital letters (108). Six three-center stereosequences and 20 five-center sequences are possible, due to the... 

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. 

Because of the close connection between stereosequence distribution and... 

The configurational sequence and stereosequence coincide in this particular case because there is only one site of stereoisomerism in each constitutional repeating unit (compare Definitions 2.1.3 and 2.1.4). 

Stereosequences terminating in tetrahedral stereoisomeric eentres at both ends, and whieh comprise two, three, four, five, etc., consecutive centres of that type, may be called diads, triads, tetrads, pentads, etc., respectively. 

The polymer stereosequence distributions obtained by NMR analysis are often analyzed by statistical propagation models to gain insight into the propagation mechanism [Bovey, 1972, 1982 Doi, 1979a,b, 1982 Ewen, 1984 Farina, 1987 Inoue et al., 1984 Le Borgne et al., 1988 Randall, 1977 Resconi et al., 2000 Shelden et al., 1965, 1969]. Propagation models exist for both catalyst (initiator) site control (also referred to as enantiomorphic site control) and polymer chain end control. The Bemoullian and Markov models describe polymerizations where stereochemistry is determined by polymer chain end control. The catalyst site control model describes polymerizations where stereochemistry is determined by the initiator. 

C NMR spectra are recorded for a low molecular weight atactic PP dissolved in a variety of solvents over a broad temperature range [293 - 393 K). Comparison of chemical shifts calculated via the y effect method with the observed resonances, whose relative chemical shifts are solvent independent, permits their assignment to most of the methyl heptad, methylene hexad, and methine pentad stereosequences. Agreement between observed and calculated chemical shifts requires y effects, he., upfield chemical shifts produced by a gauche arrangement of carbon atoms separated by three bonds, of ca. - 5 ppm for the methyl and methine carbons and ca. - 4 ppm for the methylene carbons. 

Conformational energy calculations are coupled with dipole moment measurements to derive a conformational description of P2VP. When an RIS model is used to calculate the dipole moments of P2VP chains with different stereosequences, it is found that the calculated dipole moments are nearly independent of the P2VP stereosequence. 

C NMR chemical shifts are calculated for the carbon nuclei in PVA to the pentad and hexad levels of stereosequence for the methine and methylene carbons, respectively. The RIS model developed by Wolf and Surer (A 069) is employed to calculate the frequencies. The relative orders of the observed methine pentad and methylene hexad resonances agree with the calculated chemical shifts, in addition to the agreement between the overall chemical shift dispersions measured and predicted for the methylene carbons. 

Conformational energies are calculated for chain segments in poly(vlnyl bromide) (PVB) homopolymer and the copolymers of vinyl bromide (VBS and ethylene (E), PEVB. Semlempirical potential functions are used to account for the nonbonded van der Waals and electrostatic Interactions. RIS models are developed for PVB and PEVB from the calculated conformational energies. Dimensions and dipole moments are calculated for PVB and PEVB using their RIS models, where the effects of stereosequence and comonomer sequence are explicitly considered. It is concluded from the calculated dimensions and dipole moments that the dipole moments are most sensitive to the microstructure of PVB homopolymers and PEVB copolymers and may provide an experimental means for their structural characterization. 

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