Pentad sequences

Syndiotactic polypropylene became commercially available about ten years ago with the advent of single-site catalysts. Unlike its atactic and isotactic counterparts, its manufacture presented serious challenges to polymer scientists and engineers. Even under the best conditions, its syndiotacticity rarely exceeds 75%, based on pentad sequences. It typically has both a lower melting point (approximately 138 °C relative to approximately 155 to 160 °C) and density (0.89 g/cm3 relative to 0.93 g/cm3) than isotactic polypropylene. Syndiotactic polypropylene crystallites have a much more complex structure than the isotactic form, which impedes its crystallization. Therefore, in general, the syndiotactic form of polypropylene crystallizes very slowly. 

The sequence length distribution for an ideal copolymerization with r = 5, r2 = 0.2 for an equimolar feed composition is shown in Fig. 6-9. This copolymerization has pn — p2 0.8333 andp 2— p22 — 0.1667. Both M and M propagating centers have a 5 1 tendency to add Mi over M2, but Mi pentad sequences are not the most plentiful, although they are among the most plentiful. The most plentiful sequence is Mi at 16.7% with 14.0%, 11.6%, 9.7%, 8.1%, 6.7%, 5.6%, and 4.7%, respectively, of dyad, triad, tetrad, pentad, hexad, heptad, and octad Mi sequences. There are smaller amounts of longer sequences 3.2% of... 

Seven other possible pentad sequences (mmmr, rrmr, mmrm, mmrr, mrmr, mrrm, and rmmr) are atactic. Residues R may be CH3 for polypropylene, C6H5 for polystyrene, Cl for polyvinyl chloride, CN for polyacrylonitrile, and C02CH3 for polyacrylic acid methyl ester. 

B. Using a pentad sequence, give an example of the equality of a reversible sequence. 

D. Express the pentad sequence PS BABAB in terms of first order Markovian statistics. 

From an NMR experiment it was determined that in a copolymerization of monomer A with monomer B the number fraction ofBAAAB pentad sequences was 0.04, while the number of AB diad sequences was 0.32. Calculate the number fraction of sequences of three As (AAA), A/3). 

We ll be merciful and stop after just giving you the relationships for tetrads obtained starting from reversible pentad sequences (Equations 7-31). 

Two of the three pentad sequences, mmrm and rmrr, can be assigned by consideration of the necessary pentad-pentad relations ... 

Stereochemical assignments have also been made at the pentad level for poly (vinyl fluoride) from the 19F COSY spectrum, in which the cross-peaks arise from four-bond scalar coupling (about 7 Hz) between the central pair of fluorines in the pentad sequences that share a common hexad.200... 

Similarly, the pentad sequence ABABB will have a probability of occurrence given by... 

FIGURE 7. Pentad sequences and their corresponding A and B 29Si NMR signals for a random A BV copolymer. Reproduced from Reference 145 by permission of VEB Deutscher Verlag fur Grundstoffindustrie... 

The coefficient 2 appears as there are two ways of putting A and B at the end of the triad. P( A) is the probability of-finding A in the chain and is the mole fraction XA (reported as a %). Table 6 contains the probability expressions for all triad and pentad sequences. 

TABLE 6. Probabilities of occurrence of triad and pentad sequences... 

C-NMR spectroscopy is much more suitable for structural studies. Extensive studies of the structures of homo- and copolymers of cyclic acetals were published by Schulz 134). By using a shift reagent, l,l,l,2,2,3,3-heptafluor-7,7-dimethyl-4,6-octadionatoeuropium (Eu(fod)3), the pentad sequences, and with higher Eu(fod)3 concentration the heptad sequences, were observed for both M and E units. Signals observed in the spectrum of TXN-DXL copolymer, are given in Table 7.15. 

The assignment of proton chemical shift of the pentad sequences was first established by comparison with pentad sequences of TOX/DOL copolymers (7). The counit CH2CH2CH2CH2O ( B ) in TOX/DOP copolymer is expected to show higher shielding effect than CH2CH2O ( E ) in TOX/DOL copolymer. The chemical shifts of the pentad sequences of TOX/DOP copolymers are therefore usually upfield from the corresponding pentad sequences of TOX/DOL copolymers (Table I). Six M - centered pentad sequences from TOX/DOP were... 

Table I. Proton ( H) and Carbon ( C) Chemical Shift of Pentad Sequences from TOX/DOP and TOX/DOL Copolymers...

The chemical shifts of pentad sequences in NMR spectra (Table I) were then assigned through PFG-HMQC experiments following the assignment of proton chemical shifts. The assigned pentad sequences both in and NMR spectra were further ascertained with PFG-HMBC experiments. In PFG-HMBC experiments, the pentad sequences should have multiple-bond correlation to two flanking pentads. For exan le, pentad MMMBM sequence should show correlation to two M -centered sequences MMMMB and BMMMB and to two B -centered sequences MMBMM and MMBMB. 

Figure 2. TOX/DOP copolymerization kinetic profiles of pentad sequences and central carbon of CH2CH2CH2CH2O counit, B2. Molar ratio M/B= 9 BFsOEt2/M =30 ppm...

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