Triad sequences

Fig. 1. Triad sequences for stereoregular poly(propylene oxide) where (a) shows isotactic (RRR or SSS), (b) syndiotactic (RSR or SRS), and (c) heterotactic...

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). 

Copolymer sequence analysis follows the same procedure. A computer program (HIXCO.TRIAD) was previously written for the two-state B/B model-fitting of triad sequence distributions and applied to (unfractionated) propylene-butylene copolymers and... 

Table 5.7 Observed triad sequence for copoly(St/MA). [Reprinted from H. L. Frisch and Qihua Xu, Macromolecules, 25, 5145 (1992), with kind permission from ACS.]...

TPP)MnOAc (18) is an excellent initiator of the polymerization of the ringopening polymerization of PO. An example is illustrated in Fig. 52, where 400 equiv of PO was consumed completely in about 20 h at 30 °C. The NMR spectrum of the polymer shows a simple resonance pattern due to the CH3 group (6 17.4 ppm), indicating that the polymer consists exclusively of head-to-tail linkages. The NMR spectrum was also informative concerning the stereoregularity of the polyether, where the obtained polyether was almost atactic (i/s= 0.54 0.46,1/H/S=0.28/0.50/0.22). This is in contrast to the case with the aluminum porphyrin as initiator, which produces a polymer rich in isotactic triad sequences under similar conditions. 

The distributions of isotactic (1) and syndiotactic (s) diad together with isotactic (I), heterotactic (H) and syndiotactic (S) triad sequences were determined from the resonances due to methylene and methine carbons Oguni N, Lee K, Tani H (1972) Macromolecules 5 819... 

The 300 MHz H NMR and 20 MHz 13C NMR spectra of poly(4-methyl-l-pentene) have been found to be more complex than the corresponding spectra of poly(3-methyl-l-butene) due to the presence of an additional isomer structure in the polymer. Investigation of the 20 MHz 13C NMR spectrum of the polymer has indicated that placement of units in different triad sequences is die cause of multiple methyl proton resonances which have been observed in the H NMR spectra of poly(3-methyl-l-butene) and poly(4-methyl-l-pentene). The use of a computer program for simulating and plotting spectra has enabled measurements of polymer composition to be made of poly(4-methyl-l-pentene) s prepared under a variety of synthesis conditions. 

The benzenoid C-l resonance of styrene units in acrylonitrile-styrene copolymers is particularly sensitive to the sequence of the chain relative configurations of triad sequences can be determined by quantitative evaluation of carbon-13 signals [524], Micro-structures of other vinyl polymers such as polystyrene [525], polypropylene oxide [526], and polyalkyl acrylates [527] have also been investigated by 13C NMR. 

Triad sequence assignments have been made for ethyl acrylate-centered triads. Apparent reactivity ratios have been calculated for the semi-batch copolymers using run number theory. A model has been developed to describe the power-feed systems and predict the triad distributions in the incremental and final copolymer using the experimentally determined r-j and r values. 

In the case of crosslinked EPDM, the spectral resolution was sufficiently good to carry out the monomer sequence analysis [123]. A similar technique was also applied to NBR. The triad sequence distribution of monomers was determined, by simplifying the copolymer to be consisting of acrylonitrile and butadiene, without discriminating between cis, trans, and vinyl isomeric units [123]. The results indicated the anticipated distribution in the rubber, reflecting the reliability of this technique in the analysis. Based on the... 

Our description of the expected methylene spectrum depends only on the configuration of two adjacent monomer units, a dyad. The environment of a given methyl group, on the other hand, depends on the relative configurations of both of the neighboring M—C—X groups, hence on a triad sequence. The three lines... 

A combination of carbon 13 solution and solid state NMR was used to reveal the relative amount of crystalline PVC in two PVC/diethylhexyl phthalate samples. The relative amount of different triad sequences in the crystallites of PVC were obtained. 38 refs. 

As you might guess, we can now go up to the next lever and express diad sequences in terms of triad sequences. If we consider successor and predecessor units to an AA diad, for example, we get Equations 6-16 ... 

We have alresdy shown in Equations 6-21 how you can break down a compound event, like the number fraction of (or probability of finding) a triad sequence such as AAA, into conditional probabilities of different order. Let s first write this in a slightly different form to give you a feel for the nomenclature (Equation 6-24) ... 

In the same way triad sequences can be calculated from diad sequences, as shown in Equations 7-22, and if you have a weekend or two to spare you can continue calculating higher order sequences from those of lower order (tetrad from triads, etc.). 

PVDF is just one polymer where sequence isomerism has been obseved using NMR spectroscopy. The 13C NMR spectrum of polychloroprene, for example, has also been analyzed by one of your authors (in collaboration with his coworkers) and the olefinic region is shown in Figure 7-28. It proved possible to assigu lines to triad sequence isomers of the trans-1,4 units (TH and HT) and also triad sequences containing the cis-lA... 

However, the lines due to backbone methyl groups are sensitive to triad sequences, highlighted on the schematic spectrum shown in Figure 7-35. The differences in the magnetic... 

An atactic polymer will also have isotactic and syndiotactic triad sequences, of course, but we ll get to that in a minute. First, we... 

FIGURE 7-41 Schematic ball and stick diagram depicting triad sequences. 

If the stereochemistry of addition does depend upon the configuration found at the end of the chain, whether it is m or r, then we have a terminal model, or first-order Markovian statistics. At minimum we need tetrad data from NMR—i.e., data for (mmm), (mmr), etc.—to test for the terminal model. Remember, we can always calculate triad data from tetrad data using the relationships previously given in Equations 7-26. Equations 7-34 relate the relevant conditional probabilities to observable tetrad and triad sequences. 

The same model has been used lt6) to explain the copolymerization of ethylene and propylene with TiCl+/EB/MgCl2—AlEt3, with various amounts of EB added to the cocatalyst. The triad sequence distribution calculated for the copolymer obtained without EB was in disagreement with reactivity ratios, while the values obtained with high concentrations of EB did agree. Thus, the two active species mentioned, having two and one vacancies respectively, would be characterized by... 

---