Triad resonances

Quantitative measurement shows about 11% of the monomer units to be inverted. The principal spectrum shows splitting into mm, mr, and rr triad resonances with some pentad fine structure. The polymer is nearly atactic. Assignment of inversion "defect" resonances is made easier by reference to spectrum (b), which is that of poly (vinyl fluoride) prepared by the following route (17) ... 

Polymerization of CFE in urea at -80"C has a mild stereoregulating influence compared to bulk polymerization at 60 °C, although the spectra in Figure 2 show that both polymers are substantially atactic. Three major triad resonances can be discerned more readily in spectrum (b), and these are assigned to mm, mr, and rr stereosequences as shown simply by analogy with the poly(l,2-difluoroethene) assignments (14,15). At present we have no basis to make definitive assignments, but those indicated are at least consistent with the known syndiotactic bias exerted by urea on the polymerization of complexed vinyl monomers (10). 

The low field region shows three methine carbon resonances, peaks 1-3. Referring to Terao [2], and concentrating for the moment only at PV(, peaks 1-3 in the solid state cannot simply be assigned to the tacticity induced splitting of the mm, mr and rr triads respectively the relative area intensities of the three peaks in the solid state are not consistent with the triad tacticity observed in solution [2]. Moreover a significant downfield shift of the resonances 1 and 2 occurs compared with the observed chemical shifts for mm and mr triad resonances in solution, which results in much larger mutual chemical shift differences between the methine carbon resonances in the solid. 

The methine proton resonance pattern of poly(vinyl thiophene) is the same as that observed for polystyrene. The lowest field resonance component amounts to 25 percent of the total methine proton resonance and is consistent with poly(vinyl thiophene) being atactic if this lower field component can be assigned to mm triads. Finally, the C-1 carbon resonance spectrum of poly-(vinyl thiophene) (Figure 11) seems to consist of triad resonances, with the lower field area being further subdivided into pentad resonances. This spectrum also suggests that poly(vinyl thiophene) is atactic. If we can conclude on the basis of the results presented here that poly(vinyl thiophene) is indeed atactic, then it seems reasonable to expect the same thing to be true for polystyrene, since the benzene and thiophene rings have very similar steric requirements and similar chemical behavior. 

Additionally, three resonances are observed for the a-methyl proton resonances which exhibit substantially different relative intensities between the two polymers. These three resonances are assigned to the stereoregular triad sequences. Those monomer units that are flanked on both sides by units of the same configuration are termed isotactic triads (i). Those monomer units that have units of opposite configuration on both sides are syndiotactic triads (s). Those monomer units that have a unit of the same configuration on one side and a unit of opposite configuration on the other side are heterotactic triads (h). These three triad a-methyl resonances have the same chemical shifts in the spectra of the different stereoregular polymers but have very different intensities in each spectrum. The relative intensities of these a-methyl proton triad resonances provide a measure of the triad probabilities. 

The COSY spectmm of PVC, shown in Figure 52(b), also shows evidence to help distinguish between resonances of m-and r-centered sequences. In this spectmm, correlations between methine triad resonances and methylene tetrad resonances are found. Two correlations are expected for rr (to mrr and rrr) and mm (to mmr and mmm] methines, while four correlations are expected for mr methines (to mmr, mrr, mrm, and rmr). Due to the low concentration of isotactic sequences in this sample, the correlations between mm methines and mmm methylene groups are not resolved. The large number of correlations to the central methine triad resonance is further evidence that this must be due to the mr stmcture. 

In this spectrum, correlations between methine protons on adjoining monomer units are expected. The methine-methine coupling patterns mm-to-mr-to-rr groups are clearly identified. The combination of these four 2D-NMR techniques provided unequivocal proof of the assignments for the three methine triad resonances and the six methylene tetrad resonances. This study is a classic example of applying the combination of a variety of 2D-NMR methods to polymer spectral resonance assignments. 

For the case of copolymers, suppose we consider the various triads of repeat units. There are six possibilities MjMjMj, M1M1M2, M2M1M2, M2 M2 M2, M2 M2 Ml, and Mi M2 Mi. These can be divided into two groups of three, depending on the identity of the central unit. Thus the center of a triad can be bracketed by two monomers identical to itself, different from itself, or by one of each. In each of these cases the central repeat unit is in a different environment, and a characteristic proton in that repeat unit will resonate at a different location, depending on the effect of that environment. 

Figure 7.6 Chemical shift (from hexamethyldisiloxane) for acrylonitrile-methyl methacrylate copolymers of the indicated methyl methacylate (Mj) content. Methoxyl resonances are labeled as to the triad source. [From R. Chujo, H. Ubara, and A. Nishioka, Polym. J. 3 670 (1972).]...

To investigate the triads by NMR, the resonances associated with the chain substituent are examined, since structures [XV] -[XVII] show that it is these that experience different environments in the various triads. If dyad information is sufficient, the resonances of the methylenes in the chain backbone are measured. Structures [XIII] and [XIV] show that these serve as probes of the environment in dyads. 

The nmr spectmm of PVAc iu carbon tetrachloride solution at 110°C shows absorptions at 4.86 5 (pentad) of the methine proton 1.78 5 (triad) of the methylene group and 1.98 5, 1.96 5, and 1.94 5, which are the resonances of the acetate methyls iu isotactic, heterotactic, and syndiotactic triads, respectively. Poly(vinyl acetate) produced by normal free-radical polymerization is completely atactic and noncrystalline. The nmr spectra of ethylene vinyl acetate copolymers have also been obtained (33). The ir spectra of the copolymers of vinyl acetate differ from that of the homopolymer depending on the identity of the comonomers and their proportion. 

The H-NMR spectrum of 2 in CDCI3 (Figure 1) exhibits broad unresolved resonances in the aromatic region similar to those found in the monomer. Broad signals with lack of resolution are consistent with magnetic non-equivalence of the methyl group protons resulting from a mixture of triad tacticities. 

The relative reactivities (in free-radical copolymerizations) of TBTM and MMA are 0.79 and 1.00 respectively (15). With equal concentrations of monomer, an excess of MMA in the polymer would be expected. In the following discussion A will represent the MAA or TBTM unit and B will represent the MMA unit. For A-centered triads four different arrangements are possible AAA, AAB, BAA, and BAB. Analogous sequences apply to the B-centered triads. For a random compositon, Bernoullian statistics should apply (14). With P (the proportion of TBTM) equal to 0.5, the probabilities of each of the A-centered triads is P 2 or 0.25. The AAB and BAA triads are indistinguishable and appear as a single resonance. 

As described in Section 3.5, any polar M—L bond is susceptible to backside attack by a Lewis base I. to form a linear (or near-linear) 3c/4e /ryperbonded L i- M -i L triad, equivalent to strong resonance mixing of the form... 

Because trans dispositions commonly result from co-bonding (the near-linear alignment of the hyperbonding 3c/4e X—M- L triad), it is not surprising that the origin of the trans influence can be traced to the resonance nature of co-bonding. When H is placed trans to a halide or PH3, the dominant resonance structure will be that with a 2c/2e M—H bond and a donor pair of electrons on the halide or phosphine ligand, as depicted on the left in (4.93) ... 

In the resonance representation (4.121), each Cp ring is seen from the edge (dashed lines), with the characteristic L M—X <—> L—M X resonance interactions forming the two tu-bonded triads that link the sandwich fragments. 

In contrast, resonance delocalization and bond alternation in the C—C=C—C backbone are only slightly affected by H-bond formation (namely, the C4—C5 bond order varies by only 0.008 between H-bonded and open conformers), because such resonance shifts do not intrinsically alter the charge distribution in the H-bonded O- H—O triad. This example illustrates the principle that H-bonding is not generally coupled to resonance per se, but only to such resonance as leads to effective CAHB enhancement (Section 5.2.2). 

Statistical analysis is important and relatively easy. Suppose we have a fully atactic polymer which we analyse for the triad content for isotactic polymer. Only three methyl resonances due to triads are observed, and the statistical ratio of mm, rr, and mr is 1 1 2. Thus even in the atactic polymer our isotactic content is 25% Pentad analysis, however, would give only 8% mmmm isotactic content Especially for catalysts with low enantiospecificity it is worthwhile keeping this in mind. 

The measurement of polymer configuration was difficult and sometimes speculative until the early 1960 s when it was shown that proton NMR could be used, in several instances, to define clearly polymer stereochemical configuration. Bovey was able to identify the configurational structure of poly(methylmethacrylate) in terms of the configurational triads, mm, mr and rr, in a classic example (3). In the case of polypropylene, configurational information appeared available but was not unambiguously accessible because severe overlap complicated the identification of resonances from the mm, mr and rr triads (4). Several papers appeared on the subject of polypropylene tacticity but none totally resolved the problem (5). 

The Triad Chemical Shift Sequence with Respect to an Increasing Field Strength for Some Representative Vinyl Polymer Backbone Methlne Carbon Resonances... 

There are only three unique triad combinations, mm, mr and rr thus a methyl configurational sensitivity to just nearest neighbor configurations would produce only three resonance in the methyl region of the C-13 spectrum. From an earlier spectrum of the amorphous polymer, we noted at least ten methyl resonances. We must therefore consider the situation where the next-nearest as well as nearest neighbor configurations are affecting the chemical shift, that is. 

---