Pentad level

More complex schemes have been proposed, such as second-order Markov chains with four independent parameters (corresponding to a copolymerization with penultimate effect, that is, an effect of the penultimate member of the growing chain), the nonsymmetric Bernoulli or Markov chains, or even non-Maikov models a few of these will be examined in a later section. Verification of these models calls for the knowledge of the distribution of sequences that become longer, the more complex the proposed mechanism. Considering only Bernoulli and Markov processes it may be said that at the dyad level all models fit the experimental data and hence none can be verified at the triad level the Bernoulli process can be verified or rejected, while all Markov processes fit at the tetrad level the validity of a first-order Markov chain can be confirmed, at the pentad level that of a second-order Maikov chain, and so on (10). 

In a recent study on poly(a-methyl-a-ethyl-piopiolactone) and poly(a-methyl-a-propyl-propiolactone) (112) Spassky et al. extended the earlier examination carried out on oxiranes and thiiranes (113) to the tetrad and pentad level. Because of the chirality of these polymers, Spassky avoids the use of the terms m and r as being inapplicable (21). To indicate the dyads RR (or SS) and RS (or SR) 64 he used the symbols ij and Sj where the indices stand for dyad. In their turn the triads RRR (or SSS) are indicated with i, RSR (or SRS) with s, RRS (or SRR) with hi (heterotactic triad beginning with an dyad) and RSS (or SRR) with (heterotactic triad beginning with a dyad), 65 (Scheme 14) (114). 

The NMR chemical shift of a given CH3 group is influenced by its neighbors. Most modem NMR instruments will allow resolution at the pentad level, i. e. the resonance of a methyl group is determined by the orientation of the two monomer... 

A significant improvement in resolution and information content of NMR spectra was achieved by high-field (150 MHz) NMR which allows resolution at hep-tad or even higher levels. At present, this level of resolution is still the preserve of the specialist NMR practitioner. In any case, for most purposes of structural assignments resolution at pentad level is satisfactory. For chemical shifts of high-resolution spectra see Ref [123]. 

Steric defects in isotactic polypropylene, which involve the appearance of isolated r diads or pairs of r diads, may be considered on a pentad level (Figures 3.45a and b respectively). The 13C NMR signals associated with occasional stereoerrors in the propylene isotactic polymers produced by chiral metallocene-based catalysts (pairs of r diads) indicate that the polymerisation stereochemistry is governed by the enantiomorphism of catalytic sites an error pentad distribution close to mmmr.mmrr.mmrm-.mrrm = 2 2 0 1 is observed... 

PMPS, PMHS, PMDS, and PDHS. Individual resonances are observed for each carbon type, except for the dodecyl polymer in which C-4 to C-9 in the side chain are not completely resolved. The carbon resonances can also be assigned without the use of model compounds by using standard two-dimensional NMR techniques (2). C-1 and, in some cases, C-1, C-2, or both in the three asymmetrically substituted polymers show shift dispersion, which results from the many different stereochemical sequences along the polymer chain. The methyl carbon in PMPS is resolved to at least the pentad level of stereosequences. This chemical shift information can be analyzed to provide a description of the chain statistics resulting from a particular polymerization. 

The distribution of steric defects along the polymer chain may be indicative of which kind of stereocontrol is operative. The type and amount of stereomistakes (enantioface insertion errors) is measured by solution 13C NMR spectroscopy, a sensitive technique that is able to see the steric environment of a given propylene unit up to undecads (five propylene units on each side of the observed monomeric unit). Routine analysis is usually performed at the pentad level (two propylene units on each side of the observed monomeric unit).162,179 The microstructures which result from stereomistakes are shown in Scheme 7. 

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... 

The polymerization reaction is a sequence of different events, such as monomer insertions, site isomerizations, and chain release reactions. The polymer chain can be seen as a permanent picture of the sequence of these events, and it is possible to use a statistical approach to study their distribution along the chain to increase our knowledge on polymerization mechanisms. As a consequence, a mathematical model of the polymerization can be built by assigning a probability at each event in our system. In the case of propene homopolymerization, this approach is (largely) used to study the mechanisms governing the stereoselectivity of the catalyst from the NMR spectrum of the polymer. In fact, the type and the relative amount of the stereosequences present in the chain are obtained from the methyl region of the spectrum and are usually determined at the pentad level (see section II.G). This distribution can be studied using insertion probabilities for propene enantiofaces, which depend on the type of stereocontrol mechanism active for the catalytic... 

NMR is usually performed at pentad level (J). This analysis is not adequate only in the following cases /) polymers from multisite catalysts (at least 5 adjustable parameters for only 3 different active species) //) polymers from fluctuating catalysts (at least 4 adjustable parameters) iii) weakly tactic polymers (all models degenerate into the Bernoullian statistics). In most such cases, however, unambiguous conclusions can be reached from the analysis of high-field NMR data at heptad/nonad level. 

Figure 3.17 Partial C H NMR (150MHz, (a = 0.94,%mmmm = 0.71). Stereochemical l,l,2,2-Cl4C2D2, 90°C) spectrum of the microstructure analysis at the pentad level,...

Chain end analysis of the polypentene by H NMR shows the presence of geminally disubstituted olefin resonances, indicating chain termination by /6-H elimination. NMR analysis of the polymer at the pentad level shows a remarkably high degree of isotacticity. This catalyst also initiates the polymerization of 1-hexene and afford polymers (M = 24,000) with M jM =1.75. A similar C NMR analysis showed the tacticity of more than 95%. 

The structure of polysilylenes has also been explored with H, and NMR methods. " The studies indicate that polysilylene chain configurations in solution are varied and quite complex for a wide range of asymmetrically substituted homopolymers and copolymers. Thus, at high field (500 MHz) the alkyl protons in poly(methyl-n-hexylsilylene) are well resolved and assigned using 2D spectra. The spectra provide information about the microstructure and the Si spectra appear sensitive to stereochemical configuration at least to the pentad level. 

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