Pentad relationships

Polymerization mechanism Microstructure with isolated stereoinversion triad/triad and pentad/pentad relationships... 

When the chain migrates from one site to the other without insertion we obtain two stereocentres of the same absolute configuration and our pentad analysis will show one meso (m) in the series of (r) relationships. 

The advent of high-resolution 13 C NMR allows the determination of tetrad, pentad, and even higher sequence distributions in many polymers [Bovey, 1972 Bovey and Mirau, 1996 Farina, 1987]. The tetrad distribution consists of the isotactic sequence mmm, the syndiotactic sequence rrr, and the heterotactic sequences mmr, rmr, mrm, rrm. The sum of the tetrad fractions is unity, and the following relationships exist ... 

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

Considering each insertion as an independent event (which implies enantiomorphic site control, i.e., the chirality of the catalyst is the dominating factor for stereocontrol). Hart and Rappe then calculated the intensities for the mmmm pentad by multiplication of the individual probabilities. The calculated intensities were in excellent agreement with those from NMR data for a series of substituted ethylene-bridged, C2-symmetric zirconocenes. The use of this new and more quantitative approach to analysis of the relationship between the molecular structure of the catalyst and the polymer microstructure was, however, restricted to comparison of intensities for the mmmm pentad. Application to error pentads or prediction of pentad distributions for new or modified catalysts was not attempted. 

FIGURE 4.12 Relationship between the isotactic pentad fractions estimated by Py-HGC and C-NMR. Diagonal lines represent an ideal relation. 

It might be expected that an ELPP with a block-type microstructure would display a relationship between %mmmm pentad intensity and polymer melting temperature somewhat intermediate between the blended samples and the compositionally pure, pseudorandom microstructures produced using stereorigid metallocenes. This finding supports Waymouth s contention that a very different propagation process is occurring with their catalyst system. 

FIGURE 2.24 Typical isotactic polymers generally adhere to a linear relationship between the [mmmm] pentad fraction and melting temperature (gray circles), whereas isotactic-hemiisotactic polymers can exhibit faint melting endotherms at considerably higher temperatures (black squares) than predicted by this relationship. 

To get a closer insight into the polymerization mechanism responsible for the strong dependence of stereoselectivity on monomer concentration observed for this type of asymmetric catalyst, the pentad distributions of the polypropylenes prepared with 9a were investigated using nuclear magnetic resonance (NMR) spectroscopy. The mmmm pentad content was observed to decline with increasing monomer concentration this relationship was attributed to the existence of two coordination sites in these dual-side complexes, which show different stereoselectivities for monomer coordination and insertion. Guerra et al. have supported this hypothesis in a theoretical study. 

Statistical Models. Syndlospecific stereoselective copolymerization Schemes were selected. The statistical models for the pentad probability relationships were derived by ... 

Syndiotactic Propylaie Pofymerization wM iPii.CfiFbi]ZrCli 475 Table 14. Two Site Hodel Triad and Pentad Intensity Relationships. ... 

Since there are six observable tetrads and ten observable pentads, it is a formidable task to make resonance assignments for these structxiral features. However, there are a number of general approaches for doing this. These include (a) the use of stationary or "necessary n-add relationships" to correlate the relative intensities of resonances (b) the development of empirical relationships to correlate differences in chemical shift due to structural differences ... 

The evaluation of a and p for application of the above rules is accomplished by examining the spectra of a series of copolymers. The chemical shift difference between AAAAA and BBABB pentads affords a measvire of 2(a + p) and the spectra of polymers with either low or high a contents, where only a few pentads are present in appreciable concentration, can be used for the estimation of a and p. The validity of assignments made this way can be checked by determining whether or not relative pentad concentrations evaluated from the spectra obey necessary n-add relationships [28-30]. 

The stereochemical relationship between adjacent substituents is described by the terms meso and racemU abbreviated as m and r, respectively. If five sequential substituents stand in a meso relationship, it is described as an mmmm pentad, and the probability of mmmm is often used to evaluate the isotacticity of polymers. In addition, [m] dyad for two sequential substituents and [mm] triad for three are used. Indeed,in order to achieve 99% of [mmmm], the probability of each [m] needs to be more than 99.8%. It can be understood that excellent stereoselectivity of the insertion reaction is required to produce highly stereoregular polyolefins. 

---