Polypropylene pentads

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. 

Later Rytter et al. reported possible polymer chain exchange with polypropylene produced with a combination of 8 and 11 with TMA [32], The number of stereoerrors increased in the binary system at higher TMA levels. As discussed in the case of Przybyla and Fink (vida supra), pentad analysis is less compelling evidence for reversible chain transfer. In addition, the gel permeation chromatography (GPC) data showed bimodal peaks, indicating very limited reversible transfer. 

Of course, the stereoselectivity (and hence the isospecificity) of the catalytic models strongly depends on the encumbrance of the jt-ligand, increasing along the following series 3-methyl-cyclopentadienyl (e.g., 3.7 kcal/mol for 10), indenyl (e.g., 4.9 kcal/mol for 13), 4,7-dimethyl-indenyl (e.g., 5.3 kcal/mol for 17), and tetrahydroindenyl (e.g., 5.9 kcal/mol for 18). This is in good qualitative agreement, for instance, with the percent of mmmm pentads evaluated for polypropylene samples obtained for different catalytic systems in strictly similar conditions by Resconi and co-workers.58... 

This amorphous polypropylene, therefore, has a tendency toward short blocks of meso and racemic sequences. This structural conclusion is not readily apparent from a simple inspection of the pentad distribution although the dominant pentads are mmmm, mmmr, rrrr, rrrm and mmrr as indicated by the above formula for the average configurational structure. 

Table 8.1 Experimental c NMR chemical shifts for the methyl pentads of regioregular polypropylene (in 1,2,4-trichlorobenzene, 137°C).

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. 

Spin-lattice relaxation times of carbon-13 in different polypropylene stereosequences differ slightly while nuclear Overhauser enhancements are almost identical (1.8-2.0) [533] isotactic sequences display larger Tx values than the syndiotactic stereoisomers. Other vinyl polymers behave correspondingly [534]. Carbon-13 spin-lattice relaxation times further indicate that dynamic properties in solution depend on configurational sequences longer than pentads. The ratio 7J(CH) 7J(CH2) varies between 1.6 to 1.9 thus, relaxation can be influenced by anisotropic motions of chain segments or by unusual distributions of correlation times [181],... 

The steric pentad distributions of the polypropylene with structure (14) are in accord with the symmetric Bemoullian statistical model (Eq. 30) based on stereochemical control by the last propylene unit of a growing chain end. 

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... 

Chain end stereocontrol, which gives rise to single-inversion pentads, mmrm and mmmr, as main error signals in 13C NMR spectra, is characteristic of propylene polymerisation with the Cp2TiPli2— [A Me) ] catalyst at lowered temperature (leading to stereoblock polypropylene) in this case an error pentad distribution is observed close to mmmr mmrr mmrm mrrm= 1 0 1 0 (Figure 3.35a) [1,30]. 

Syndiotactic polypropylene produced by typical c/ .SY/-metalloccnc [ (Me) ], catalysts is highly stereoregular with rrrr > 0.9 (the ratio of the rrrr pentad integral to the integral sum of all pentad signals observed) a single 13C NMR signal for syndiotactic rrrr pentads appears at ca = 20.3 ppm [30,418],... 

Spaleck et al. (154) reported a large number of chiral zirconocenes with different bridges and substitutions on the indenyl ligand (Table VII). Some C2-symmetric metallocenes give polypropylenes with a high melting point (162°C) and tacticities (mmmm pentades) of 97-99%, measured by 13C-NMR spectroscopy (155,156). 

The dependence the polymer microstructure on the propylene concentration in the reactor is shown in Table XVI. By varying the monomer concentration in operation with the supported catalyst, one can obtain polypropylenes with relatively low concentration of rrrr pentads. At low propylene concentrations, it is possible that m insertions are formed by epimerization. The yield of these isolated m diads is significantly increased when the catalyst is supported. Similar effects were shown by catalysts producing isotactic polymer. 

Discussion Point DPS Typical stereoerror patterns occurring in isotactic polypropylene are represented in Figures 27 and 28. Try to delineate the stereoerror patterns and the associated pentad signals which are to be expected for... 

Figure 27 Pentad NMR signals of essentially isotactic polypropylene (top) and of atactic polypropylene (bottom).

D-Limonene and ot-pinene have been used as renewable solvents and chain transfer agents in metallocene-methylaluminoxane (MAO) catalysed polymerization of ot-olefins. Chain transfer from the catalyst to the solvent reduces the achieved in limonene compared with toluene and also reduces the overall catalyst activity. This was confirmed, as in the ROMP studies, by performing identical reactions in hydrogenated limonene. However, an increase in stereospecificity was seen when D-limonene was used as the solvent. This is measured as the mole fraction of [mmmm] pentads seen in NMR spectra of the polymer. 100% isotactic polypropylene would give a value of 1.0. On performing the same propylene polymerization reactions in toluene and then in limonene, the mole fraction of [mmmm] pentads increased from 0.86 to 0.94, indicating that using a chiral solvent influences the outcome of stereospecific polymerizations. Unfortunately, when a-pinene was used, some poly(a-pinene) was found to form and this contaminates the main polymer product. 

D INADEQUATE (incredible natural abundance double-quantum transfer experiment) enables direct observation of neighbouring l3C-l3C correlation and has been applied to poly(vinyl alcohol)199 and polypropylene.206 For example, the CH carbon in the rrrr pentad has two cross-peaks with hexad methylenes of rrrrr and rrrrm. 

---