Polypropylene stereochemistry

Polypropylene made by free-radical polymerization is generally atactic , that is to say, there is no pattern to the stereochemistry. On the other hand, both isotactic polypropylene (in which all the stereocenters are the same) and syndiotactic polypropylene (in which the stereocenters alternate) may be made via the Ziegler-Natta process (see Chapter 18, Problem 4). Experimentally, both isotactic and syndiotactic polypropylene generally have higher melting points than atactic polypropylene. 

Examine three different strands ofpolypropylene. For each strand, assign R/S stereochemistry to each stereocenter. (All three strands have as their terminal monomer perfluoropropane in order to facilitate assignment of stereochemistry.) Which of the three strands corresponds to atactic polypropylene, isotactic polypropylene and syndiotactic polypropylene ... 

In the following pages we shall treat, in detail, the NMR analysis of two polymers, poly(methyl methacrylate) and polypropylene, which have epitomized, in consecutive epochs, the most classical and fhiitful examples of interaction between NMR spectroscopy and macromolecular stereochemistry. 

As in many other aspects of polymer stereochemistry, polypropylene also plays a central role in NMR spectroscopy. Since 1962 numerous articles have dealt with the interpretation of its proton spectrum (125-128) the state of knowledge at the end of that decade has been well described by Woodbrey (117). The difficulty in this study stems fiom two factors The narrow frequency range comprising the entire spectmm and the large homonuclear coupling between CH2, CH, and CH3 protons. The whole spectrum is within a range of <1.5... 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

There are three principal stereochemical types of poly(l-alkene)s, illustrated in Scheme 8.38 for polypropylene. In isotactic polypropylene 80 (i-PP) all methyl substituents have the same relative orientation (m). The scheme shows the stereochemistry with the usual Fischer projection underneath. In syndiotactic PP (81, s-PP) every second CHMe unit has the opposite stereochemistry to the first, while in atactic PP (82, a-PP) the orientation of the methyl substituents is random. In some polymers there is partial order, i. e. only every second monomer orientation is random (83, hemi-isotactic PP). 

The stereochemistry of the products is often controlled through control of the reaction temperature. For instance, use of low temperatures, where the alkyl shift and migration is retarded, favors formation of syndiotactic polypropylene (sPP). Commercial iPP is produced at room temperatures. 

Three stereoisomers are possible in the cholestanylindene-derived zir-conocene complexes illustrated in Scheme 67. Two are racem-like, and the other is meso-like depending on the geometry of the metallocene moiety. The stereochemistry of the reaction is controlled by both the structure of the metallocene skeleton and steroidal substituent. Polymerization of propylene with 0-C activated with MAO gave polypropylene of 240,000, about 40% mmmm approximately 70% is due to enantiomorphic site control and the rest is due to chain-end control. Use of the catalyst derived from a /3-A-B mixture produced a mixture of polymers. The a-A and a-B/MAO catalysts afforded isotactic poly-... 

As mentioned in section 4.1, the kinetics of the living polypropylene synthesis have been interpreted in terms of a coordination polymerization mechanism represented by Eq. (22). We discuss here the mechanism of chain propagation on the basis of the structure and stereochemistry of the synthesized polypropylenes. 

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... 

It is important to note that high molecular weight trans-isotactic poly(methy-lene-1,3-cyclopentane) contains no mirror or mirror glide planes of symmetry and is thus chiral by virtue of its main chain stereochemistry (it exhibits optical activity) this is in contrast to high molecular weight polypropylene and other poly(a-olefin)s, which contain an effective mirror plane perpendicular to the molecular axis in the middle of the molecule and are thus achiral [30,497],... 

Figure 6.2 Polypropylene of different stereochemistries. In (a) the orientation of the methyl group with respect to the polymer backbone is highlighted. In (b) the stereochemical relationship (meso or racemic) between two adjacent methyl group is shown.

Figure 6.9 Effect of symmetry and substituents on the stereochemistry of the resultant polypropylene. 6.32 has Cs symmetry. 6.33 is chiral, but the effect of Me is moderate. 6.34 is also chiral, and the effect of bulky Bu is more marked.

Recently, Doi152) speculated on the presence of two types of bimetallic active centers, based on 13C NMR analysis of the structure and stereochemistry of polypropylene fractions obtained with different Ziegler-Natta catalyst systems (see Fig. 44). Site A produces highly isotactic polypropylene, site B atactic polypropylene consisting of isotactic and syndiotactic stereoblocks. The formation of the latter fraction would be due to the reversible migration of the aluminum alkyl, made... 

The revolutionary discoveries by Ziegler and Natta, relating to the low pressure polymerization, respectively, of ethylene and of propylene and other a-olefins onto the previously unknown crystalline polymers, opened a new era in science and technology. Since then, remarkable progress has been made in the fields of coordination catalysis, macromolecular science and stereochemistry. With the discovery and development of the new generation catalytic systems for polyethylene in the late 1960 s, and more recently for polypropylene, enormous progress was made in terms of polymerization process as to economics and product quality Further process simplification and, above all, ever more accurate product quality control by taylor made catalytic systems is the aim of the 1980 s. 

Early metal-metallocene-alkene polymerization catalysts permit the synthesis of highly isotactic polypropylene . They rely on controlling the stereochemistry of alkene insertion by the use of chiral C2 symmetric metallocenes . Late metal systems for alkene polymerization , and copolymerization of alkenes and CO , have also been developed. 

Figure 14 The most relevant elementary steps observed at the (R,/ -enantiomer of a chiral, C2-symmetric, isospecific zirconium center with a primary growing chain end (top) and a secondary growing chain end (bottom). The (S,S)-enantiomer produces the opposite stereochemistry of each single event, but overall the same polymer chains and the same insertion mistakes. In practice, in the case of C2-symmetric metallocenes, the racemic mixture (R,R+S,S) is always used. P = growing polypropylene chain [C] = concentration of active primary centers pC] = concentration of active secondary centers.

With metallocene catalysts, not only homopolymers such as polyethylene or polypropylene can be synthesized but also many kinds of copolymers and elastomers, copolymers of cyclic olefins, polyolefin covered metal powders and inorganic fillers, oligomeric optically active hydrocarbons [20-25]. In addition, metallocene complexes represent a new class of catalysts for the cyclopolymerization of 1,5- and 1,6-dienes [26]. The enantio-selective cyclopolymerization of 1,5-hexadiene yields an optically active polymer whose chirality derives from its main chain stereochemistry. 

Syndiotactic propagation of propylene is know to be catalyzed by homogeneous vanadium catalyst (1 ). In the polypropylene samples prepared with the homogeneous catalysts, the relative population of iso-, hetero- and syndiotactic triads is in accordance with that predicted from the first order Markov model (25, 26). There is no chiral structure around the homogeneous vanadium species. The stereochemistry of the entering monomer is controlled by the chirality of the growing chain end, in contrast with the isotactic propagation. 

Inoue et al. ( ) found that a porphyrin-Zn alkyl catalyst polymerized methyloxirane to form a polymer having syndio-rich tacticity. The relative population of the triad tacticities suggests that the stereochemistry of the placement of incoming monomer is controlled by the chirality of the terminal and penultimate units in the growing chain. There is no chirality around the Zn-porphyrin complex. Achiral zinc complex forms syndio-rich poly(methyloxirane), while chiral zinc complex, as stated above, forms isotactic-rich poly(methyloxirane). The situation is just the same as that for propylene polymerizations. Achiral vanadium catalyst produces syndiotactic polypropylene, while chiral titanium catalyst produces isotactic polypropylene. 

Both the isotactic and the syndiotactic forms of polypropylene are known as stereoregular polymers, becanse each is characterized by a precise stereochemistry at the carbon atom that bears the methyl group. There is a third possibility, shown in Figure 7.17c, which is described as atactic. Atactic polypropylene has a random orientation of its methyl groups it is not a stereoregular polymer. 

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