Syndiotactic polypropylene symmetry

Figure 2.4 Line repetition symmetries, according to equivalence principle, and corresponding possible conformations, according to minimum conformational energy, for (a) isotactic and (b) syndiotactic polypropylene (T = trans, G = gauche).

TT G G TT G G+ TT and G G- TT G G TT G G the third (, = 2 = 180°) to the zigzag planar form lTTlTT TT TT (symmetry cm) (Figure 12). The dimorphism of syndiotactic polypropylene, therefore, finds an adequate explanation on a conformational basis It is interesting to observe that this phenomenon was first recognized theoretically (173) and only later verified experimentally (169, 170). 

Considerations of the polymerisation stereochemical behaviour of catalysts belonging to class V with pseudohelical symmetry (based on lopsided metallocene precursors) still warrant more in-depth investigations. However, an assertive statement can be made that, the more unsymmetrical the catalysts are, the more m diads there are in syndiotactic polypropylene. On the other hand,... 

In 1962. Natta and Zambelli reported a heterogeneous. vanadium-based catalyst mixture which produced partially syndiotactic polypropylene at low polymerization temperatures. " The regiochemistry of the insertion was determined to be a 2.1-insertion of propylene, and a chain-end control mechanism determined the s mdiospecificity of monomer insertion. This catalyst system suffered from both low activity and low stereoselectivity. Highly active single-site olefin polymerization catalysts have now been discovered that make syndiotactic polypropylene with nearly perfect stereochemistry. Catalysts of two different symmetry classes have been used to make the polymer, with Cs-symmetric catalysts typically outperforming their Q -symmetric counterparts due to different mechanisms of stereocontrol (Figure 10). 

The insertion step consists of an alkyl migration to the olefin ligand at the same time, a new free coordination site is generated at the vacant position of the former alkyl ligand. A catalyst molecule with a center of asymmetry (Q symmetry) at the metal is predestinated to produce syndiotactic polypropylene a catalyst with Cz symmetry, isotactic polypro-... 

Unbridged (fluorenyl) (cyclopentadienyl) metallocene dichloride complexes are potential catalyst precursors for the production of syndiotactic polypropylene. There are three prerequisites the catalyst precursors have to fulfill Cg symmetry and a ligand ensemble with one ligand being much smaller (Cp) than the other (Flu). Moreover, the hindered rotation... 

Tacticity describes the symmetry of the monomer orientation or stereochemistry of the monomer. In polybutadiene, atactic polymers have no steric order along the polymer. Isotactic polymers have the monomer pendent groups at the same relative position. Syndiotactic 1,2- polybutadiene has pendent groups alternating along the polymer chain. For illustrative purposes atactic, isotactic, and syndiotactic polypropylene are shown in Figure 4.2 [2, 4]. 

Not all syndioselective precatalysts are C -symmetric indeed, many of the metallocenes in Figure 2.7 are Ci-symmetric. Usually symmetry is broken by distal substituents on thefiuorenyl ring or on the bridge. But, even proximally substituted Ci-symmetric species i -TO and s-83, with cyclo-pentadienyl substitution, give syndiotactic polypropylene with melting points (TniS) of 129 ([r] =... 

Complexes with C -symmetry (e.g., 2, Figure 9.6) have an internal vertical mirror plane bisecting the ligand from back to front. The two coordination sites formed after activation are mirror images, and therefore show opposite selectivity for the coordination and insertion of the prochiral monomer. This means that the preferred propylene face for coordination changes after every insertion step, which affords a syndiotactic polypropylene microstructure. 

One of the most exciting recent advances in organic and organometallic chemistry has been the development of new catalysts that produce polypropylene with high stereochemical purity. Both isotactic and syndiotactic polypropylene are now made commercially with a new class of metallocene catalysts, prototypes of which are shown below. The mechanism of the polymerization reaction is discussed in Chapter 17. Here we will focus on the stereochemistry, because symmetry principles of the sort we discussed above were crucial in the design of this chemistry. 

The changes in ligand symmetry alter the microstrucmre of the prepared polymer. Catalysts with Cs-symmetry enable the production of highly syndiotactic polypropylene [48]. An example of this is iPr[Cp][Flu]ZrCl2 (7). Based on the published reactivity ratios, this catalyst has better copolymerization ability than rac-Me2Si[Ind]2ZrCl2 (9) [5, 35, 49]. 

In the case of the homopolymerization of ethylene, there is no tacticity, but with propylene polymerizations the catalyst symmetry can influence the tacticity. Bis-cyclopentadienyl zirconocenes are symmetric and give atactic polypropylene. The isopropyl bridged cyclopentadienyl fluorenyl zirconocene has a center of asymmetry at the metal (Q symmetry) and provides syndiotactic polypropylene. The same catalyst, but with a t-butyl substituent on the cyclopentadienyl ligand has C2 symmetry and yields isotactic polypropylene [11]. 

Catalyst 12.9 is highly selective for forming syndiotactic polypropylene, (12.7). Each binding site is locally chiral, but, because the whole molecule has a plane of symmetry, the two binding sites have opposite local chirality. Because each successive propylene occupies opposite sites, they can therefore be incorporated into the growing chain with opposite chirality, leading to syndiotactic polymer. 

A third factor influencing the value of Tg is backbone symmetry, which affects the shape of the potential wells for bond rotations. This effect is illustrated by the pairs of polymers polypropylene (Tg=10 C) and polyisobutylene (Tg = -70 C), and poly(vinyi chloride) (Tg=87 C) and poly(vinylidene chloride) (Tg =- 19°C). The symmetrical polymers have lower glass transition temperatures than the unsymmetrical polymers despite the extra side group, although polystyrene (100 C) and poly(a-meth-ylstyrene) are illustrative exceptions. However, tacticity plays a very important role (54) in unsymmetrical polymers. Thus syndiotactic and isoitactic poly( methyl methacrylate) have Tg values of 115 and 45 C respectively. 

Figure 2.12 Maps of conformational energy of various syndiotactic polymers as function of backbone torsion angles 0 and 0227 (a) syndiotactic polystyrene, (b) polypropylene, (c) poly (1-butene), and (d) poly(4-methyl-l-pentene). Succession of torsion angles. .. 0i 0i 0202 - - -[s(M/N)2 symmetry] has been assumed. Isoenergetic curves are reported every 5 kJ/mol of monomeric units with respect to absolute minimum of each map assumed as zero. Values of energies corresponding to minima (x) are also indicated. Experimental conformations observed for different polymorphic forms of polymers are indicated by triangles. (Reproduced with permission from Ref. 27. Copyright 1992 by the Socicta Chimica Italiana.)...

Two examples clearly illustrate the relationship between molecular structures of the metallocene catalysts on the one hand, and the tacticity of the resultant polymers on the other. As shown in Fig. 6.9, complexes 6.32, 6.33, and 6.34 have very similar structures. In 6.33 and 6.34 the cyclopentadiene ring of 6.32 has been substituted with a methyl and a f-butyl group, respectively. The effect of this substitution on the tacticity of the polypropylene is remarkable. As already mentioned, 6.32, which has Cs symmetry, gives a syndiotactic polymer. In 6.33 the symmetry is lost and the chirality of the catalyst is reflected in the hemi-isotacticity of the polymer, where every alternate methyl has a random orientation. In other words, the insertion of every alternate propylene molecule is stereospecific and has an isotactic relationship. In 6.34 the more bulky t-butyl group ensures that every propylene molecule inserts in a stereospecific manner and the resultant polymer is fully isotactic. 

Work in the application of metallocene-based catalysis to olefin polymers has become a research topic of growing interest in recent years. A great number of symmetrie and chiral zirconocenes have been synthesized to give totally different structures of isotactic, syndiotactic, atactic or block polymers. The isotactic sequence length of polypropylene is influenced by the nature of the ligands of the metallocene. New ring or bridge substituted metallocene/methylalumoxane catalysts for the olefin polymerization are described. 

Polypropylene is a versatile polymer used in applications from films to fibers, with a worldwide demand of over 21 million Ib. It is similar to polyethylene in structure except for the substitution of one hydrogen group with a methyl group on every other carbon. On the surface, this change would appear trivial, but this one replacement changes the symmetry of the polymer chain. This allows for the preparation of different stereoisomers, namely, syndiotactic, isotactic, and atactic chains. These configurations are shown in the introduction. 

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