Stereoregular polypropylenes

The observation in 1949 (4) that isobutyl vinyl ether (IBVE) can be polymerized with stereoregularity ushered in the stereochemical study of polymers, eventually leading to the development of stereoregular polypropylene. In fact, vinyl ethers were key monomers in the early polymer Hterature. Eor example, ethyl vinyl ether (EVE) was first polymerized in the presence of iodine in 1878 and the overall polymerization was systematically studied during the 1920s (5). There has been much academic interest in living cationic polymerization of vinyl ethers and in the unusual compatibiUty of poly(MVE) with polystyrene. 

Polymerizations catalyzed with coordination compounds are becoming more important for obtaining polymers with special properties (linear and stereospecific). The first linear polyethylene polymer was prepared from a mixture of triethylaluminum and titanium tetrachloride (Ziegler catalyst) in the early 1950s. Later, Natta synthesized a stereoregular polypropylene with a Ziegler-type catalyst. These catalyst combinations are now called Zieglar-Natta catalysts. 

Ziegler-Natta catalysts currently produce linear polyethylene (non-branched), stereoregular polypropylene, cis-polybutadiene, and other stereoregular polymers. 

Do you see any way to make a stereoregular polypropylene displaying optical activity ... 

Figure 3. Experimental and theoretical plots of the crystalline mass fraction vs. temperature for a stereoregular polypropylene oxide fraction X—four-parameter model [H-four-parameter model with "car parking correction.

The metallocenes generally used for ethylene polymerization are achiral, and yield atactic polypropylene. Stereoregular polypropylene requires the useof chiral metallocene catalysts, such as [Et(Indenyl)2ZrMc] [BiCeFj] . 

In reality various direct experimental evidences exist to support such an hypothesis first of which, in the case of polypropylene, is the well known heterogeneity according to stereospecific properties of centres Actually, the stereoregular polypropylene isotactic fraction and the amorphous one generally show broad MWD too, thus proving the probable existence of different active centres although similar in stereoregulating ability... 

Figure 6.3 Structures of metallocene single site catalysts used to produce stereoregular polypropylene (M. P. Stevens, Polymer Chemistry, S Edition, Oxford University Press, p 246,1999).

Polypropylene chains associate with one another because of attractive van der Waals forces. The extent of this association is relatively large for isotactic and syndiotactic polymers, because the stereoregularity of the polymer chains permits efhcient packing. Atactic polypropylene, on the other hand, does not associate as strongly. It has a lower density and lower melting point than the stereoregular forms. The physical properties of stereoregular polypropylene are more useful for most purposes than those of atactic polypropylene. 

In 1953, Ziegler [38] first employed aluminum trialkyl-titanium tetrachloride (RsAl-TiCU) catalyst to prepare stereo-irregular polypropylene. Later in 1954, Natta [39,40] refined the Ziegler catalyst with aluminum trialkyl-titanium trichloride (RsAl-TiCls) catalyst and successfully prepared highly stereoregular polypropylene. The new catalyst rendered polypropylene the possibility of practical usefulness. In 1957, it led to the construction of the first production facility for polypropylene by Montecatini Co. in Italy. 

When cross-metathesis was first discovered, propene enjoyed only limited use and the reaction was viewed as a potential source of ethylene. Once methods were developed for the preparation of stereoregular polypropylene, however, propene became more valuable and cross-metathesis of ethylene and 2-butene now serves as a source of propene. 

The polyallomers constitute the class of block copolymers where both components are capable of crystallizing independently (Coover et al, 1966 Hagenmeyer and Edwards, 1966, 1970 Eastman Chemical Products, n.d.). The most important member of this family contains crystalline, stereoregular polypropylene as the major component and polyethylene as the minor component. As expected for a block copolymer, these products differ greatly in behavior from mechanical blends of polyethylene and polypropylene, and also from their random copolymers, poly(propylene-co-ethylene). When crosslinked with a diene monomer, the latter copolymers are known as EPDM rubbers (Lee et a/., 1966 Rodriguez, 1970, Chapter 13), while the former blends are of apparently little interest. In Figure 6.28 and 6.29 the... 

If every other methyl group in polypropylene is placed on the other side of the plane, another stereoregular polypropylene is formed (XVI). This was also first synthesized by Natta et al. 

Stereorepeating unit n. A configurational repeating unit having defined configuration at all sites of stereoisomerism in the main chain of a polymer molecule (lUPAC). In stereoregular polypropylene, the two simplest possible stereorepeating units are ... 

Natta used the optically active isomeric terminology DD, DL, etc. when he first described his stereoregular polypropylene. However, his wife Rosita Beati Natta had a better idea and coined the term isotactic "DD", syndiotactic (DL) and atactic (DLLDLDD) to designate these stereospecific polymers. Later, one pundit called her the real creator of isotactic polypropylene. 

A specfrum of polymer microsfrucfures exists. There are two crystalline, stereoregular polypropylenes, one in which each methyl group has an m configuration with respect to its neighbors... 

This chapter will focus on four classes of awsa-metallocene catalysts C2-symmetric bis-Cp metallocenes, C2-symmetric bis(indenyl) metallocenes, Ci-symmetric Cp-indenyl mixed metallocenes, and lastly, Ci-symmetric bis(indenyl) metallocenes containing two different indenyl moieties (Figure 3.2). There are several other classes of catalysts that have been shown to produce stereoregular polypropylene. These systems will not be reviewed herein, but are discussed in other chapters of this book. 

A peculiar feature of CGCs is that they are quite aselective catalysts out of the many structures known, only a few are able to produce stereoregular polypropylenes (PPs). However, CGCs based... 

The objective of this paper is to provide a fundamental presentation of well established aspects of the synthesis of stereoregular macromolecules and related stereochemical features. Since the discovery of stereoregular polypropylene by Natta and coworkers 40 years ago the number of stereoregular polymers has increased enormously and similar concepts have been extended to natural macromolecules. Therefore, this presentation cannot be really exhaustive and we Umit ourselves to polymers of 1-olefins, which were the first studied systems and can be sites of steric isomerism in the main chain (formed during polymerization) and in the side chains (present in the monomer). 

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