Stereoblock polypropylene

Two random copolymers of this type are of importance, ethylene-propylene copolymers and ethylene-but-l-ene copolymers. The use and properties of polypropylene containing a small quantity of ethylene in stereoblocks within the molecule has already been discussed. Although referred to commercially as ethylene-propylene copolymers these materials are essentially slightly modified polypropylene. The random ethylene-propylene polymers are rubbery and are discussed further in Section 11.9. 

It is known the case of i-PP, for which the copolymerization with small amounts of ethylene tends to stabilize the y form [84] for instance, by melt crystallization of a copolymer with 6% by mol of ethylene more than 80% of the crystalline phase is in the y form [85], It is also known that the obtainment of the y form by melt crystallization, is also favored for samples of low molecular mass [86, 87] and for stereoblock fractions [88]. This seems to suggest that, whenever the preferential crystallization of the y-form is observed, there is the concomitant occurrence of a reduction in the polymer of the length of the chain stretches with polypropylene head to tail constitution and isotactic configuration. 

Waymouth and coworkers reported a unique system where the unbridged bis(2-phenylindenyl)zirconium-based catalysts (129) gave elastomeric, isotactic-atactic stereoblock polypropylene, controlled by rotation of the 2-phenylindenyl as shown in Scheme VII [173]. 

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

Figure 8 Metallocenes for iso-, syndic-, and stereoblock polypropylene synthesis.

Scheme 6 Formation of stereoblock polypropylenes with an oscillating catalyst.

The atactic amorphous portion (9-16% of the total) contained in the obtained polypropylene has been separated by treating the raw polymer with n-heptane at room temperature. When operating in such a way, we have not separated the stereoblock fraction (extractable in boiling n-hep-tane) from the isotactic (not extractable in boiling n-heptane) fraction of polymer. The results reported in this paper are generally referred to the crystalline fraction, named non-atactic, which contains also some stereoblock polymers (at the considered polymerization temperatures, the latter generally correspond only to 5-7 % of the total) (9)... 

The same methods employed with raw polymers were used for fractions of isotactic polypropylene (ether extraction residue), containing also some stereoblock polymers 32). 

Isoblock polypropylene is characterised by the appearance of stereosequences of m diads bridged by pairs of r diads [20,21], Such polypropylene, containing a small number of long blocks, is usually referred to as isotactic polypropylene. Note that the polymer chains in this case contain helix sequences of the same handedness (right or left) in the chain, which is due to the same relative configuration of tertiary carbon atoms in these sequences. Stereoblock polypropylene, which is also called block-isotactic polypropylene,... 

During the last decade, a variety of new catalysts have been presented for the stereospecific polymerisation of a-olefins, based on non-bridged metallocene or stereorigid ansa-metallocene as the procatalyst and a methylaluminoxane activator [29,30,37,105-107,112-114,116-135], Apart from isotactic [118,119,124, 131,132] and syndiotactic [23,118,124,133] polypropylenes and other poly(a-olefin)s [121], hemiisotactic [112,121,124], isoblock [131,132,134], syndioiso-block (stereocopolymer) [127], stereoblock isotactic [135] and stereoblock isotactic atactic [116,128,129] polypropylenes have been obtained using these new catalysts. 

Propylene polymerisation with class I and class II catalysts gives rise, in principle, to atactic polypropylene, with class III catalysts to isotactic polypropylene [22] and with class IV catalysts to syndiotactic polypropylene [23], while for the less symmetric class V catalysts no general prediction is possible. In specific cases, however, isotactic, hemiisotactic, stereoblock isotactic attactic as well as syndiotactic polypropylenes can be obtained with class V catalysts, depending on their kind [107,112,116,124,127,137]. 

Class I catalysts obtained from achirotopic metallocene precursors of the Cp2MtX2 type, which produce atactic polypropylene at elevated temperature in the range of ca 50-70 °C (characteristic of propylene polymerisation in the presence of heterogeneous catalysts), can yield stereoblock isotactic polypropylene at lowered temperature, e.g. — 45 °C (Table 3.1) [22]. 

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]. 

Polypropylenes with low amounts of ohgomers atactic, isotactic, isoblock, stereoblock, and syndiotactic... 

Polypropylene. Isotactic polypropylene for all experiments was prepared in the Research Institute for Macromolecular Chemistry in Brno. Technical data MW = 510,000 (calculated from viscosity measurements in Tetralin), atactic fraction = 0.68%, stereoblocks = 6.75%, ash = 0.29%, Ti = 0.0172% (no compounds with Ti—Cl bonds), A1 = 0.11% free of stabilizers. The polymer was stored in an inert atmosphere in sealed ampoules at low temperature in the dark. 

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... 

Stereoblock polypropylene is synthesized by unbridged metallocenes such as bis(neomenthyl)zirconium dichloride (7) with cyclopentadienyl or phenyl substituted indenyl ligands [55, 56]. The stereoblock length increases at lower polymerization temperatures. Products containing isotactic and atactic blocks are elastomeric if the isotactic block length is short. Rieger [57] obtained similar polypropylenes by bridged fluorenyl-substituted indenyl zirconium complexes. 

A sample of commercial atactic polypropylene (Montecatini resin) was purified by extraction with hot toluene followed by precipitation from the toluene solution with methanol to remove isotactic PP contamination. The purified material showed no detectable crystalline melting endotherm by DSC (< 0.5% crystalline content). However, IR indicated the presence of some stereoblock material. The atactic PP was cast into thin films ( 50 pm) on glass from heptane solution, vacuum dried and peeled from the glass with a sharp blade. 

Chiral titanocenes, zirconocenes, and hafnocenes in combination with methylalu-minoxane [A1(CH3)—0] , can lead to highly isotactic propylene. Nonchiral metallocenes like (Cp)2ZrCl2 or other similar compounds produce only pure atactic polypropylenes. Molecular mass of 590,000 for atactic polypropylenes can be achieved by low polymerization r. The activities of these hydrocarbon soluble catalysts are extremely high. Different structures of polypropylenes are obtained when the rr-bonded ligand of the transition metal is varied (Fig. 1). With no other catalyst can atactic, isotactic, stereoblock, isoblock, and syndiotactic polypropylene of such purity be produced. 

The propene units are bonded mainly head to tail. A small part (0.7%) is incorporated in 1,3 enchainment . MW of the polypropylene depends strongly on reaction T. At T between -20 and 60°C, MW range from 300,000 to 1500 . Hafnocenes give polypropylenes with a higher MW °. Stereoblock and isoblock polypropylenes were obtained by bis(neomenthyl) ZrCl2 or dimethyl silyl(cyclopentadienylindenyl)ZrCl2. 

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