Stereoblock propylenes

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. 

Chien JCW, Llinas GH, Rausch MD, Lin YG, Winter HH, Atwood JL, Bott SG (1992) Metallocene catalysts for olefin polymerizations. XXIV. Stereoblock propylene polymerization catalyzed by rac-anri -ethylidene(l-T 5-tetramethylcyclopentadienyl)(l-r 5-indenyl) dimethyltitanium A two-state propagation. J Polym Sci A 30 2601-2617... 

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... 

Isotactic polystyrenes (IPS), 10 180 23 365 Isotactic propylene polymers, 17 703, 704 Isotactic-syndiotactic stereoblock PP, 16 110... 

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]. 

The syndiospecific polymerisation of propylene with soluble vanadium-based Ziegler Natta catalysts is not completely regiospecific [389 392], i.e. the monomer unit enchainment is not entirely head-to-tail. In addition to syndiotactic stereoblocks, the polymer also contains sterically irregular stereoblocks. The whole polymerisation can be thus described as a copolymerisation with four head-to-tail and tail-to-tail stages [2,379]. 

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]. 

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. 

As discussed earlier, ethylene propylene rubber (EPR or EPM) has been blended with PP and PE to improve the impact strength and to render the materials softer. Recently, metallocene catalysts or postmetallocene catalysts provide new pathways to generate elastic copolymers that can replace EPR. These pathways possess cheaper manufacturing cost and generate new materials with better compatibility to PP or PE. Such new materials included ethylene-propylene random copolymers with dominant ethylene component (33-34) or propylene-dominant component (35 1), propylene-ethylene block copolymer (42), ethylene-octene copolymer (43), poly(propylene-co-ethylene) (44), ethylene-hexene copolymer (45), ethylene-butene copolymer (46), low isotactic PP (47), and stereoblock PP (48). These materials are generally compatible with PP or PE, thus can be used to tailor the toughness (or the softness) of... 

MAJOR APPLICATIONS The polymers referred to in this chapter include those families of homopolymers of propylene which are known to have elastomeric recovery properties at reasonable molecular weight and for which properties have been attributed to a crystallizable-noncrystallizable (e.g., isotactic-atactic) stereoblock structure, or to a major component with a stereoblock structure, whether or not the compositions are homogeneous by solvent fractionation tests. Copol)rmers and blends are not deliberately included in the data presented, but are described in some of the references. (See also some of the closely related elastomeric polymers presented in the entry on Polypropylene, atactic in this handbook.) The criterion of multiple crystallizable blocks per polymer chain may be met in significant fractions of low-tacticity, low-stereoregularity polymers of very high molecular weight. 

The catalyst structures presented in the previous section have been used to prepare many types of polyolefins. The following several sections of this chapter provide an overview of the synthesis of polypropylenes containing various microstructures. These sections describe a selection of the catalysts that have been used to generate isotactic, syndiotactic, hemiisotactic, and stereoblock copolymers. Subsequent sections of this chapter describe catalysts that give rise to various copolymers of ethylene and propylene. Again, representative examples are provided, and the reader should consult other sources for comprehensive coverage of the synthesis and properties of each type of copolymer. ... 

More importantly, while it is very difficult to control the nature of the site t)rpes on conventional heterogeneous Ziegler-Natta catalysts, metallocene catalysts can be designed to synthesize PP with different chain microstructures. PP chains with atactic, isotactic, isotactic-stereoblock, atactic-stereoblock and hemiisotactic configurations can be produced with metallocene catalysts (Figure 2). It is also possible to s)mthesize PP chains that have optical activity by using only one of the enantiomeric forms of the catalyst. Additionally, copolymers of propylene, ethylene and higher a-olefins made with metallocene catalysts have random (or near random) comonomer incorporation and narrow chemical composition distributions (CCD). 

For the synthesis of stereospecific metallocene catalysts for propylene polymerization, C2 symmetric precursors are necessary to obtain a catalyst for isospecific polymerization, and C5 symmetric precursors to produce a catalyst for syndiospecific polymerization. Asymmetric precursors can be used to synthesize metallocene catalysts that produce hemiisotactic and isotactic-stereoblock PP. Farina et al. [7] have proposed an useful classification of metallocene catalysts based on their symmetry (Figure 3). 

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