Polypropylene block-isotactic

Polypropylene block and graft copolymers are efficient blend compatibilizers. These materials allow the formation of alloys, for example, isotactic polypropylene with styrene-acrylonitrile polymer or polyamides, by enhancing the dispersion of incompatible polymers and improving their interfacial adhesion. Polyolefinic materials of such types afford property synergisms such as improved stiffness combined with greater toughness. 

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,... 

Figure 6.10 Temperature-dependent equilibrium between chiral and achiral isomers of a metallocene catalyst and the resultant polypropylene with isotactic and atactic polymer-blocks.

In 1995 Coates and Waymouth ° reported a catalyst that produced polypropylene-containing blocks of atactic polypropylene and isotactic polypropylene. This novel elastomeric material was referred to as elastomeric homopolypropylene (EHPP). The unit cell of the crystal structure of the EHPP catalyst precursor bis(2-phenylindenyl)zirconium dichloride was observed to contain two distinct conformers see Figure 1 for the structures. In one conformer. 8M. the indenyl ligands were syn to one another (a meso stereochemistry), while in the other conformer. 8R. the indenyl ligands were anti to one another (a rac stereochemistry). Production of isotactic blocks of polypropylene could be explained by polymerization from the rac active site and production of atactic blocks by polymerization from the meso active site. In 1996 Pietsch and Rappe published a molecular... 

FIGURE 7.21 Superposition of ion pair (solid line) and bare cation (dashed line) insertion transition state models for isotactic polypropylene block formation with 6r/6r MA09. 

FIGURE 8.13 Sawhorse representation of two isotactic polypropylene blocks with same (a) or opposite (b) relative configurations separated by a short stereoirregular sequence. 

Busico, V., Cipullo, R., Friederichs, N. etal. (2003) The first molecularly characterized isotactic polypropylene-block-polyethylene obtained via quasi-hving insertion polymerization. Macromolecules, 36,3806-3808. 

Radulescu, A., Mathers, R.T., Coates, G.W. etal. (2004) A SANS study of the self-assembly in solution of syndiotactic polypropylene homopolymeis, syndiotactic polypropylene-block-poly(ethylene-co-propylene) diblock copolymers, and an alternating atactic-isotactic multisegment polypropylene. Macromolecules, 37,6962 971. 

Bar et al. [71] characterized the morphology of blends of poly(styrene)-WocA -poly(ethene-co-but-l-ene)-WocA -poly(styrene) with isotactic and atactic polypropylene block copolymers by ICAFM. Samples deposited from solution onto a glass substrate, dried, and annealed or quenched from the melt and by samples cut by an ultramicrotome were compared with earher TEM results. The polymer film on the side of the film-glass interface was studied rather than the free surfaces of the polymer. 

Similarly, the random introduction by copolymerization of stericaHy incompatible repeating unit B into chains of crystalline A reduces the crystalline melting point and degree of crystallinity. If is reduced to T, crystals cannot form. Isotactic polypropylene and linear polyethylene homopolymers are each highly crystalline plastics. However, a random 65% ethylene—35% propylene copolymer of the two, poly(ethylene- (9-prop5lene) is a completely amorphous ethylene—propylene mbber (EPR). On the other hand, block copolymers of the two, poly(ethylene- -prop5iene) of the same overall composition, are highly crystalline. X-ray studies of these materials reveal both the polyethylene lattice and the isotactic polypropylene lattice, as the different blocks crystallize in thek own lattices. 

As shown in the previous section the mechanical and thermal properties of polypropylene are dependent on the isotacticity, the molecular weight and on other structure features. The properties of five commercial materials (all made by the same manufacturer and subjected to the same test methods) which are of approximately the same isotactic content but which differ in molecular weight and in being either homopolymers or block copolymers are compared in Table 11.1. 

A new generation coordination catalysts are metallocenes. The chiral form of metallocene produces isotactic polypropylene, whereas the achiral form produces atactic polypropylene. As the ligands rotate, the catalyst produces alternating blocks of isotactic and atactic polymer much like a miniature sewing machine which switches back and forth between two different kinds of stitches. 

In the polymer filed, new-generation metallocenes, which are currently used in many polyethylene and polypropylene processes, can polymerize proplylene in two different modes alternating blocks of rigid isotactic and flexible atactic. These new developments and other changes and approaches related to polymerization are noted in Chapters 11 and 12. 

Blend with Isotactic Polypropylene and Styrene-Ethylene-Butylene-Styrene Block Copolymer... 

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... 

The effect of heterogeneous nucleation on the crystallization of isotactic polypropylene from the melt can be easily established as follows. A small amount of powdered polypropylene is well mixed with about 0.1 wt% of sodium benzoate in a mortar or by means of an analytical mill. Some of the mixture is transferred with a spatula to a microscope slide and melted at about 250 °C on a hot block. A cover slip is pressed on to the melt with a cork to obtain as thin a film as possible.The sample is held at 200-250 °C for some minutes and then allowed to crystallize at about 130 °C on the hot stage of the microscope an unadulterated polypropylene sample is crystallized in the same way. Both samples are observed under a polarizing microscope during crystallization,the difference in spherulite size between nucleated and untreated polypropylene can be seen very clearly. An ordinary microscope can also be used by placing polarizers on the condenser and eyepiece, and adjusting these to give maximum darkness. 

The data from this table illustrate the semicompatibility of the phase between isotactic polypropylene and the high density polyethylene with block copolymer without gross interference in the domain structure or the crystalline phases that exist in these TPR s. 

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. 

Toughened polypropylene may be prepared by block copolymerization in which ethylene monomer is added during the final stages of the polymerization of propylene (4). Thus, some polypropylene chains would contain an end block of rubbery ethylene-propylene copolymer. Alternatively, a blend of an elastomeric copolymer of ethylene and propylene (EPR or EPDM) with isotactic polypropylene (PP) can produce an impact-resistant polymer (5). 

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