Isotactic domains

The differences in the properties of polypropylene obtained with homogeneous and supported catalysts are also illustrated by their 13C-NMR spectra (Fig. 8). The spectrum of the polymer formed by C-l is identical to that of isotactic polypropylene (Fig. 8a), whereas the spectra of the polymers produced by supported catalysts (Fig. 8b,c) are typical of the polypropylene in which isotactic domains alternate with many stereodefects [41]. 

The polymerization of propylene using complex 14 activated by MAO (Al Zr ratio=500, solvent toluene, 25 °C) yielded 80 g polymer-mol Zrl-hrl with a molecular weight Mw= 115,000 and polydispersity=2.4 [119]. The reaction was carried out in liquid propylene to avoid, as much as possible, the epimerization of the last inserted monomer unit and to allow rational design of the elastomeric polymer. The formation of elastomeric polypropylene is consistent with the proposed equilibrium between ds-octahedral cationic complexes with C2 symmetry inducing the formation of the isotactic domain, and tetrahedral complexes with C2v symmetry responsible for the formation of the atactic domain (Scheme 7). The narrow polydispersity of the polypropylene obtained supports the polymerization mechanism in which the single-site catalyst is responsible for the formation of the elastomeric polymer. 

Natta also serendipitously isolated polypropylene fractions that exhibited novel elastomeric behavior that he proposed were a manifestation of properties linked to an unique isotactic-atactic stereoblock polypropylene (sbPP) microstructure (Vll in Figure 3.1) [13]. In this model, the elastic properties of sbPP were hypothesized to originate with interchain associations of hard, crystalline isotactic) domains that function as nonbonded physical crosslinks within an amorphous atactic) matrix, with the former serving to dimensionally restore the material upon the removal of a deforming strain. Unfortunately, this sbPP material was not the principal product of a controlled polymerization for which a sound mechanism could be established to account for chain growth that, in this case, must proceed in alternating stereoselective and nonselective fashion. Indeed, both sPP and sbPP... 

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. 

In rubber-plastic blends, clay reportedly disrupted the ordered crystallization of isotactic polypropylene (iPP) and had a key role in shaping the distribution of iPP and ethylene propylene rubber (EPR) phases larger filler contents brought about smaller, less coalesced and more homogeneous rubber domains [22]. Clays, by virtue of their selective residence in the continuous phase and not in the rubber domains, exhibited a significant effect on mechanical properties by controlling the size of rubber domains in the heterophasic matrix. This resulted in nanocomposites with increased stiffness, impact strength, and thermal stability. 

In contrast to isotactic polystyrene, syndiotactic polystyrene crystallises very rapidly (it contains ca 72% crystalline domains) with a crystallisation rate... 

For most polymers, the yield of hydroperoxides is relatively low even in the presence of oxygen excess. The relatively high values were, e.g., obtained during oxidation of atactic polypropylene [79], In the initial phases of oxidation, the yield of hydroperoxide related to 1 mol of oxygen absorbed is 0.6 at 130 °C when passing the maximum concentration it decreases considerably. In isotactic polypropylene, the maximum yield of hydroperoxides attains the value 0.2, only [80]. This may be probably related with a local accumulation of hydroperoxides in domains of defects in the crystalline structure which leads to an increased ratio of participation of hydroperoxide groups in the chain reaction of an oxidation process (induced decomposition of hydroperoxides) and finally to a lower yield of hydroperoxides... 

Isotactic polypropylene is a rather stiff and tough solid material with a melting point of 164°C. Closely packed, CHs-studded helices (Figure 17), rigidly interwoven in crystalline domains (Figure 18), account for the mechanical and thermal resistance of isotactic polymers. Syndiotactic polypropylene has a related crystalline structure, but atactic polymers are amorphous and form oily or waxy materials depending on chain lengths. 

NMR spectroscopy has been applied to investigate the behavior under uniaxially mechanical deformation. A study of drawn fibers prepared from an isotactic polypropylene modified by an ethylene-aminoalkyl acrylate copo-l)uner has been done using the broad line of H NMR. NMR spectra were measured on the set of fibers prepared with a draw ratio X from q to 5.5 at two temperatures, one of them corresponding to the onset of segmental motion and the other one is the middle of the temperature interval as determined by decrease of the second moment 2D time-domain H NMR was used to... 

Figure 2. Influence of atactic (aPM) and isotactic (iPM) PP-g-MA blend compatibilizers on the morphological and mechanical properties of PP—PA6 (70/30) PA6 domain size (a), Youngs modulus (b), yield stress (c), and notched Charpy impact strength (d).

However, as we shall see, the results in the asymptotic domain should reveal a different property of polymer chains. Criticism came from Yoon and Flory,10 disturbed by the absence of local chemical structure effects. These authors modelled the atactic polystyrene chain of N beads as realistically as they could (see Chapter 1). Thus, Yoon and Flory accounted for interactions between nearest neighbour monomers in particular, these interactions are responsible for the fact that the three orientations of a bond j, relative to bonds j — 1,/ — 2, are weighted differently in relation to the stereochemical composition. Moreover, we have seen in Chapter 1 that, for atactic polystyrene, pairs of successive benzene rings are slightly more frequent in syndiotactic than in isotactic positions. 

IPP-EPM blends were generally considered to be immiscible (92-95). Chen et al. (96) have reported miscibility and a LCST in blends of isotactic polypropylene and an ethylene-propylene terpolymer (EPDM). Seki et al. (97) reported that iPP was miscible with EPM with both 19 and 47 mol% of ethylene based on both transmission electron microscopy (TEM) and SANS. However, the molecular weight of the iPP was below 10, OOOgmoP. More recently both Nitta et al. (98) and Kamdar et al. (99) reported that the miscibility between iPP and EPM, whose molecular weight is higher than 100,000 g moP, is strongly dependent on ethylene content in EPM. The miscibility increases with decrease of ethylene content. IPP was miscible with EPM with ethylene content lower than around 17 mol% based on both dynamic mechanical analysis (DMA) and TEM. The EPM domain sizes decrease with decrease of ethylene content as shown in Fig. 2.3. 

The blends of EPDM terpolymers and isotactic PP with curing agents, such as peroxide, phenol resins, and sulfur, are termed as thermoplastic vulcanized elastomer (TPV) since the rubber domains are vulcanized. Polyolefin copolymers, such as random copolymer of propylene with ethylene, copolymers of other olefins, elastomeric PP, and elastomeric PE, are developed with recent advances of... 

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