Atactic domains

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. 

This category includes such polymers as atactic polystyrene (25-291 or poly(vinylchloride) (30.31 and references therein). A closely related problem is the gelation of non-block copolymers (5), which share with atactic polymers the feature that chemically and conformationally homogeneous sequences may be relatively short, so that when two or more chains interact, large crystalline domains are prevented from forming. 

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. 

Fig. 1.6 TEM image of a RuOi-stained thin film of an atactic polystyrene/Kraton G1650 blend containing 30% polystyrene. Swollen styrene domains can be seen within the block copolymer together with the larger phase-separated polystyrene regions.

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. 

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... 

In the above process, the formation of crystalline domains involves consecutive insertions from one of the lateral coordination sites of the catalyst so as to give rise to isotactic sequences, whereas consecutive insertions at the other site (2) give rise to atactic amorphous sequences. Interconversion between these two states must occur within the lifetime of a given polymer chain in order to generate a physically cross-linked network and is believed to occur via occasional isomerizations of the polymer chains (i.e., interconversion at the metal center). Preparation of an oscillating catalyst that yields an elastomeric polypropylene was also reported by others [441]. 

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