Growth, isotactic

Fig. 7.13, this shifts the vacancy—represented by the square-in the coordination sphere of the titanium to a different site. Syndiotactic regulation occurs if the next addition takes place via this newly created vacancy. In this case the monomer and the growing chain occupy alternating coordination sites in successive steps. For the more common isotactic growth the polymer chain must migrate back to its original position. 

Figure 8.1. (a) Spherulites growing in a thin film of isotactic polystyrene, seen by optical microscopy with crossed polars (from Bassett 1981, after Keith 196.3). (b) A common sequence of forms leading to sphertililic growth (after Bassett 1981). The fibres consist of zigzag polymer chains. 

Isotactic (Section 31.2) A chain-growth polymer in which the substituents are regularly oriented on the same side of the backbone. 

Figure 6 Spherulites of isotactic poly-l-butene (a, during growth) and of polyethylene (b, after completion) by optical microscopy (OM) under crossed polars. Reproduced from Ref. [3] with permission of John Wiley Sons, Inc.

In a system with site control ideally the mistake leads to a single odd insertion in the chain the site enforces for instance the growth of an isotactic chain with all m configurations and after one mistake has occurred it will return to producing the same configurations. In other words, an isospecific catalyst will produce a polymer chain with all methyl groups pointing towards us (as in... 

Beers KL, Douglas JE, Amis EJ, Kaiim A (2003) Combinatorial measurements of crystallization growth rate and morphology in thin films of isotactic polystyrene. Langmuir 19 3935-3940... 

Figure 1.4- Dependence of spheralitic growth rate (cm/s) on supercooling (AT) for isotactic polystyrene [27].

In this case, the value 0.4 X 10 mol. of C2H6 mol. of a-titanium trichloride was found for C. Such active centers, although they represent only % of the whole amount of centers present in the polymerization, generate nearly 93 % of the whole amount of polymer. Such a result may be interpreted by the assumption that the growth rate of the amorphous polymeric chains (having a molecular weight which is lower than that of the isotactic chains) is about % of the growth rate of isotactic chains. 

In the present paper we report results obtained with t-butyl-and phenyl-magnesium compounds which under suitable conditions give highly isotactic polymers. We are in particular looking to see the extent to which the character of the polymerization is predetermined during the initiation stage when the stable and persistent growth centres are formed, and whether monomer is coordinated to the active centre in these cases. 

The influence "of THF is shown in Figure 1, where the frequencies of the iso-, syndio- and hetero-tactic triads, i, s and h, of each sample can be read off along the appropriate median (e.g. the apex i corresponding to i = 1-0, the base opposite i corresponding to i = 0). The curve represents the Bernoullian triad distribution (h = 2i1/2s1 2). The black circles refer to samples prepared in toluene solution containing the optimum trace of THF (Xthf s . ) for initiation of isotactic growth centres. 

Isotactic polymerization requires the presence of halide, and is critically dependent on the nature of the alkyl or aryl group. At least one of each of these groups must be present in the vicinity of the active centre. The requirement of a minimum amount of THF for the formation of an isotactic growth site implies that this must also be coordinated at this site. We have evidence that monomer is complexed at the active centre, which has been postulated for many years (10,15). 

Abstract The fracture properties and microdeformation behaviour and their correlation with structure in commercial bulk polyolefins are reviewed. Emphasis is on crack-tip deformation mechanisms and on regimes of direct practical interest, namely slow crack growth in polyethylene and high-speed ductile-brittle transitions in isotactic polypropylene. Recent fracture studies of reaction-bonded interfaces are also briefly considered, these representing promising model systems for the investigation of the relationship between the fundamental mechanisms of crack-tip deformation and fracture and molecular structure. 

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