Defects, stereochemical

The sterlc defect shown In the Fischer projection formulae for structure I is a consequence of reversed enantioface selectivity in an enantiomorphic site stereochemical control mechanism. The defective stereochemical placement represents a monomer unit that has been accidentally enchained "backwards . A random distribution of mm defects in an otherwise stereoregular syndiotactic chain is the... 

Tacticity or stereochemical arrangement of atoms in three-dimensional space in relation to each other along the polymer chain cannot really be termed a structural defect. But researchers have shown that tacticity has an important bearing on the reactivity and thermal stability of PVC. For this reason tacticity is being discussed under this section. 

Several isospecific Ci-symmetry catalysts have also been described including (12-15). When activated with [Ph3C]+ [B(C6F5)4]-, (12) affords highly regioregular i-PP (mmmm = 95%) with the stereochemical defects predominantly being isolated rr triads, consistent with a self-correcting enantiomorphic site-control pathway. 2,73 The isospecificity was therefore explained by a mechanism... 

The possible occurrence of a back-skip of the chain for catalytic systems based on C2-symmetric metallocenes would not change the chirality of the transition state for the monomer insertion and hence would not influence the corresponding polymer stereostructure. On the contrary, for catalytic systems based on Cs-symmetric metallocenes, this phenomenon would invert the chirality of the transition state for the monomer insertion, and in fact it has been invoked to rationalize typical stereochemical defects (isolated m diads) in syndiotactic polypropylenes.9 376 60 This mechanism of formation of stereoerrors has been confirmed by their increase in polymerization runs conducted with reduced monomer concentrations.65 In fact, it is reasonable to expect an increase in the frequency of chain back-skip by reducing the monomer concentration and hence the frequency of monomer insertion. 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

The evidence for the various types of defect structures is (it is hardly necessary to repeat) provided by X-ray diffraction patterns. The unit cell dimensions, the chemical analysis, and the density settle the composition of the unit cell, and the intensities of the reflections settle the positions of the atoms. Those who studied these structures were forced to the rather surprising conclusions by this evidence. The moral of this tale is that the implications of X-ray diffraction patterns (in conjunction with reliable chemical analyses and densities) should be accepted boldly, even if they conflict with geometrical ideals (the application of the theory of space-groups) or with stereochemical preconceptions. Only in this way is new knowledge and a deeper comprehension of the crystalline state attained. 

When the stereocontrol occurs by a chain end control mechanism, a stereochemical defect results in the stereochemistry of the defect being propagated along the chain until the next defect occurs (polymer 1 in Fig. 7). If a stereochemical defect occurs in a polymerization using an initiator that exhibits enantiomeric site control, the mistake will be rectified with the next incoming lactide unit (polymer 2 in Fig. 7). This is because it is the chirality of the metal centre which determines the PLA tacticity and not that of the last inserted lactide unit. 

Efficient and defect-free folding of the polymer will, at least, require a control of the regioselectivity of monomer incorporation and polymer tacticity. If, for example, the degree of tacticity control is poor, the stereochemical defects are irreversibly incorporated into the polymer, and the subsequent folding may fail or produce defective structures. By contrast, dynamic self-assembly may allow defects to be corrected and the hierarchical structure to be controlled or fine-tuned using external parameters (solvent, additives, temperature) prior to covalent fixation by polymerization. 

How can one explain the occurrence of steric defects in tactic poly(ot-olefin)s Explain why high-resolution nuclear magnetic resonance is the most convenient method for determining the chain micro structure in poly(a-olefin)s. Consider how 3H and 13C NMR spectroscopy can provide stereochemical information concerning a-olefin polymer chains on the diad level (m, r) and the triad level (mm, rr, mr). Explain why /1-olefins, which do not homopolymerise (without isomerisation) in the presence of Ziegler-Natta catalysts, undergo copolymerisation with ethylene in the presence of these catalysts. 

Supported metallocene catalyst systems are preferred to soluble versions in conventional polyolefin plants, which were designed to use supported Ziegler-Natta or Cr203-based catalysts. Metallocenes can be supported on a number of substrates, such as Si02, MgC or AI2O3. Supported catalysts also provide polypropylene with fewer stereochemical defects. 

There are four possible combinations of regiochemistry and stereochemistry within diad units. The olefins can join in a head-to-head or head-to-tail fashion. Most polyolefins formed by early metal catalyts are formed by strict head-to-tail enchainment, but this head-to-tail enchainment can occur by a series of 1,2-insertions in which the a-olefin substituent is located P to the metal in tihe insertion product or 2,1-insertions in which the a-olefin substituent is located a to the metal in the insertion product. In addition, the olefins can join to give rise to a diad unit containing identical (meso, m) or opposite (racemo, r) stereochemical relationships to the last inserted monomer unit. Site control of polymerization to form isotactic polymer gives rise to rr-defects m the polymer from stereoerrors, but cham-end control of polymerization to form isotactic polymer gives rise to r-defects from stereoerrors. 

C12H22O11 342.299 Evidence for the proposed struct, and stereochem. of the nat prod. Coyolosa was highly defective and has been shown to be incorrect by synthesis. Claimed constit. of the roots of Acro-comia mexicana. Hypoglycaemic agent (nat. prod.). Cryst. solid (MeOH). 

Stereochemical defects, e.g., syndiotactic configurations in an otherwise isotactic chain. 

It is obvious that, because of intramolecular interactions, the introduction of some kind of chemical and/or stereochemical defects forces also the introduction of conformational defects. 

Copolymerization of L-lactide with other analogous cyclic lactones, such as DL-lactide, D-lactide, glycolide, or 8-capro-lactone, produces polymers with relatively random distribution of comonomers. The Tg of PL A copolymers decreases proportionally to the content of the glycolide or 8-caprolac-tone comonomer to some extent. Moreover, the presence of stereochemical defects in PLLA reduces rate of crystallization, and extent of crystallization of the resulting... 

The NMR spectrum for polypropylene produced at 50°C with this catalyst reveals a high degree of stereocontrol (81% rrrr pentads). Moreover, analysis of the stereochemical defects (predominantly rmmr pentads) were indicative of a site control mechanism. For a site control mechanism to operate in syndiospecific polymerization, the olefin must alternately bind to coordination sites with opposite enantioface selectivity. The model for this polymerization is shown in Scheme HI. 

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