Steric defects

In practice, there is no such thing as a pure isotactic or syndiotactic polymer. Once again, we find that polymers comprise a statistical distribution of chemical structures. Polymers that contain steric centers inevitably incorporate a certain number of steric defects that prevent us from obtaining 100% isotacticity or syndiotacticity. Polymer manufacturers vary the catalyst type and reaction conditions to control the tacticity level and the resulting properties. 

There is no such thing as a pure polymer. All polymers comprise molecules that exhibit chemical and physical distributions of many variables these include molecular weight, branching, steric defects, molecular configuration, preferential chain orientation, and crystallite size and shape. The properties and characteristics that we exploit in polymers are controlled by the overall balance of these distributions. 

In the simpler cases, the discrimination between the two faces of the prochi-ral monomer may be dictated by the configuration of the asymmetric tertiary C atom of the last inserted monomer unit (chain-end. stereocontrol) or by the chirality of the catalytic site (chiral site stereocontrol). The distribution of steric defects along the polymer chain may be indicative of which kind... 

Scheme 1.1 Typical steric defects in a (mainly) isotactic poly-l-alkene chain (adapted Fisher projections) for (a) chain-end-stereocontrol (b) enantiomorphic site stereocontrol.

Steric Defects in Stereospecific Chain growth Reactions and Analysis of Polymer Stereoregularity... 

Let us consider typical steric defects in (mainly) isotactic poly(a-olefin) chains. One can imagine two ways in which a minor configurational error may be introduced into an isotactic polymer chain a steric propagation error, denoted as... 

Steric defects in isotactic polypropylene, which involve the appearance of isolated r diads or pairs of r diads, may be considered on a pentad level (Figures 3.45a and b respectively). The 13C NMR signals associated with occasional stereoerrors in the propylene isotactic polymers produced by chiral metallocene-based catalysts (pairs of r diads) indicate that the polymerisation stereochemistry is governed by the enantiomorphism of catalytic sites an error pentad distribution close to mmmr.mmrr.mmrm-.mrrm = 2 2 0 1 is observed... 

Figure 3.47 Typical steric defects (pentad distribution of stereoerrors) in a (mainly) syndiotactic poly(a-olefin) chain (a) isolated m diad, characteristic of chain end stereocontrol (b) pair of m diads, characteristic of enantiomorphic site stereocontrol during the propagation...

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. 

For the catalysts of different composition two polypropylene fractions can be singled out which differ greatly in their stereoregular structure (Table 6) a) a fraction insoluble in boiling n-heptane with an insignificant amount (1-2%) of steric defects of the type... 

The distribution of steric defects along the polymer chain may be indicative of which kind of stereocontrol is operative. The type and amount of stereomistakes (enantioface insertion errors) is measured by solution 13C NMR spectroscopy, a sensitive technique that is able to see the steric environment of a given propylene unit up to undecads (five propylene units on each side of the observed monomeric unit). Routine analysis is usually performed at the pentad level (two propylene units on each side of the observed monomeric unit).162,179 The microstructures which result from stereomistakes are shown in Scheme 7. 

Again, NMR spectroscopy represents the method of choice to distinguish between chain-end and catalytic-site control mechanisms using the PP microstructure as fingerprint. Recently, Randall reported the characterization of steric defects in PP by means of NMR spectroscopy for iPP as well as sPP [27]. As outlined in Figiue 12, false insertion affords a change in chirality of the stereogenic carbon atom of... 

However, the examination of the steric defects alone in the homopolymerization cannot be considered as fully demonstrative. In fact, it is difficult to deduce probative data on the stereospecific polymerization by examining just those units that are inserted in the chain in a disordered way. In order to obviate this difficulty we started research on stereoregular copolymers of propylene with a low amount of 60% enriched 1 ethylene. 

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