Stereochemistry polymers and

Polymer Stereochemistry and Optical Activity.—Although stereoregular products from carbocation propagations are not common, there are some notable examples, perhaps the most important being the isotactic materials from alkyl vinyl ethers. Recently, novel catalysts based on phosphoryl and thionyl chlorides with vanadium pentoxide have been added to those initiator systems capable of producing stereospecific reactions. 

Gin DL, Conticello VP, Grubbs RH (1994b) Stereoiegular precursors to polylp-phenylene) via transi-tion-metal-catalyzed polymerization. 2. The effects of polymer stereochemistry and acid catalysts on precursor aromatization a characterization study. J Am Chem Soc 116 10934-10947 Goldschmiedt G (1886) Monatsh Chem 7 40... 

This section defines the basic concepts of polymer stereochemistry, and describes how the stereochemistry is manifest in NMR spectra. 

Historically vinyl polymers were the first to be classified stereochemically and the first to be studied by NMR. In some respects they are also the simplest. The basic concepts of polymer stereochemistry and its influence on NMR spectra are developed first with specific reference to vinyl polymers. The ideas introduced are then extended to other systems. 

The final part of this book features tactic polymerizations of functional and nonolefinic (ring-opening) monomers— materials for which many aspects of polymer stereochemistry and microstructure control are very different than for simple polyolefins. Acrylate, epoxide, and lactide polymerizations are addressed in this part, along with tactic olefin/carbon monoxide co- and terpoly-mers. These final chapters provide an expanded view of polymer microstructures and stereocontrol strategies, such as enantiomer-selective polymerization, that may be less familiar to the polyolefin-minded chemist and serve to enhance the reader s overall understanding of stereoregular polymers and polymerization. 

It should be stressed that this treatment of polymer stereochemistry only deals with relative configurations whether a substituent is "up or down" with respect to that on a neighboring unit. Therefore, the smallest structural unit which contains stereochemical information is the dyad. There are two types of dyad meso (m), where the two chiral centers have like configuration, and racemic /-), where the centers have opposite configuration (Figure 4.1). 

Microstructure (see also Stereochemistry and Tacticity) 114,115.128.138,139 Miscibility (see also Compatibility) 12, 53, 68 Model networks 163 Modification of a polymer, chemical 154 Mold release 71, 74 Molecular weight, control 147. 154... 

This chapter is concerned with aspects of the structure of polymeric materials outside those of simple chemical composition. The main topics covered are polymer stereochemistry, crystallinity, and the character of amorphous polymers including the glass transition. These may be thought of as arising from the primary structure of the constituent molecules in ways that will become clearer as the chapter progresses. 

The [Ni(NCS)f,]4 ion is almost perfectly octahedral, with Ni—N distances of around 209.5 pm and N—Ni—N angles around 89.5°. The Ni—N—C and N—C—S entities are practically linear.438,439 In [Ni(NCS)2L2] where L is a R-substituted pyridine, stereochemistry and spin state depend on the type and positions of R.431 While for 2-Me- and 2-Et-pyridine square planar complexes are observed, other pyridins lead to coordination polymers with pseudo-octahedral Ni11 due to N,S-bridging thiocyanate. Ni11 thiocyanato complexes have been studied quite intensively as hosts for inclusion compounds.440"442... 

The aim of this work was to find out how to get more information about stereochemistry and molecular motion of polymer methyl- and methyl-phenyl-siloxanes by measuring longitudinal relaxation times, Tj, and nuclear Overhauser effects, NOE, of the individual building blocks. 

Figure 7.12 Plots of qc vs. T for cholesteric aqueous solutions of short fragments of DNA, 5-dGMP, and dG4 Filled symbols refer to heating scans, while open symbols to cooling scans. While DNA and the G-wire of dG4 are right-handed, the G-wire of 5 -dGMP is left-handed Slopes and intercepts reflect the polymer stereochemistry. (Reprinted with permission of Wiley— VCH from Chemistry—A European Journal, Vol. 6, p. 3249 ad ff., copyright 2000.)...

A most interesting extension of this type of reaction was performed by Addadi and Lahav (175). Their aim was to obtain chiral polymers by performing die reaction in a crystal of chiral structure. They employed monomers 103. The initial experiments were with a chiral resolved 103 where R1 is (R)- or ( -sec-butyl and R2 is C2H3. This material indeed crystallizes in the required structure, and yields photodimers and polymers with the expected stereochemistry, and with quantitative diastereomeric yield. It was possible to establish that the asymmetric induction was due essentially only to the chirality of the crystal structure and not to direct influences of the sec-butyl. Subsequently they were able, using sophisticated crystal engineering, to obtain chiral crystals from nonchiral 103, and from them dimers and polymers with high, probably quantitative enantiomeric yields. This may be described as an absolute asymmetric polymerization. 

Given the vastness of the subject matter I have limited myself to dealing with the structural (or static) aspects of macromolecular stereochemistry. An adequate treatment of the stereochemistry of polymerization, with specific regard to the polymerization of olefins and conjugated diolefins, would have occupied so much space and called for such a variety of additional information as to make this article excessively long and complex. I trust that others will successfully dedicate themselves to this task. However, the connection between polymer structure and polymerization mechanism is so important that the fundamentals of dyruunic macromolecular stereochemistry cannot be completely ignored in this chapter. 

The above analysis represented the first example of the combined use of spectroscopic results and statistical calculations to define the polymerization mechanism, a combination which, today, is the norm for those who work in polymer stereochemistry. 

As in many other aspects of polymer stereochemistry, polypropylene also plays a central role in NMR spectroscopy. Since 1962 numerous articles have dealt with the interpretation of its proton spectrum (125-128) the state of knowledge at the end of that decade has been well described by Woodbrey (117). The difficulty in this study stems fiom two factors The narrow frequency range comprising the entire spectmm and the large homonuclear coupling between CH2, CH, and CH3 protons. The whole spectrum is within a range of <1.5... 

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. 

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