Poly stereochemistry

CHR) , formed, e g. from the reaction of diazomethane and alcohols or hydroxylamine derivatives in the presence of boron compounds or with metal compounds. Poly-methylene is formally the same as polyethene and the properties of the various polymers depend upon the degree of polymerization and the stereochemistry. 

The reaction of the allylic acetate with a diene system 784 affords the poly-fused ring system 785 by three repeated alkene insertions[487]. An even more strained molecule of the [5.5.5.5] fenestrane 788 has been constructed by a one-pot reaction in a satisfactory yield by the Pd-catalyzed carbonylation-cycliza-tion of 786 without undergoing elimination of /3-hydrogen in the cr-alkylpalla-dium intermediate 787 owing to unfavorable stereochemistry for syn elimination[488]. 

The poly(vinyl alcohol) made for commercial acetalization processes is atactic and a mixture of cis- and /n j -l,3-dioxane stereoisomers is formed during acetalization. The precise cis/trans ratio depends strongly on process kinetics (16,17) and small quantities of other system components (23). During formylation of poly(vinyl alcohol), for example, i j -acetalization is more rapid than /ra/ j -acetalization (24). In addition, the rate of hydrolysis of the trans-2iQ. -A is faster than for the <7 -acetal (25). Because hydrolysis competes with acetalization during acetal synthesis, a high cis/trans ratio is favored. The stereochemistry of PVF and PVB resins has been studied by proton and carbon nmr spectroscopy (26—29). 

When poly(propylene) was first made, it was found to exist in two possible forms. One was similar to poly(ethylene), but had greater rigidity and hardness the other was found to be amorphous and of little strength. The first of these is now known to be isotactic, that is with a regular stereochemistry at each alternating carbon atom. The other is now known to be atactic, that is with a random distribution of different stereochemical arrangements... 

Given that all of the optically active epoxides in Tables 1 and 2 synthesised using poly-(l)-amino acids are laevorotatory, their absolute stereochemistry has... 

In the following pages we shall treat, in detail, the NMR analysis of two polymers, poly(methyl methacrylate) and polypropylene, which have epitomized, in consecutive epochs, the most classical and fhiitful examples of interaction between NMR spectroscopy and macromolecular stereochemistry. 

The same type of addition—as shown by X-ray analysis—occurs in the cationic polymerization of alkenyl ethers R—CH=CH—OR and of 8-chlorovinyl ethers (395). However, NMR analysis showed the presence of some configurational disorder (396). The stereochemistry of acrylate polymerization, determined by the use of deuterated monomers, was found to be strongly dependent on the reaction environment and, in particular, on the solvation of the growing-chain-catalyst system at both the a and jS carbon atoms (390, 397-399). Non-solvated contact ion pairs such as those existing in the presence of lithium catalysts in toluene at low temperature, are responsible for the formation of threo isotactic sequences from cis monomers and, therefore, involve a trans addition in contrast, solvent separated ion pairs (fluorenyllithium in THF) give rise to a predominantly syndiotactic polymer. Finally, in mixed ether-hydrocarbon solvents where there are probably peripherally solvated ion pairs, a predominantly isotactic polymer with nonconstant stereochemistry in the jS position is obtained. It seems evident fiom this complexity of situations that the micro-tacticity of anionic poly(methyl methacrylate) cannot be interpreted by a simple Bernoulli distribution, as has already been discussed in Sect. III-A. 

There are three principal stereochemical types of poly(l-alkene)s, illustrated in Scheme 8.38 for polypropylene. In isotactic polypropylene 80 (i-PP) all methyl substituents have the same relative orientation (m). The scheme shows the stereochemistry with the usual Fischer projection underneath. In syndiotactic PP (81, s-PP) every second CHMe unit has the opposite stereochemistry to the first, while in atactic PP (82, a-PP) the orientation of the methyl substituents is random. In some polymers there is partial order, i. e. only every second monomer orientation is random (83, hemi-isotactic PP). 

When all of the double bonds in the polymer molecule have the same configuration, the result is two different ordered polymer structures—transtactic and cistactic. Figure 8-5 shows the structures of the completely cis and completely trans polymers of isoprene. The stereochemistry of these polymers is indicated in their names. For example, the trans polymer (IX) is named as trans-1,4-polyisoprene or poly( -l-methylbut-l-ene-l,4-diyl). The first name is the IUPAC-recommended trivial name the second name is the IUPAC structure-based (Sec. l-2c) [IUPAC, 1966, 1981, 1996],... 

In principle, bifunctional aldehydes should be able to engage in twofold enzymatic aldol additions to both of their acceptor carbonyls in a fashion to be classified as a tandem reaction, that is, without the need for isolation of intermediates. Depending on the specificity of the enzyme used and on the functionalization in the starting material, the isomeric constitution as well as the absolute and relative stereochemistry should be deliberately addressable. Therefore, we engaged in a program to evaluate the scope and the Hmitations of such two-directional chain elongation processes for the construction of extended poly functional molecules [36]. 

NMR spectroscopy is sensitive to the polymer composition and stereochemistry, branching, isomerism, head-to-head and tail-to-tail additions, and chain ends. Figures 13.4 and 13.5 show two types of poly(ethylene) [29]. Figure 13.4 is Chevron HDPE LX-1159, which is a high-density poly(ethylene), whereas Fig. 

Scheme 68 illustrates cyclopolymerization of 1,5-hexadiene catalyzed by a homogeneous chiral zirconocene complex to form optically active poly(methylenecyclopentane), whose chirality derives from configurational main-chain stereochemistry (757). This polymer is predominantly isotactic and contains predominantly trans cyclopentane rings. 

Three stereoisomers are possible in the cholestanylindene-derived zir-conocene complexes illustrated in Scheme 67. Two are racem-like, and the other is meso-like depending on the geometry of the metallocene moiety. The stereochemistry of the reaction is controlled by both the structure of the metallocene skeleton and steroidal substituent. Polymerization of propylene with 0-C activated with MAO gave polypropylene of 240,000, about 40% mmmm approximately 70% is due to enantiomorphic site control and the rest is due to chain-end control. Use of the catalyst derived from a /3-A-B mixture produced a mixture of polymers. The a-A and a-B/MAO catalysts afforded isotactic poly-... 

Davidjan et. al.166) have made a study of the influence upon the propagation of poly(isoprenyl)lithium in n-hexane of very small additions of 1,2-dimethoxyethane (DMEractive centers = 0.01). Analysis of polymer obtained at 10% conversion by size exclusion chromatography coupled with a determination of the dependence of the stereochemistry upon molecular weight led them to the conclusion that complexation reduces the reactivity of what they assumed to be the most reactive species, i.e., the non-associated active center. 

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