Stereospecific polymerization structures

He was a Professor of Industrial Chemistry, School of Engineering, Polytechnic Institute of Milan, Milan, Italy since 1937. He became involved with applied research, which led to the production of synthetic rubber in Italy, at the Institute in 1938. He was also interested in the synthesis of petrochemicals such as butadiene and, later, oxo alcohols. At the same time he made important contributions to the understanding of the kinetics of some catalytic processes in both the heterogeneous (methanol synthesis) and homogeneous (oxosynthesis) phase. In 1950, as a result of his interest in petrochemistry, he initiated the research on the use of simple olefins for the synthesis of high polymers. This work led to the discovery, in 1954, of stereospecific polymerization. In this type of polymerization nonsymmetric monomers (e.g., propylene, 1-butene, etc.) produce linear high polymers with a stereoregular structure. 

In the polymer field, reactions of this type are subject to several limitations related to the structure and symmetry of the resultant polymers. In effect, the stereospecific polymerization of propylene is in itself an enantioface-diflferen-tiating reaction, but the polymer lacks chirality. As already seen in Sect. V-A there are few intrinsically chiral stractures (254) and even fewer that can be obtained from achiral monomers. With two exceptions, which will be dealt with at the end of this section, optically active polymers have been obtained only from 1- or 1,4-substituted butadienes, fiom unsaturated cyclic monomers, fiom substituted benzalacetone, or by copolymerization of mono- and disubstituted olefins. The corresponding polymer stmctures are shown as formulas 32 and 33, 53, 77-79 and 82-89. These processes are called asymmetric polymerizations (254, 257) the name enantiogenic polymerization has been recently proposed (301). 

Polymerizations that yield tactic structures (either isotactic or syndiotactic) are termed stereoselective polymerizations. The reader is cautioned that most of the literature uses the term stereospecific polymerization, not stereoselective polymerization. However, the correct term is stereoselective polymerization since a reaction is termed stereoselective if it results in the preferential formation of one stereoisomer over another [IUPAC, 1996]. This is what occurs in the polymerization. A reaction is stereospecific if starting materials differing only in their configuration are converted into stereoisomeric products. This is not what occurs in the polymerization since the starting material does not exist in different configurations. (A stereospecific process is necessarily stereoselective but not all stereoselective processes are stereospecific.)... 

Full understanding of the stereospecific polymerization, especially of the asymmetric one, is one of the most fascinating problems in synthetic organic chemistry as well as in synthetic polymer chemistry. Requirement posed on the stereospedfic polymerization may be severer than that on the stereospedfic reaction of low molecular wdght compound, because any side reaction, if occurred, puts the structurally and stereochemically irregular units, which cannot be separated from the regular one, in the polymer molecule. 

We should like to consider a molecular mechanism of stereoregulation in the stereospecific polymerization of aliphatic monoaldehydes by carrying out a molecular model examination based on the structure, chemical behavior and catalytic behavior of the catalyst. 

Yasuda,H., Araki,T., Tani,H. Monomer-catalyst complex in the stereospecific polymerization of aliphatic aldehyde. The structure and chemical behavior of the aldehyde complex of [R2 A10CR NPh]2. J. Organometal Chem. in press. 

Some information is available on other acrylates. N,N-disubstituted acrylamides form isotactic polymers with lithium alkyls in hydrocarbons (12). t-Butylacrylate forms crystallizable polymers with lithium-based catalysts in non-polar solvents (65) whereas the methyl, n-butyl, sec-butyl and isobutyl esters do not. Isopropylacrylate also gives isotactic polymer with lithium compounds in non-polar solvents (34). The inability of n-alkylacrylates to form crystallizable polymers may result from a requirement for a branched alkyl group for stereospecific polymerization. On the other hand lack of crystallizability cannot be taken as definite evidence of a lack of stereoregulating influence, as sometimes quite highly regular polymer fails to crystallize. The butyllithium-initiated polymers of methylmethacrylate for instance cannot be crystallized. The presence of a small amount of more random structure appears to inhibit the crystallization process1. 

Stereospecific Polymerization. In the early 1950s, Ziegler observed that certain heterogeneous catalysts based on transition metals polymerized ethylene to a linear, high density material at modest pressures and temperatures. N atta showed that these catalysts also could produce highly stereospecific poly-a-olefins, notably isotactic polypropylene, and polydienes. They shared the 1963 Nobel Prize in chemistry for their work. More recently, metallocene catalysts that provide even greater control of molecular structure have been introduced. 

Heterobicyclic compounds often display specific behavior in their ring-opening polymerization. This is attributed to their rigid and bulky structures which contain, in most cases, two or more asymmetric carbon atoms. Sometimes, stereoelectronic effects involving heteroatoms also play an important role in regulating polymerization processes. For example, racemic bicyclic acetals such as 6,8-dioxabicy-clo[3.2.1]octane and its derivatives often undergo stereospecific polymerization even in the presence of conventional Lewis acid initiators. 

By polymerizing the trans isomer of 1,3-pentadiene two different types of crystalline cis-1,4 polymers have been obtained, one with an isotactic, the other with a syndiotactic structure. The isotactic polymer was obtained by homogeneous systems from an aluminum alkyl chloride and a cobalt compound, the syndiotactic one by homogeneous systems from an aluminum trialkyl and a titanium alkoxide. Some features of the polymerization by Ti and Co catalysts are examined. IR and x-ray spectra, and some physical properties of the crystalline cis-1,4 polymers are presented. The mode of coordination of the monomer to the catalyst, and possible mechanisms for the stereospecific polymerization of pentadiene to cis-1,4 stereoisomers are discussed. 

The possibility of obtaining, from the same monomer, different stereoregular polymers having an isotactic or a syndiotactic structure is, perhaps, the most challenging feature of the stereospecific polymerization. 

Previously, we synthesized and studied various Group 4 complexes with different ligations as alternatives to the cyclopentadienyl ligand. Here we present an overview of the synthesis, structure, and catalytic properties in the polymerization of a-olefins of several zirconium octahedral complexes. We show how the stereoregular polymerization of a-olefins using these octahedral zirconium complexes can be modulated by pressure. These results raise conceptual questions regarding the general applicability of ds-octahedral C2-sym-metry complexes to the stereospecific polymerization of a-olefins. 

According to this classification, the polymerization type can usually be easily determined. The structure of the initiator, the manner of its reaction with the monomer, the effects of the medium and last, but not least, sensitive spectroscopic or resonance methods usually, but not always, provide sufficiently convincing information. We know systems containing radical ions. Several years ago it was sometimes assumed that stereospecific polymerizations (now classified as coordination polymerizations) proceed by a radical or cationic mechanism. 

The second approach is a popular route to cationic lanthanide alkyl complexes, which have proven to be the important intermediates for ethylene polymerization and the stereospecific polymerization of diene [5]. Various monocationic lanthanide monoalkyl complexes have been synthesized by the alkyl abstraction/elimination reaction of lanthanide dialkyl complexes. The reaction of a bisbenzyl scandium complex supported by P-diketiminate with B(C6Fs)3 affords the cationic complex with a contact ion pair structure, in which a weak bonding between the cation and the anion exists (Figure 8.21) [77]. The reaction of an amidinate... 

Miyashita, A., Mabika, M., Suzuki, T., presented at International Symposium on Synthetic, Structural and Industrial Aspects of Stereospecific Polymerization Proceeding, Milan, 1994. 

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