Definition, stereoregular polymer

The size-dependence of the intensity of single shake-up lines is dictated by the squares of the coupling amplitudes between the Ih and 2h-lp manifolds, which by definition (22) scale like bielectron integrals. Upon a development based on Bloch functions ((t>n(k)), a LCAO expansion over atomic primitives (y) and lattice summations over cell indices (p), these, in the limit of a stereoregular polymer chain consisting of a large number (Nq) of cells of length ao, take the form (31) ... 

Summarising, in the chain-end control mechanism the last monomer inserted determines how the next molecule of 1-alkene will insert. Several Italian schools [7] have supported the latter mechanism. What do we know so far Firstly, there are catalysts not containing a stereogenic centre that do give stereoregular polymers. Thus, this must be chain-end controlled. Secondly, whatever site-control we try to induce, the chain that we are making will always contain, by definition, an asymmetric centre. As we have mentioned above, the nature of the solid catalysts has an enormous influence on the product, and this underpins the Cossee site-control mechanism. Thus both are operative and both are important. Occasionally, chain-end control alone suffices to ensure enantiospecifity. 

The butadiene polymers represent another cornerstone of macromolecular stereochemistry. Butadiene gives rise to four different types of stereoregular polymers two with 1,2 linkage and two with 1,4. The first two, isotactic (62) and syndiotactic (25), conform to the definitions given for vinyl polymers, while the latter have, for eveiy monomer unit, a disubstituted double bond that can exist in the two different, cis and trans, configurations (these terms are defined with reference to the polymer chain). If the monomer units all have the same cis or trans configuration the polymers are called cis- or trans-tactic (30 and 31). The first examples of these stereoisomers were cited in the patent literature as early as 1955-1956 (63). Structural and mechanistic studies in the field have been made by Natta, Porri, Corradini, and associates (65-68). 

As the definition of a stereoregular polymer (see Definitions 1.4 and 1.9) requires that the configuration be defined at all sites of stereoisomerism, structures (8) and (10) do not represent stereoregular polymers. The same is true of (11) and (12), which differ from (8) and (10) in that the sites of specified and unspecified configuration have been interchanged. 

The first publication of the lUPAC in the area of macromolecular nomenclature was in 1952 by the Sub-commission on Nomenclature of the then lUPAC Commission on Macromolecules, which drew on the talents of such remarkable individuals as J. J. Hermans, M. L. Huggins, O. Kratky, and H. F. Mark. That report [1] was a landmark in that, for the first time, it systematized the naming of macromolecules and certain symbols and terms commonly used in polymer science. It introduced the use of parentheses in source-based polymer names when the monomer from which the polymer is derived consists of more than one word, a practice that is now widely followed, and it recommended an entirely new way of naming polymers based on their structure that included the suffix amer , a recommendation that has been almost totally ignored. After ten years, the Sub-commission issued its second report [2], which dealt with the then-burgeoning field of stereoregular polymers. A revision [3] of definitions in the original report appeared four years later. In 1968, a summary report [4] of the activities of the Subcommission was published. 

Our overall plan in this paper is this. Initially, we shall set down a few definitions, and a point of view. We shall then describe a number of principles or causes of stereoselection associated with bonding, steric, thermodynamic, electrical, mechanical, etc., factors, inherent in the stereoprocess. The relation between the properties of intermediates, e.g. carbanion or syn-anti isomers, etc., rate and equilibrium conditions and stereoselectivity will be dealt with. Throughout, we shall examine representative systems and reactions to see how relevant the principles of stereoselection are. No attempt will be made to venture into several important areas, e.g. enzyme stereospecificity, stereoregular polymers, etc. 

It should be mentioned, in connection with definitions of stereoregular and tactic polymers, that a stereoregular polymer is always a tactic polymer, but... 

However before dealing with the main object of the present paper it is necessary to recall some basic definitions concerning stereoregular polymers (the product) and stereospecific polymerization (the reaction). 

This definition refers to ideal structures, but it is conceded that it may be applied also to practical cases where deviations from the ideality are not too large. Despite this, the lUPAC definition is very restrictive. Indeed, stereoregular polymers are not only those which fall under this definition, but also any tactic polymer and any copolymer that contains in the main chain stereoisomerism sites of the same type in a configurationally regular sequence. 

THE CONCEPT OF STEREOREGULARITY IN LINEAR POLYMERS 2.1. Definition of Stereoregular Polymers... 

Scheme 1. Types of polymers considered as stereoregular in this paper, but not conforming to the lUPAC definition of stereoregular polymers, and types of stereoirregular and non-stereoregular polymers (The R s are substituents containing one site of stereoisomerism, and the polymers which contain them are considered to exhibit a regular pattern of configurations for these sites).

In 1992/1994, Grubbs et al. [29] and MacDiarmid et al. [30] described an improved precursor route to high molecular weight, structurally regular PPP 1, by transition metal-catalyzed polymerization, of the cyclohexa-1,3-diene derivative 14 to a stereoregular precursor polymer 16. The final step of the reaction sequence is the thermal, acid-catalyzed elimination of acetic acid, to convert 16 into PPP 1. They obtained unsupported PPP films of a definite structure, which were, however, badly contaminated with large amounts of polyphosphoric acid. 

Polymer whose molecular structure has a definite spatial arrangement. Also known as stereoregular. 

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