Tactic polymer, definition

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

According to the lUPAC definition, a tactic polymer is a regular polymer the molecules of which can be described in terms of only one species of configurational repeating unit in a single sequential arrangement. 

It is clear that a determination of any two triad fractions allows a complete definition of both the triad and dyad structures of a polymer via Eqs. 8-10 through 8-13. An atactic polymer is one in which (r) = (m) = 0.5 and mm) = (rr) = 0.25, mr) = 0.5 with a random distribution of dyads and triads. The (aU-) isotactic polymer has (w) = mm) = 1. The (all-) syndiotactic polymer is defined by (r) = (rr) — 1. For random distribution with (m) 7 (r) 7 0.5 or mm) / rr) / 0.25, one has different degrees of syndiotacticity or isotacticity. Isotacticity predominates when (m)>0.5 and mm) >0.25 and syndiotacticity predominates when (r)>0.5 and (rr)>0.25. These pol3miers are random tactic polymers, containing random placement of isotactic and syndiotactic dyads and triads. When the distribution of dyads and triads is less than completely random, the polymer is a stereoblock polymer in which there are block (which may be short or long) of isotactic and syndiotactic dyads and triads. 

Tactic polymer n. A polymer in which there is an ordered structure with respect to the configurations around at least one main-chain site of steric isomerism per conventional base unit. Note— The carbon (or other) atom in the chain at the site of the steric isomerism need not in a strict sense be an asymmetric atom, since in a chain of infinite length the two chain portions may be considered as equivalent however, for the purpose of this definition, such atoms are referred to as asymmetric atoms. 

For basic definitions of stereochemical terms relating to tactic polymers and polymerizations, see... 

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. 

According to the definition, a stereospecific polymerization is a polymerization in which a tactic polymer is formed. 

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). 

In these types of polymer reaction, quantitative conversion is a definite requirement otherwise, there would be a possibility that the tacticity of the derived polymer differs from the original one owing to the tacticity dependence of the reaction. 

We adopt the definition of stereospeclfic polymerization to refer to the process leading to a tactic (stereoregulai) polymer pro duced with a stereoselective catalyst. The terms stereoselective (or enantioselective) and regioselective will also be used to refer to the single insertion event. Given their widespread use, we will also use the terms aspecific, isospecific, and syndiospecific, referring to the type of enantioselectivity of a catalyst. 

Kubisa et al. [64] have been exploring the use of chiral ionic liquids in polymer synthesis. Using ionic liquids with a chiral substituent on the imidazolium ring for the ATRP of methyl acrylate gave a small but definite effect on polymer tacticity, with more isotactic polymer formed than in simple [BMIM][PF6]. They also found that the use of ionic liquids led to fewer side reactions. Ionic liquids have been used as solvents in biphasic ATRP to facilitate the separation of the products from the catalysts [65]. 

This kind of approach, particularly if it could be used to control polymer tacticity, might be a useful alternative to polymerizations involving mononuclear initiators as propagation is definitely more rapid than that involving mononuclear, cationic enolate complexes of similar structure. 

Stereospecific n. Of polymerization catalysts, implying a specific or definite order of spatial arrangement of molecules in the polymer resulting from the catalyzed polymerization. This ordered regularity of molecules (tacticity), in contrast to the branched or random structure found in other plastics, permits close packing of the molecular segments and leads to high crystallinity, as in polypropylene. The adjective is sometimes applied imprecisely to polymers to mean tactic. 

The first extensive lUPAC publications on stereochemistry in high polymers were published in the early 1960s, and subsequently in 1966 as a single article (39). In addition to more conventional polymer names, eg, polyethylidene and polypropylene, the -amer nomenclature was introduced in 1952 (10). lUPAC basic definitions relating to stereochemistry, eg, tacticity and isotactic polymer, were published in 1974 (1). 

A major 1981 lUPAC document (9) addresses polymer stereochemistry in depth. Key chapters of this document discuss basic definitions (configurational unit, configurational base and repeating units, stereorepeating imit, different types of tacticity, tactic block polymers, and stereoblock polymers) sequences conformations and supplementary definitions. Most of the illustrations use the rotated Fischer projections, but some three-dimensional representations are included. 

The better definition of spectra from crystalline polymers can give an indication of polymer tacticity since atactic polymers are generally non-crystalline and there are certain polymers such as poly(methyl methacrylate) and polypropylene in which specific absorption bands can be assigned to the presence of molecules with particular types of tacticity. Some idea of the tacticity for samples of these polymers can, therefore, be obtained from measurements of the strength of the absorption of the relevant bands, but in general this method is not as accurate as n.m.r. for tacticity determination. 

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