Syndiotactic, definition

Although the definitions of isotactic, syndiotactic, and atactic polymers according to International Union of Pure and Applied Chemistry (IUPAC) rules are well established in terms of succession of mesa (m) or racemic (r) dyads,12 the symbolism of (+) and (—) bonds allows the easy treatments of possible configurations in cases of any complexity.1 Moreover, the (+) or (—) character of the bonds in a polymer chain is strictly related to the accessibility of gauche+ or gauche conformations of the bonds and, therefore, to the formation of right-handed or left-handed helical conformations.1... 

In dilute solution, the dependance of LnRj on solvent polarity for copolymers is definitely measurable, but it is significantly reduced with respect to that of the model compounds, by a factor of about 3 for a predominantly syndiotactic chain bearing keto-2-picolyl functions in the form of isolated units (DSm = 0.129,... 

The contrast between formulas 20 and 21, both pertaining to isotactic polyethylidene, should be noted This contrast occurs because the polymer repeating unit has only one carbon atom in the chain and thus there is no correspondence between such periodicity and that of the zigzag representation. The classical definition of an isotactic polymer (as one in which all substituents are on the same side of the chain) holds true, in general terms, only if the polymer is represented in the Fischer projection. Analogous considerations pertain to syndiotactic polyethylidene 22 and 23. 

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 order to present clear concepts it is necessary that idealized definitions be adopted but it is recognized that the realities of polymer science must be faced. Deviations from ideality arise with polymers at both molecular and bulk levels in ways that have no parallel with the ordinary small molecules of organic or inorganic chemistry. Although such deviations are not explicitly taken into account in the definitions below, the nomenelature recommended can usefully be applied to the predominant structural features of real polymer molecules, if necessary with self-explanatory, if imprecise, qualifications such as almost completely isotactic or highly syndiotactic . Although such expressions lack the rigour beloved by the purist, every experienced polymer scientist knows that communication in this discipline is impossible without them. 

These definitions are clarified by considering a portion of a polymer chain such as XVII. Chain segment XVII has a total of 9 repeating units but only 8 dyads and 7 triads. There are 6 meso dyads and 2 racemic dyads (m) —, (r) —, There are 4 isotactic, 2 heterotactic, and 1 syndiotactic triads mm) = (mr) — j, (rr) — A. 

Prepare a plot of 7r/c versus c for these results and evaluate (7r/c)0. Calculate M and B for this system. Define what is meant by an atactic polymer and compare with syndiotactic and isotactic polymers. List reference(s) consulted for these definitions. 

Polymerization of CFE in urea at -80"C has a mild stereoregulating influence compared to bulk polymerization at 60 °C, although the spectra in Figure 2 show that both polymers are substantially atactic. Three major triad resonances can be discerned more readily in spectrum (b), and these are assigned to mm, mr, and rr stereosequences as shown simply by analogy with the poly(l,2-difluoroethene) assignments (14,15). At present we have no basis to make definitive assignments, but those indicated are at least consistent with the known syndiotactic bias exerted by urea on the polymerization of complexed vinyl monomers (10). 

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. 

Atactic polymers are also regular polymers. They contain, by definition, the possible configurational monomeric units in equal proportions, but with an ideally random distribution from molecule to molecule. Such distributions are caused by symmetric Bernoulli mechanisms during polymerization (see Section 15). They are distinguished by having equal numbers of iso- and syndiotactic diads (A i = ATJ, iso-, hetero-, and syndiotactic triads (Na = A is =... 

Definition Thermoplastic consistg. of 50-65% syndiotactic polybutadiene with > 90% of the monomers linked at their 1,2-positions polymer is partially cryst., but transparent film has high gas permeability, high tear/puncture resist. 

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