Atactic, 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... 

Note As the definition above indicates, a regular polymer, the configurational base units of which contain one site of stereoisomerism only, is atactic if it has equal numbers of the possible types of configurational base units arranged in a random distribution. If the constitutional repeating unit contains more than one site of stereoisomerism, the polymer may be atactic with respect to only one type of site if there are equal numbers of the possible configurations of that site arranged in a random distribution. 

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. 

The character of the polymethyl methacrylate data is essentially similar to that found for systems atactic polystyrene-benzene at 25°, 35°, and 50° C. [Kishimoto, Fujita, Odani, Kurata and Tamura (1960) Odani, Kida, Kurata and Tamura (1961)] and also atactic polystyrene-methyl ethyl ketone at 25° C. [Odani, Hayashi and Tamura (1961)], and appears to be fairly general for amorphous polymer-solvent systems in the glassy state. On the other hand, the cellulose nitrate data shown in Fig. 8 appear to manifest features characteristic of crystalline polymer-solvent systems. For example, the earlier data of Newns (1956) on the system regenerated cellulose-water (in this case, water is not the solvent but merely a swelling-agent) and recent studies for several crystalline polymers all show essentially similar characters [see Kishimoto, Fujita, Odani, Kurata and Tamura (I960)]. To arrive at a more definite conclusion, however, more extensive experimental data are needed. 

An atactic polymer is a regular polymer with macromolecules composed of a certain number of statistically distributed configurational units. The constitutional unit is a type of atom or group of atoms composing the macromolecule (e. g. —[CH2—CHPhJ- or —[CHPh— in polystyrene). The configurational unit is a constitutional unit with one or several stereoisomer-ic centres. These definitions would require a more detailed explanation. In this volume they will only rarely be used, the stereochemistry of jjolymers is a special branch of macromolecular chemistry. More information can be found in the original literature [2]. 

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 Polymer of propylene monomers three forms isotactic (fiberforming), syndiotactic, atactic (amorphous)... 

Give a definition of and illustrate examples of isotactic, syndiotactic, and atactic arrangement of macromolecules. This should include Fischer and Newman projections. 

Synonyms Acrylic acid homopolymer Acrylic acid polymer Acrylic acid, polymers Acrylic acid resin Acrylic polymer Acrylic polymer resins Acrylic resin Atactic poly(acrylic acid) PAA Polyacrylate Poly (acrylic acid) 2-Propenoic acid, homopolymer Propenoic acid polymer Propenoic acid, polymers, homopolymer Definition Polymer of acrylic acid Empiricai (C3H402)x Formuia [CH2CHCOOH]x Properties M.w. 50,000-250,000 dens. 1.02 kg/l anionic... 

Synonyms Atactic polypropylene Isotactic polypropylene Polypropene PP Propathene 1-Propene, homopolymer Propene polymer Propene polymers Propylene polymer Syndiotactic polypropylene Classification Thermoplastic polymer Definition Polymer of propylene monomers three forms isotactic (fiber-forming), syndiotactic, atactic (amorphous)... 

The probabilities represent the dyad tactic fractions m and r. The triad tacticity represent isotactic, syndiotactic, and heterotactic (or atactic) arrangements. They are designated as mm, rr, and nor respectively. One way to understand these definitions is by examining a representation of a section of a polymeric chain which has a total of 9 repeating units but only 8 dyads and 7 triads, as shown below ... 

However, by making use of this definition, one has to conclude that many systems which look like a gel are in fact not covered by this definition aqueous poly(vinyl alcohol)/borate systems, which are known to show liquid-like behaviour at low frequencies [7], solutions of phase separated atactic polystyrene at temperatures above the glass transition temperature of the swollen polystyrene crosslinks, solutions of ABA block copolymers above the glass transition temperature of the swollen A-blocks, and even gelatin, which also shows creep behaviour, as shown by Ross-Murphy et al. [8,9] and by Kramer et al. (private communication), and a relaxation mechanism at extremely low frequencies, as is shown in Fig. 7 of the Section on gelatin, and possibly ako poly(vinyl chloride) in plasticizers [10-13]. The advantage of the approach of Kramer et al. [3] is that these systems certainly are covered by their practical definition. 

---