Atactic polymers, 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... 

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. 

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]. 

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 =... 

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)... 

Sometimes it is stiU debated whether the glass transition is a purely kinetic transition or a second-order thermodynamic transition (van Krevelen 2003). On one hand, it is true that the crystallization process for a number of (atactic) polymers would not take place even at infinite time, and this transition possesses the characteristics of a second-order thermodynamic transition (at least formally, in the Ehrenfest sense see definition of the phase transition in Section 2.2). But the absence of crystallization does not prove that the glass transition is a thermodynamic second-order transition, and it is also true that the glass transition does not occur as a definite sharp transition as would be required by equilibrium thermodynamics. Therefore, the glass transition must be considered a kinetic transition. 

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 by 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. See Syndiotactic Polymer, Isotactic Polymer, and Atactic Polymer. 

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. 

We have recently completed what we conceived as a definitive review of T and related liquid state transitions (relaxations) in the T>Tg region of atactic polymers and the 7>T region of crystalline polymers. A table of contents appears on page 233 of that article. In general the subject matter covered definition of terms, history, theories for Tw experimental techniques, practical significance, and a guide to the literature. 

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. 

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. 

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). 

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

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... 

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