Atactic arrangement

In an atactic arrangement, substituents are in an unordered sequence along the polymer chains. 

The polymer of 13C-labeled 2-phenyl-1-methylenecyclopropane by the same catalyst contains the isotope-labeled carbon exclusively at the exo-methylene group. The polymer consists of the 1-phenyl-2-exo-methylenepropan-1,3-diyl structure, exclusively. The 13C 1H NMR signals of the ipso carbon of the phenyl group are split into four due to the four triads, rr, rm, mr, and mm, based on comparison of the spectrum with that of poly(styrene-co-CO) with isotactic, syndiotactic, and atactic arrangement of the monomer units [160] (Scheme 16). Thus, the polymer produced from 2-phenyl-1-... 

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

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 ... 

Clearly, the 3-dimensional periodicity of a crystal could not be achieved without some kind of regularity in the tacticity. Consequently, only the isotactic and syndiotactic arrangements can form crystals, whilst the atactic arrangement will make amorphous materials. 

CpTiCbactivated by MAO polymerizes (Z)-l,3-pentadiene, affording a polymer with high cis content (>99%) at room temperature. In this case, the polymer does not show crystallinity, suggesting an atactic arrangement of the methyl groups." Notably, in this case the (A) isomer is more reactive, but its polymerization results in a mixture of 1,4-cis (50%), 1,4-trans (40%), and 1,2 (10%) units. Similar results have also been obtained using bis(phenoxyimino) titanium catalysts 2 activated by MAO. ... 

Polymerization. Pd(PhCN)2Cl2 catalyzes the polymerization of alkoxyallenes via polymerization of the (7-substituted double bond. Isotactic, syndiotactic, and atactic arrangements of OR groups are all possible (eq 208). ... 

Any of the four monomer residues can be arranged in a polymer chain in either head-to-head, head-to-tail, or tail-to-tail configurations. Each of the two head-to-tail vinyl forms can exist as syndiotactic or isotactic stmctures because of the presence of an asymmetric carbon atom (marked with an asterisk) in the monomer unit. Of course, the random mix of syndiotactic and isotactic, ie, atactic stmctures also exists. Of these possible stmctures, only... 

When poly(propylene) was first made, it was found to exist in two possible forms. One was similar to poly(ethylene), but had greater rigidity and hardness the other was found to be amorphous and of little strength. The first of these is now known to be isotactic, that is with a regular stereochemistry at each alternating carbon atom. The other is now known to be atactic, that is with a random distribution of different stereochemical arrangements... 

What is it about the monomer that promotes a syndiotactic arrangement of the pendant styrene group Why is the polymer still considered atactic if the syndiotactic addition is more energetically favorable ... 

In an atactic structure the substituent groups are arranged irregularly above and below the plane of the main chain. 

Hesse and Heusinger130 studied the ESR signal due to Am = 2 transition of radical pairs in a number of /-irradiated polymers including 1,2-polybutadiene (both atactic and isotactic) and 3,4-polyisoprene. It was found that the distance between the radicals in the pair is 0.53 0.04 pm 1.0 0.5% of the radicals in 1,2-polybutadiene and 3,4-isoprene are arranged in pairs. 

Gutta Percha is an example of this arrangement. The third type atactic (Greek without order) the groups are arranged randomly above and below the chain. This type of radical is obtained by free radical polymerisation. 

Both isotactic and syndiotactic polymethyl methacrylate have been prepared. However, the commercial polymer is generally atactic because of the random arrangement of the bulky side groups. 

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