Repeating Unit Isomerism

There is one important class of polymers that do exhibit strong optical activity, as opposed to the above tactic structures. These are the polymers in which the chiral center is surrounded by different atoms or groups, and a true local center of asymmetry exists. 

The most important examples in this class are the cis and tram isomerism about double bonds. Take polybutadiene as an example. 

The cis-trans isomerism arises because rotation about the double bond is impossible without disrupting the structure. Thus the formula on the left of equation (2.15) is written ds-polybutadiene. The reader should note that the cis-trans isomerism is entirely different from the trans-gauche structures written in equation (2.4). 

The cis and trans formulas are both crystallizable when appearing in pure form, but with different melting temperatures. If a mixture of cis and trans isomers occurs, crystallization may be suppressed, similar to the atactic polymers. 

In the synthesis of diene type polymers, yet another type of isomerism may occur, that of 1,2 versus 1,4 addition  

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Table V. Kinetics of Repeating Unit Isomerization in Poly[(chloromethyl)thiirane] and Poly(3-chlorothietane)...

Repeating unit isomerization is similar in several respects to isomerization polymerization (26,27). Isomerization polymerization may be defined as a process whereby a monomer of structure A is converted to a polymer of repeating unit structure B, wherein the conversion of A to B represents a structural change which is not a simple ring opening or double bond addition ... 

We define repeating unit isomerization as a process subsequent to polymerization, in which an intramolecular rearrangement of the repeating unit leads to a thermodynamically preferred structure ... 

C NMR analysis, 356,360,3611-362/ Reduction, 2,2-bis[4 -(4"-phenylsulfonyl phenoxyl)phenyl] propane, H NMR, 9 Repeating unit isomerization vs. 

In this chapter we describe a new method of copolymer synthesis which is analogous to the method of isomerization polymerization, in that a copolymer is prepared from a single monomer The method is repeating unit isomerization, which we define as a polymerization followed by intramolecular rearrangement of the polymer repeating unit to a thermod3mamically more favorable structure (Eqn 4) ... 

In this chapter, we review the repeating unit isomerization of polymeric 3-chlorosulfides, with emphasis on the kinetics and mechanism of the reaction. We then discuss some preliminary observations concerning the isomerization of poly(chlorobutylthiirane), a... 

Isomerization of Polymeric P-Chlorosulfides. The first example of a repeating unit isomerization was found when we examined the carbon-13 NMR spectrum of a sample of poly(chloromethyIthiirane) which had been stored at room temperature for three months. Instead of the expected three lines at approximately 39, 51 and 54 ppm down-field from tetramethylsilane, we foimd two major lines at 40 and 61 ppm the expected signals were present, but of low intensity. The proton NMR spectrum was also unexpected, showing in addition to the backbone and chloromethyl signals, a downfield (6 4.28) quintet which could not be rationalized on the basis of the simple CMT repeating unit structure. When a freshly-prepared sample was analyzed, both the and the NMR spectra were as predicted. 

The repeating unit isomerization of poly(chloromethylthiirane) occurs in bulk and in non-nucleophilic solvents such as chloroform, dichloromethane and nitrobenzene. Regardless of the medium, the rearrangement appears to stop after isomerization of about 60% of... 

In a first test of this expectation, we sought evidence for repeating unit isomerization in poly[(2-chloroethyl)oxirane] (PCEO, IV), according to the mechanism shown in Eqn 9. 

The importance of repeat unit isomerism in poly (1,3-dienes) is very clearly demonstrated by the naturally-occurring polyisoprenes. Gutta... 

Tirrell et al. ° showed that poly(epichlorohydrin) does not undergo repeating unit isomerization however substituted polyether carrying functional groups placed five bonds away from the backbone heteroatom could undergo isomerization due to anchimeric assistance. 

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