Configuration in Polymers

Due to energetic and steric reasons, the head-to-tail union is the preferred structure. In spite of this, radical addition polymers always contain a small proportion of head-to-head unions. The properties of these isomers are considerably different in particular, the presence of head-to-head unions gives rise to irregularities that make it difficult for polymer chains to crystallize. 

A wide range of polymers are obtained from the addition of conjugate diene monomers, notably 1,3-butadiene, isoprene, and chloroprene  

The addition of successive monomers can be made to the 1-2 bond the polymerization is then called 1-2 addition and forms chains with the configuration 

In the case of 1,3-butadiene, 1-2 addition and 3-4 addition are equivalent, but for isoprene and chloroprene, as is evident, 1-2 and 3-4 additions give different products. The third possibility is referred to as 1-4 addition and produces chains with the configuration 

The 1-4 addition can take the cis orientation, leaving the substituents on the same side of the double bond, or the trans orientation, leaving them on opposite sides. These isomers are known as geometric isomers 

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B. Yan, J. He, X. Du, K. Zhang, S. Wang, C. Pan, and Y. Wang, Control of hquid crystal droplet configuration in polymer dispersed hquid crystal with macro-iniferter polystyrene. Liq. Cryst. 36, 933-938 (2009). 

The same type of polymerization reaction carried out under conditions where solvated ion pairs were expected yielded random arrangements of methylene configurations in polymers, which were either atactic or largely syndiotactic. 

In polymers made of dis-symmetric monomers, such as, for example, poly(propylene), the stmcture may be irregular and constitutional isomerism can occur as shown in figure C2.1.1(a ). The succession of the relative configurations of the asymmetric centres can also vary between stretches of the chain. Configuration isomerism is characterized by the succession of dyads which are named either meso, if the two asymmetric centres have the same relative configurations, or racemo if the configurations differ (figure C2.1.1(b )). A polymer is called isotactic if it contains only one type of dyad and syndiotactic if the dyad sequence strictly alternates between the meso and racemo fonns. 

In conclusion RAIRS, which affords high spectral resolution, is a very versatile nondestructive optical technique which does not depend on a vacuum environment. Vibrational spectra also serve as characteristic fingerprints for adsorbate molecules, adsorption configurations, and structures on metallic and dielectric substrates. Extension to include dielectric substrates opened new fields of application in polymer and biochemical research. 

Isotactic Type of polymeric molecular structure that contains sequences of regularly spaced asymmetric atoms that are arranged in similar configuration in the primary polymer chain. Materials having isotactic molecules are generally in a highly crystalline form. 

P. L. Luisi and F. Ciardelli, Configuration and Conformation in High Polymers in. Reactivity, Mechanism and Structure in Polymer Chemistry, (A. D. Jenkins and A. A. Ledwith, eds.), John Wiley Sons, London (1979). 

The composition of the copolymer determines its electroluminescence efficiency. Optimal efficiency (0.3%) was achieved in system 34 when the feed ratio of monomer 4 to monomer 34 was 9 1. This represents a 30-fold improvement in luminescence efficiency relative to PPV in the same device configuration (AlALOj/polymer/Al) 58, 62. Copolymer 33 has found uses as waveguides and... 

More detailed theoretical approaches which have merit are the configurational entropy model of Gibbs et al. [65, 66] and dynamic bond percolation (DBP) theory [67], a microscopic model specifically adapted by Ratner and co-workers to describe long-range ion transport in polymer electrolytes. 

Polymer stereochemistry, sometimes referred to as tacticity, is not the only source of variation in polymer configuration. For the monosubstituted butadiene isoprene, the structures shown in Figure 3.2 are possible. 

Typical materials that exhibit liquid crystalline behaviour are made up of long, thin molecules. Hence in principle polymers ought to show the basic requirement for liquid crystal behaviour. Conventional polymers, however, are too flexible and tend to adopt random coil configurations in the melt. They are thus not sufficiently anisotropic to exhibit a mesophase. 

The configuration of the polymer molecule must depend also on its environment. In a good solvent, where the energy of interaction between a polymer element and a solvent molecule adjacent to it exceeds the mean of the energies of interaction between the polymer-polymer and solvent-solvent pairs, the molecule will tend to expand further so as to reduce the frequency of contacts between pairs of polymer elements. In a poor solvent, on the other hand, where the energy of interaction is unfavorable (endothermic), smaller configurations in which polymer-polymer contacts occur more frequently will be favored. 

An important departure from the previous theory arises from the fact that portions only of the polymer molecules, and never an entire molecule, occur within a volume element. It will be recalled that, in the estimation of the total number of configurations in the entire solution, one segment (the first one in the chain) of each molecule was allowed the opportunity to locate in any vacant cell of the lattice. 

The problem of the influence of intramolecular interactions on the configurations of polymer molecules has been the subject of much controversy, which need not be reviewed here. For treatment of the problem by methods other than the one presented in the following pages, the reader is referred to papers by F. Bueche, J. Chem. Phys., 21, 205 (1953), and B. H. Zimm, W. H. Stockmayer, and M. Fixman, J. Chem. Phys., 21, 1716 (1953). These papers include references to other literature on the subject. 

In polymer electrolytes (even prevailingly crystalline), most of ions are transported via the mobile amorphous regions. The ion conduction should therefore be related to viscoelastic properties of the polymeric host and described by models analogous to that for ion transport in liquids. These include either the free volume model or the configurational entropy model . The former is based on the assumption that thermal fluctuations of the polymer skeleton open occasionally free volumes into which the ionic (or other) species can migrate. For classical liquid electrolytes, the free volume per molecule, vf, is defined as ... 

Another quasi-thermodynamical model of ion transport in polymers is based on the concept of minimum configurational entropy required for rearrangement of the polymer, giving practically identical o—T and D — T dependences as the preceding model. 

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