Hindered Polymer Chains

Hindered Polymer Chains.—Molecular dimensions predicted by calculations made under the assumption of free rotation are found to be appreciably in error. Although the freely rotating chain of fixed valence angle d conforms more nearly to reality than the freely jointed chain, it is nevertheless quantitatively unsatisfactory. The reason 

The threefold symmetry of rotation about the C—C bond of ethane disappears when substituents are introduced on both of the carbon 

This analysis of hindrances to rotation is oversimplified even for a polymethylene chain. However, with the tacit assumption that only interactions between atoms or groups directly connected to the C—C bond under consideration affect the value of 0, the hindrance potential should be symmetrical about 0 = 0 regardless of the precise nature of the interactions. That is to say, the repulsion for an angle 0 should be the same as for —0. When this is true, the resultant of bond (3) in Fig. 74 (p. 400) averaged over all angles 02, each angle occurring with a probability proportional to can be resolved into two 

Failure to take account of interactions with elements of the chain other than those bonded directly to the atoms of the bond under consideration constitutes an approximation which is not justified physi- 

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Antioxidants markedly retard the rate of autoxidation throughout the useful life of the polymer. Chain-terminating antioxidants have a reactive —NH or —OH functional group and include compounds such as secondary aryl amines or hindered phenols. They function by transfer of hydrogen to free radicals, principally to peroxy radicals. Butylated hydroxytoluene is a widely used example. 

Polar substituents such as chlorine increase the interchain forces and hinder free rotation of the polymer chain. Hence polydichlorostyrenes have softening points above 100°C. One polydichlorostyrene has been marketed commercially as Styramic HT. Such polymers are essentially self-extinguishing, have heat distortion temperatures of about 120°C and a specific gravity of about 1.40. 

A hindered phenol is one of the most common antioxidants used in urethanes. This antioxidant traps radicals, which can degrade the polymer chain. The proposed mechanism is as follows [87] ... 

The formation of the microstructure involves the folding of linear segments of polymer chains in an orderly manner to form a crystalline lamellae, which tends to organize into a spherulite structure. The SCB hinder the formation of spherulite. However, the volume of spherulite/axialites increases if the branched segments participate in their formation [59]. Heterogeneity due to MW and SCB leads to segregation of PE molecules on solidification [59-65], The low MW species are accumulated in the peripheral parts of the spherulite/axialites [63]. The low-MW segregated material is brittle due to a low concentration of interlamellar tie chains [65] and... 

In a real polymer chain, rotation around backbone bonds is likely to be hindered by a potential energy barrier of height AEr. If AEr < RT, the population of the... 

The changes with steric hindrance and temperature must be due to conformational effects along the polymer chains. It is now generally agreed that the thermochromism results from an increase in the proportion of trans- to gauche conformations in the polymer chain as the temperature is decreased. Similarly, the introduction of sterically hindering substituents could increase the amount of trans junctions. Evidence in favor of this model is presented in several recent papers, (28i) as well as the chapter by Michl in this volume. 

Polymers with triflate groups react with alcohols to form alkoxysubstituted polysilanes. This reaction occurs readily in the presence of bases. The best results were obtained using triethylamine and hindered pyridine. In Fig. 3c the NMR spectrum of the reaction mixture containing the excess of triethylamine is shown, the methyl groups from the polymer chains absorb in the range typical for alkoxysilanes. Reaction in the presence of unsubstituted pyridine leads to the formation of insoluble polymer probably by attack at the p-C atom in the silylated pyridine. 

Stereodefects reduce the overall regularity of an isotactic polymer chain and hinder its ability to crystallize. As the concentration of defects increases, the degree of crystallinity falls, resulting in reduced density, reduced melting temperatures, lower heat distortion temperatures, reduced modulus, and reduced yield stress. 

Polymers crystallize when the crystalline state is more thermodynamically stable than the molten one. The system progresses towards a low energy state, but lull crystallization is hindered by chain entanglements, branches, side group interactions, and viscous drag on chains. 

Now, let s look at a polymeric system. To begin with, the motion in polymer chains is hindered. The massive size of the polymer itself and the intermolecular forces within the chains create an inflexible system, especially when compared to the aqueous systems with which we are most familiar. Secondly, the entropy of mixing is not actually as great as that seen in typical solution formation. Polymers are inherently highly entropic, so the benefit of mixing them together is modest. Therefore, any two polymers that form a miscible blend depend primarily... 

The relative rates of polymerization of a series of substituted e-caprolactones initiated by (246) demonstrate that methyl groups, particularly adjacent to the acyl oxygen, retard polymerization.757 In addition, the rate of polymerization of the parent unsubstituted CL at 25 °C was found to be 4 x 102 times greater than L-LA at 70 °C. The slower propagation of LA is usually attributed to coordination of the nearest inserted carbonyl of the polymer chain to the A1 center, leading to formation of a stable 5-membered chelate, which hinders monomer uptake.758... 

Chien already postulated that C,-symmetric ansa-bridged complexes exist in two isomeric states, which interconvert during the course of the polymerization reaction [14, 15, 21, 22], Different stereoselectivities for monomer coordination and insertion are found for the two coordination sites of the asymmetric metallocene catalysts (Fig. 6,1 and IV). The migration of the polymer chain to the monomer, coordinated at the isoselective site f I—>11), followed by a consecutive chain back-skip (at higher temperatures) to the sterically less hindered side (II >111) leads to isotactic [mmmm] sequences [11],... 

By the time the concentration of monomer is low, the back-skip of the polymer chain to the less-hindered site is faster than the formation of the high-energy alkene coordinated intermediate (IV). For this reason, at low propene concentrations and elevated temperatures isotactic sequences are formed. The probability of monomer coordination at the aspecific site (IV) is enhanced when the propene concentration increases. The consequence is that single stereoerrors [mrrm] are introduced in the isotactic polymer chain. 13C-NMR was able to prove the mechanism because a... 

Antioxidants act so as to interrupt this chain reaction. Primary antioxidants, such as hindered phenol type antioxidants, function by reacting with free radical sites on the polymer chain. The free radical source is reduced because the reactive chain radical is eliminated and the antioxidant radical produced is stabilised by internal resonance. Secondary antioxidants decompose the hydroperoxide into harmless non-radical products. Where acidic decomposition products can themselves promote degradation, acid scavengers function by deactivating them. 

Among the spectrum of melt-spinnable fibers such as polyolefins and nylons, PET stands at the upper end in terms of crystalline melt temperature and glass transition temperature. This provides superior dimensional stability for applications where moderately elevated temperatures are encountered, e.g. in automobile tires or in home laundering and drying of garments. The high thermal stability results from the aromatic rings that hinder the mobility of the polymer chain. 

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