Short oligomer

Polymers can be crystalline, but may not be easy to crystallize. Computational studies can be used to predict whether a polymer is likely to crystallize readily. One reason polymers fail to crystallize is that there may be many conformers with similar energies and thus little thermodynamic driving force toward an ordered conformation. Calculations of possible conformations of a short oligomer can be used to determine the difference in energy between the most stable conformer and other low-energy conformers. 

Radicals generated from water-soluble initiator might not enter a micelle (14) because of differences in surface-charge density. It is postulated that radical entry is preceded by some polymerization of the monomer in the aqueous phase. The very short oligomer chains are less soluble in the aqueous phase and readily enter the micelles. Other theories exist to explain how water-soluble radicals enter micelles (15). The micelles are presumed to be the principal locus of particle nucleation (16) because of the large surface area of micelles relative to the monomer droplets. 

The transfer efficiency decreases from a value close to 100% for short oligomers to 16% for the n = 12 oligomer. The efficiency of energy transfer as a function of distance is shown in Figure 6.7. 

A significant feature of this work was the demonstration that short oligomers can be representative of the vibrations in polymers, even when disorder is taken into account. 

PLA is a compostable plastic material. Chemical hydrolysis is considered to be the main degradation route for PLA [14]. This hydrolysis process takes place at high humidity and at elevated temperatures (for example in industrial composting facilities). The fragments that result from the hydrolysis process, i.e., short oligomers and monomers, then can be metabolized by microorganisms. Therefore,... 

FIGURE 11.1 Chemical scheme for degradation of poly(esters). Water reacts with the hydrolytic unstable ester bonds and finally produces monomers or short oligomers of the monomer. 

The first important question to be answered concernes the lower limit of molecular weight down to which the concepts obtained for long Gaussian or latticelike chains were applicable. It is clear that a Gaussian chain does not adequately describe the conformational properties of short oligomer chains. Other models e.g. a model of the wormlike chain may be more suitable. The introduction of this model may lead to considerable mathematical complications and the determination of Kd may become difficult. 

Oligothioureas have been prepared on cross-linked polystyrene from Boc-pro-tected oaminoisothiocyanates, which were synthesized in solution from symmetric diamines [258]. The formation of thioureas from support-bound amines and isothiocyanates is a rather sluggish reaction (e.g. 3 d, 45 °C, mean coupling yield 90% [258]), and only short oligomers can be prepared using this strategy. 

Experiments indicate that for a fixed ionic strength 7m is very sensitive to the number N of base nucleotides for short oligomers, while for long double hehces it reaches a saturated, yet rather high, value (see Fig. 4). For 1 M NaCl, Tm ranges from 7m 20-25°C for /V 6. to 7m 80-90°C for /V > 25. This behavior can be reproduced fairly well by combining the thermodynamic parameters introduced... 

Only the nitro-substituted oligothiophenes display large bathochromic shifts, large Stokes shifts, high fluorescent quantum yields, and long lifetimes for excited states. As for the other substituents, the trend is mostly noticeable for the short oligomers like terthiophenes and seems to disappear for sexithiophenes. As can be inferred from their solvatochromic effect, an intramolecular charge transfer takes place in the excited states of these molecules. 

Another issue under investigation is the evolution of the second-order hyperpolarizability upon elongation of the 7r-electron path in a molecular backbone. Measurements and quantum-chemical calculations provide an empirical power law of the second-order hyperpolarizability y versus conjugation length (or the number of monomer units n) in short oligomers. 

With the second pure PTA (TES) series longer oligomers with n=8, 12, 16 could be measured and the onset of the saturation covered (Fig. 26) [74]. The power law for short oligomer shows an exponent of a=2.46 0.10 in good agreement with the first pure PTA (TMS) series. The saturation is again around ten monomer units. The change from the power law to the saturated nonlinearity per number of monomers reveals to be smooth. 

The Polyisobutenes, homopolymers of purified isobutene and available on a very wide range of molar masses from liquid short oligomers to high or ultra-high MM rubbery materials. 

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