Oligomers Versus Polymers

Polymers exhibit certain disadvantages, however, when being subjected to the methods of radical ion characterization outline in Sect. 1  

1) Polymers exist as polydisperse mixtures with different molecular weights. The resulting physical pro( rties such as, e.g., a redox potential cannot be ascribed to a specific length of the extended r-chain [53]. This and the large size of the molecules create an element of uncertainty in the physical description. 

2) Conjugated polymers with their rigid Jt-systems often possess a low solubility even for low molecular weights, which seriously inhibits full structural 

3) The most important drawback in the electronic description of conjugated polymers is the occurrence of structural defects such as sp -hybridized carbon (inters in an extended 7t-chain [58, 59]. This leads to an interruption of 7i-conjugation which may determine the effective conjugation length within the polymer. The scientific literature is full of examples in which one does not pay proper attention to a complete structural elucidation and to the synthesis of defect-free, homogeneous polymeric materials instead, the consideration of conjugated jt-systems is often restricted to idealized, oversimplified or even partially decomposed structures. 

The synthetic organic chemist, on the other hand, who is used to deal with structurally defined, but small molecules should be aware that avoiding structural defects in conjugated polymers is by no means a trivial task. A side reaction in any of the polymer forming steps does not lead to a separable side product, but produces an inherent structural defect of the macromolecule. 

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Figure 6 shows the swelling capacity of polymer particles containing different amounts of an oligomer versus in this case r is... 

The most widely used molecular weight characterization method has been GPC, which separates compounds based on hydrodynamic volume. State-of-the-art GPC instruments are equipped with a concentration detector (e.g., differential refractometer, UV, and/or IR) in combination with viscosity or light scattering. A viscosity detector provides in-line solution viscosity data at each elution volume, which in combination with a concentration measurement can be converted to specific viscosity. Since the polymer concentration at each elution volume is quite dilute, the specific viscosity is considered a reasonable approximation for the dilute solution s intrinsic viscosity. The plot of log[r]]M versus elution volume (where [) ] is the intrinsic viscosity) provides a universal calibration curve from which absolute molecular weights of a variety of polymers can be obtained. Unfortunately, many reported analyses for phenolic oligomers and resins are simply based on polystyrene standards and only provide relative molecular weights instead of absolute numbers. 

Fig. 2. Swelling capacity of po mer-oligomer particles versus for different vatum of Vi. To = radius of polymer-oligomer particles prior to swelling with Z,. =...

Infrared studies of the radiation-induced degradation of PMPS by Bowden et al. (19) supports the oligomerization process and also shows that the oligomers can be removed by post-exposure baking. These effects have not been seen for other poly (olefin sulfone)s (2.3). Figure 8 and Figure 9 show the yield versus dose curves for irradiation of poly(l-butene sulfone) and poly (cyclohexene sulfone) respectively (20). No comparable shift of the S02/olefin ratio towards unity is observed in the radiolysis of these polymers. 

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