Oligomers polydisperse

The UF-resin itself is formed in the acid condensation step, where the same high molar ratio as in the alkaline methylolation step is used (F/U = 1.8 to 2.5) the methylolureas, urea and the residual free formaldehyde react to form linear and partly branched molecules with medium and even higher molar masses, forming polydispersed UF-resins composed of oligomers and polymers of different molar m.asses. Molar ratios lower than approx. 1.7-1.8 during this acid condensation step might cause resin precipitation. 

These equations predict that for oligomers with degree of polymerization less than 10, polydispersities significantly less than 1.5 will be obtained - Figure 5.2. 

We have demonstrated a new class of effective, recoverable thermormorphic CCT catalysts capable of producing colorless methacrylate oligomers with narrow polydispersity and low molecular weight. For controlled radical polymerization of simple alkyl methacrylates, the use of multiple polyethylene tails of moderate molecular weight (700 Da) gave the best balance of color control and catalyst activity. Porphyrin-derived thermomorphic catalysts met the criteria of easy separation from product resin and low catalyst loss per batch, but were too expensive for commercial implementation. However, the polyethylene-supported cobalt phthalocyanine complex is more economically viable due to its greater ease of synthesis. 

In direct insertion techniques, reproducibility is the main obstacle in developing a reliable analytical technique. One of the many variables to take into account is sample shape. A compact sample with minimal surface area is ideal [64]. Direct mass-spectrometric characterisation in the direct insertion probe is not very quantitative, and, even under optimised conditions, mass discrimination in the analysis of polydisperse polymers and specific oligomer discrimination may occur. For nonvolatile additives that do not evaporate up to 350 °C, direct quantitative analysis by thermal desorption is not possible (e.g. Hostanox 03, MW 794). Good quantitation is also prevented by contamination of the ion source by pyrolysis products of the polymeric matrix. For polymer-based calibration standards, the homogeneity of the samples is of great importance. Hyphenated techniques such as LC-ESI-ToFMS and LC-MALDI-ToFMS have been developed for polymer analyses in which the reliable quantitative features of LC are combined with the identification power and structure analysis of MS. 

We have been recently studying new pathways leading to polysilanes with low polydispersity and controlled structures. Our research is focused on three areas. The first one is low temperature reductive coupling in the presence of ultrasound. This leads to monomodal polymers with molecular weights in the range from Mn=50,000 to Mn=300,000 and polydispersities as low as Mw/Mn=1.20 (in addition to usually formed cyclic oligomers). 

In order to enhance the understanding of the properties in polymers, iterative pathways have been chosen for the synthesis of structurally perfect molecules. Data obtained from the analysis of precisely defined oligomers and polymers may relate chain length and conformation to physical, electronic and optical properties. Statistical polymerization processes are not suitable as they yield polydisperse material. 

However, with initial monomer concentrations of less than 0.2 moll-1, only low molecular weight cyclic oligomers are obtained. The polydispersity index Mw/M of the resulting polymers is approximately 2. 

The fn value provides information on the average functionality of oligomers, but does not characterize at all their functional polydispersity. For instance, fn = 2, in some cases an ideal indicator for a telechelic oligomer, can be simulated by the presence of an equal number of mono- and trifunctional macromolecules. 

Type 1 — oligomers with a strictly defined target functionality. In an ideal case, oligomers of this kind must have fw/fn = 1. They are usually synthesized by different methods special techniques of initiation, telomerization, etc. In practice, however, oligomers of this type almost invariably are polydisperse in functionality, i.e. fw/fn > 1. The reasons and sources of functional defectiveness in different methods of synthesizing oligomers are briefly discussed in the review u. 

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