Rapid heterogeneous degradation

Chemically, elastomers are either thermoplastic or thermoset. Thermoplastics present advantages such as easy fabrication and heterogeneous degradation because of their semicrystalline nature. However, this results in a rapid nonlinear loss of mechanical properties, as well as in large deformations upon degradation. Conversely, thermoset polymers are more challenging to fabricate than thermoplastics are. However, thermosets are considerably superior compared to thermoplastics in terms of imiform biodegradation, favorable mechanical properties, and durability [76]. 

Since these reactions are relatively rapid, i.e., phenolic acids are rapidly degraded aerobically, their presence in the soil under these conditions appears transitory. It has been difficult to detect unbound phenolic acids in the soil solution and the compounds do not appear to accumulate in appreciable amounts under aerobic conditions. However, the soil is a heterogeneous medium consisting of loci or microenvironments that are at times completely opposite in character, i.e., anerobic microsites in a well-aerated soil (57). The phytotoxicity problem should be viewed in the context of a specially variable environment. 

Hemicelluloses, the second group of plant-cell constituents, are water-insoluble polysaccharides. During the first stage of attack, hemicelluloses disappear more rapidly than cellulose. Later, hemicelluloses slow down in degradation owing to their heterogeneity. Fungi, bacteria, and actinomycetes are able to break down hemicelluloses by... 

Concurrent with the above research, we were considering reaction conditions for racemization. An initial test experiment based on Prof. Drueckhammer s conditions [26] convinced us that the search for acceptable racemizing conditions would not be arduous. A chiral thioester was prepared from (R)-l with thionyl chloride followed by 2-butyl mercaptan (94% yield). When this was mixed with toluene, l,8-diazabicyclo[5.4.0]undec-l-ene and Triton X-100 in phosphate buffer at pH 9.2, the heterogeneous mixture racemized completely in only 25 min. While the substrate had also degraded, such rapid racemization convinced us that we would be able to discover optimized conditions in a reasonable time period. 

This heterogeneity in the kinetics can make a rate constant deceptive since it will change with the extent of reaction. This is just one challenge in making extrapolations of degradation with time a rapid rate due to a reactive API form may not continue once the reactive API form is consumed. Fortunately, the matter is somewhat simplified in pharmaceutical stability testing since only a small amount of degradation determines the shelf-life. 

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