Macromolecular degradation

Fig. 11.2 The effect of the PP controlled rheology (CR) or viscracking process on the viscoelastic grid each curve represents a PP macromolecular degradation path. [Reprinted by permission from R. C. Kowalski, Fit the Reactor to the Chemistry, in Reactive Extrusion, M. Xanthos Ed., Hanser, New York, 1992.]...

Probiotic bacteria differ on the basis of genus, species, and strains. Indeed, strains of the same species vary widely in traits such as expression of enzymes, types of inhibitors produced, carbohydrate fermentation patterns, resistance to acid and bile, ability to colonize the gastrointestinal tract, and clinical efficacy (Berg, 1996 Norat et al., 2002). Furthermore, the active principle has not always been associated with live bacteria, as some immune system modulation activities and macromolecular degradation have been linked to nonviable bacterial components such as enzyme activities or fermentation products (Huttner and Bevins, 1999). 

Biodegradable polymers can be defined as those which suffer macromolecular degradation in vivo by hydrolysis or the action of enzymes, microorganisms or cells. The term describes the biodegradation processes involving chain scission and modifications of the molecular weight and polymer solubility. Several biodegradable polymers, either natural or synthetic, are used to prepare IDDS (Table 16.2). 

Isotactic polypropylene (iPP) is a major commodity plastic material which cannot be utilized without thermal stabilizers. With a moderately complex structure, iPP is frequently used as a model system to test the different theoretical and experimental approaches to macromolecular degradation. 

Almost all parameters calculated for samples of starch/glycerol mixtures without water addition showed lower values than with 5% added water [26]. Much lower values in SME and in shear stress multiplied by time were noted. According to previous data [20] it may be concluded that increasing moisture content should lower values of maximal shear stress and should thus reduce the macromolecular degradation. This can be confirmed by intrinsic viscosity measurement There are some differences in the extrusion behavior of potato and of cornstarch, as reported by Della Valle et al. [27]. Molten potato starch under the same conditions... 

There were essentially three reasons for this opposition. Firstly, many macromolecular compounds in solution behave as colloids. Hence they were assumed to be identical with the then known inorganic colloids. This in turn implied that they were not macromolecular at all, but were actually composed of small molecules bound together by ill-defined secondary forces. Such thinking led the German chemist C. D. Harries to pursue the search for the rubber molecule in the early years of the twentieth century. He used various mild degradations of natural rubber, which he believed would destroy the colloidal character of the material and yield its constituent molecules, which were assumed to be fairly small. He was, of course, unsuccessful. 

Raff The difference between macromolecular synthesis and degradation. Synthesis must be greater than degradation for the cell to grow. 

The term "degradation of macromolecules" concerns the processes that are accompanied by deterioration in polymer properties. Chemical processes related to the worsening polymer properties may lead to both a reduction of average molar mass due to the scission of bonds in the macromolecular chain, or to an increase of the molar mass due to the crosslinking causing the polymer to become insoluble. 

Murthy N, Xu M, Schuck S et al (2003) A macromolecular delivery vehicle for protein-based vaccines Acid-degradable protein-loaded microgels. Proc Natl Acad Sci USA 29 4995-5000... 

As far as polymer supports for microwave-assisted SPOS are concerned, the use of cross-linked macroporous or microporous polystyrene (PS) resins has been most prevalent. In contrast to common belief, which states that the use of polystyrene resins limits reaction conditions to temperatures below 130 °C [14], it has been shown that these resins can withstand microwave irradiation for short periods of time, such as 20-30 min, even at 200 °C in solvents such as l-methyl-2-pyrrolidone or 1,2-dichlorobenzene [15]. Standard polystyrene Merrifield resin shows thermal stability up to 220 °C without any degradation of the macromolecular structure of the polymer backbone, which allows reactions to be performed even at significantly elevated temperatures. 

---