Polymer chain extension

FIGURE 7.6 Polymer chain extensions in different solvents. 

If polymer chains extensively interwind in the solution, the change in the internal conformation of each chain is severely restricted by entanglements with other chains. In such a case the segment distribution may not be cylindrical. We consider highly entangled solutions in Sect. 6.3.3. 

Therefore, size control is possible. For example, if there are many termini (i.e., z is large), then a limited amount of UDP-sugars will be distributed among many molecules and thus result in many short polymer chain extensions. Conversely, if there are few termini (i.e., z is small), then the limited amount of UDP-sugars will be distributed among few molecules and thus result in long polymer chain extensions. With this it became possible to synthesize high MW well-defined HA polymers [208]. 

The mechanism, developed by Penczek, in the presence of an excess of hydroxyl groups, is very similar to a solvolysis reaction [55] and is characterised by the presence of the active cationic centre to the monomer in the form of a secondary oxonium cation. The polymer chain extension takes place by the SN2 reaction of hydroxyl groups with the activated epoxide. This mechanism is called activated monomer mechanism (AM mechanism), characterised by the relationship [OH] [PO] (reaction 7.21). 

Incidentally, the ISPC formula high-solids polyester-melamine paint (PERMACLAD 2500) showed an initial increase in absolute viscosity at room temperature, owing most probably to the acid catalyzed polymer chain extension of polyester molecules in the formula. This is a classic problem with melamine baking enamels, and usually requires the addition of a volatile base to temporarily neutralize the acid (J4). 

Pre-polymer Chain Extension Polymer Properties and Applications Refs. 

Diisocyanate is often used in the chain extension reactions of biopolymers such as PEA, PCL, and their copolymers [97-101]. The combination of hard segment and soft segment may confer the resulting polyurethanes with shape-memory property [100,101]. Polyurethane is an important type of elastomeric polymer for biomedical applications [1,9,11]. Chain extension or cross-linking by diisocyanate can be adapted to many -OH-terminated or H-containing polymers or prepolymers [102]. The convenience of the urethane chemistry has made it into a very popular way of polymer chain extension method in biomaterial designs. 

H. H. Chuah, T. Kanamoto and R. S. Porter, Measures for Polymer Chain Extension, in High Modulus Polymers Approaches to Design and Development (A. E. Zachariades and R. S. Porter Eds.), Marcel Dekker, New York, 1988, p. 259... 

Models of polymer chain extension were first used to compare the effect of the glycosidic linkage geometry of simple polysaccharide chains, eg cellulose and amylose (43). Both polymers are 1,4-linked glucans the only difference is in the anomeric configuration on the C-1 atom of the monomeric unit, a for amylose and for cellulose. The calculated data show a remarkable pseudohelical chain... 

In 1965, researchers at DuPont discovered a new method of producing an almost perfert polymer chain extension. Hie polymers PBA and PPTA were found to form lyotropic LC solutions due to rigidity of their macromolecules. 

Formation of the addition product may provide two stereoisomers when the polymer chain extensions occur in trans conformation ((a) and (b)) or in gauche arrangement ((c) and (d)), as shown in the Newman projection in Scheme 42. 

Recent studies have shown that structural differences among components, eg differences in tacticity, affect phase behavior (33-35). The inability of the Flory-Huggins theory to account for these experimental observations is mainly a result of neglecting the particular molecular structure of the polymer chains. Extensive studies have been undertaken in order to be able to link polymer phase behavior to molecular structure (36). The polymer reference interaction site model (PRISM) represents one of the successful approaches in understanding the relationship between molecular structure, local packing and thermod3mamics. 

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