Ester cross-links, breaking

The two principal methods for processing hemicellulose to increase the accessibility of cellulose to penetrating agents are to put the hemicellulose into a liquid phase and/or reduce its molecular weight (Figure 6). Such aqueous solvents as alkali solutions solubilize hemicellulose by sodium salt formation and by breaking ester cross-links. Either true solutions or colloidal dispersion may be the mechanism by which the attack the lignin that blocks accessibility to cellulose. 

Thus, the swelling behavior in water of liquid NH3-treated hardwood is consistent with that of dilute NaOH-treated hardwood. In both, the marked increase in swelling characteristics in water is caused by the breaking of uronic esters functioning as cross-links. Other cross-links are undoubtedly present that limit the additional swelling of wood substance. These other cross-links are stable toward treatments described here some, however, are broken during conventional pulping. This... 

Polyester is a group of polymers that contain the ester functional group in their chain. Esters are chemical compounds derived from a carboxyUc acid and a hydroxyl compound, nsnally an alcohol. Most esters are considered biocompatible since they are endogenous to the human metaboUsm and able to break down to natural metabohc products by simple hydrolysis. Elastomers composed of aliphatic polyester chains cross-linked with each other by ester bonds, snch as poly(diol citrates) and poly(glyc-erol sebacate) (PGS), have received mnch attention because they are soft, elastic, and biocompatible [26,27]. Yang et al. synthesized the first citrate-based biodegradable elastomer (CABE), poly(diol citrates), in 2004 nsing a convenient and cost-effective polycondensation reaction [26,28]. 

A cleavable, temperature-responsive polymeric cross-linker was utilized by Xu and cowoikers [111] to stabilize micelles from PEO-b-PAPMA-b-poly((Af,Af-diisopropylamino)ethyl methacrylate) triblock copolymer. The PNIPAm cross-linker contained activated ester end groups that were reacted with the primary amines on the PAPMA middle block. The trithiocarbonate moiety located at the middle of PNIPAm cross-linker could then be degraded by aminolysis to break the cross-links. Temperature-responsive micelles and vesicles from diblock and triblock copolymers were shell cross-linked via interpolyelectrolyte complexation [108, 112]. The cross-links formed by the electrostatic interactions of oppositely charged polyelectrolytes could be disrupted by the addition of SME. 

---