Polymer-analogous reactions
You can replace groups on the units and keep the chain itself. That is how acetate is made from cellulose and polyvinyl alcohol from polyvinyl acetate.
Polymer-analogous reactions affect functional groups while keeping the main chain and the degree of polymerisation: esterification and etherification of cellulose (in practice often with some loss of DP), saponification (industrially methanolysis) of polyvinyl acetate to polyvinyl alcohol. Group reactivity in a polymer may differ from that in low molecular weight analogues because of neighbouring units, accessibility and conformations.
Crosslinking and grafting
Chains can be joined by bridges, and the polymer becomes a network. That is how raw rubber becomes vulcanised rubber. New branches of another polymer can also be grown onto a chain.
Crosslinking forms bonds between macromolecules: sulfur vulcanisation of rubber, curing of resins, crosslinking of cellulose with formaldehyde and polycarboxylic acids in crease-resistant textile finishing. Grafting attaches side chains of another composition to a macromolecule, for example in graft copolymerisation of vinyl monomers onto cellulose.
Degradation and its causes
Chains are broken by heat, light, oxygen, acids, alkali, mechanical force and microbes. Each break shortens the chains, and the material weakens.
By agent there is thermal, mechanical, photochemical, oxidative, hydrolytic (acid, alkaline) and biological degradation. By mechanism there is random scission and depolymerisation from chain ends. For cellulose the main routes are acid and enzymatic hydrolysis of glycosidic bonds, oxidative degradation, alkaline stepwise loss of units from the reducing end and alkaline hydrolysis at high cooking temperatures.
Random or from the end
If a chain breaks at random places, the average length drops very fast while hardly any mass is lost. If units come off one by one from the end, the length drops slowly but a lot of mass goes. The diagram on this page shows both routes.
In random scission of a linear chain the degree of polymerisation falls quickly with little mass loss: the number of scissions per original molecule is s = DP0/DPt - 1 (using number-average DP). In end-wise depolymerisation (the peeling reaction of polysaccharides in alkali, unzipping of polymethyl methacrylate to monomer) DP changes slowly while mass loss is large. In alkaline pulping peeling stops when the end unit is converted into a stable metasaccharinic acid (the stopping reaction).
Special features of cellulose reactions
Cellulose reacts unevenly: first in the loose amorphous regions, and later, if at all, in the dense crystalline ones. So in acid hydrolysis chains are quickly shortened to a certain length, and then the process slows sharply.
Cellulose reactions are usually heterogeneous, with rates set by accessibility: amorphous regions react faster than crystalline ones. In heterogeneous acid hydrolysis DP falls quickly to the level-off degree of polymerisation, after which degradation slows; for wood cellulose the level-off DP is of the order of 100-300, for cotton 200-300. For uniform conversion, cellulose is activated (swelling in alkali, dissolution).
Key facts
- polymer-analogous
- DP kept
- crosslinking
- DP grows to a network
- degradation
- DP falls
- random scission
- s = DP0/DPt - 1
- end-wise
- much mass, DP slowly
- level-off DP
- about 100-300
Sources for this chapter
- Азаров В.И., Буров А.В., Оболенская А.В. Химия древесины и синтетических полимеров: учебник. 2-е изд. СПб.: Лань, 2010.
- Евстигнеев Э.И. и др. Химия древесины и синтетических полимеров. Ч. 1. Строение, свойства, химические реакции и производные целлюлозы: учебно-методическое пособие. СПб.: СПбГТУРП, 2010.
- Sjöström E. Wood Chemistry: Fundamentals and Applications. 2nd ed. San Diego: Academic Press, 1993.