Macromolecule and repeat unit
The small molecule a polymer is built from is called a monomer. When monomers join, each becomes a unit of the chain. The number of units in a chain is the degree of polymerisation: for wood cellulose it runs into thousands.
A monomer is the low molecular weight compound from which a polymer forms. The constitutional repeating unit is the smallest fragment whose repetition describes the chain. The degree of polymerisation equals the number of units in a macromolecule; the molar mass of a macromolecule equals the degree of polymerisation times the unit mass plus the mass of the end groups. Oligomers contain few units, and their properties change noticeably with each added unit.
Natural, artificial, synthetic
Some polymers are made by nature: cellulose, lignin, starch, proteins. Others are made by chemically changing natural ones: that is how acetate and viscose come from cellulose. Others are built entirely by chemists from monomers: polyethylene, nylon, phenol-formaldehyde resins.
By origin, polymers are natural (polysaccharides, lignin, proteins, natural rubber), artificial, obtained by chemical modification of natural ones (cellulose esters, viscose), and synthetic, made from monomers. By the main chain they are carbon-chain polymers (a chain of carbon atoms, as in polyethylene) or heterochain polymers (oxygen, nitrogen and other atoms in the chain, as in cellulose, polyesters, polyamides).
Chain shape
A chain can be a straight thread, branch like a tree, or fuse with its neighbours into a continuous network. The shape decides the essentials: whether the polymer melts and whether it dissolves.
Polymers are linear, branched or network (crosslinked, three-dimensional). Linear and branched polymers dissolve under suitable conditions and many also melt (cellulose decomposes before melting); network polymers do not dissolve, only swell to a limited extent, and never reach the viscous flow state. Wood holds all three side by side: linear cellulose, branched hemicelluloses and lignin, which textbooks describe as a three-dimensional (highly branched, partly crosslinked) polymer.
Homopolymers and copolymers
If all units are the same, it is a homopolymer. If the chain has two or more different units, it is a copolymer. The units can alternate, mix at random, run in long blocks or hang as side chains.
Copolymers are classified by unit distribution: random (statistical), alternating, block and graft copolymers (side chains of one composition grafted onto a main chain of another). Natural heteropolysaccharides such as glucomannans are in effect copolymers. Grafting synthetic chains onto cellulose is used to modify its properties.
Thermoplastics and thermosets
Some plastics soften when heated and can be remelted: these are thermoplastics. Others harden for good when heated once, because their chains crosslink into a network: these are thermosets. Resins for plywood and particleboard belong to the second group.
Thermoplastic polymers (polyethylene, polyvinyl chloride, polyvinyl acetate) soften reversibly on heating. Thermosetting polymers form a network on curing and irreversibly lose the ability to melt and dissolve; cured phenol-formaldehyde, urea-formaldehyde and epoxy resins belong here.
Key facts
- monomer → polymer
- repeating unit
- cellulose
- linear
- hemicelluloses
- branched
- lignin
- 3D, partly crosslinked
- thermoplastic
- can be remelted
- thermoset
- irreversibly crosslinked
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.
- Odian G. Principles of Polymerization. 4th ed. Hoboken, NJ: Wiley-Interscience, 2004.
- IUPAC. Compendium of Polymer Terminology and Nomenclature (IUPAC Recommendations 2008). Cambridge: RSC Publishing, 2009.