Chain polymerisation
You need a monomer with a double bond, such as ethylene or vinyl acetate, and a particle to start the process. It opens the double bond, adds a monomer and passes the activity to the chain end. Within a fraction of a second to a few seconds the chain grows to thousands of units, and then growth stops.
The process consists of initiation (forming the active centre), propagation and termination; chain transfer is possible. By the nature of the active centre there is radical polymerisation (initiators: peroxides, azo compounds), ionic (cationic and anionic) and coordination polymerisation on Ziegler-Natta catalysts. Polyethylene, polypropylene, polyvinyl chloride, polystyrene and polyvinyl acetate are made this way. In radical polymerisation high molar mass chains appear early in the process, and with conversion it is mainly the polymer yield that grows; in living polymerisation molar mass grows in proportion to conversion.
Polycondensation
Here monomers carry reactive groups at their ends, for example an acid and an alcohol group. They link in pairs, then pairs link with pairs, and the chain grows step by step. Often a small molecule, such as water, is released.
In polycondensation, functional groups of monomers, oligomers and polymers of any length react, often releasing a low molecular weight product (water, alcohol, HCl). Polyesters, polyamides, phenol-formaldehyde and urea-formaldehyde resins form this way. Step-growth reactions without a by-product are called polyaddition (polyurethanes form this way).
The Carothers equation
Step growth has a catch: long chains only appear at the very end. When half the groups have reacted, the average chain is just two units long. To get a hundred units, 99% of the groups must react.
For linear polycondensation of equimolar bifunctional monomers, the number-average degree of polymerisation is linked to the conversion of functional groups p by the Carothers equation: Xn = 1/(1 - p). At p = 0.5 Xn = 2, at p = 0.99 Xn = 100. High molar mass therefore needs very pure monomers, an exact group ratio and removal of the by-product.
Networks and resins for wood
If a monomer has more than two reactive sites, chains start linking to each other, and at some point the whole mass turns into a single network. That is how the glue in plywood and particleboard hardens.
With monomer functionality above two, step growth leads to branching and, at the gel point, to a three-dimensional network. Phenol is trifunctional towards formaldehyde (two ortho and one para position). Resols are made in alkaline conditions with excess formaldehyde and cure on heating; novolacs are made in acid with excess phenol and need a hardener to cure. Urea-formaldehyde resins cure in acidic conditions and are widely used for particleboard.
Controlling the process
A polymer can be made in bulk, in solution, in water as droplets or as an emulsion. PVA glue, for example, is a water-based dispersion of polyvinyl acetate (PVAc) made by emulsion polymerisation.
Polymerisation is run in bulk, solution, suspension or emulsion; polycondensation in the melt, in solution or at an interface. The method determines heat removal, molar mass and product form. Polyvinyl acetate (PVAc) dispersion (PVA glue) is made by emulsion polymerisation of vinyl acetate.
Key facts
- chain growth
- initiation, propagation, termination
- step growth
- groups react
- Carothers
- Xn = 1/(1 - p)
- p = 0.99
- Xn = 100
- phenol
- trifunctional
- PVA (PVAc) glue
- emulsion polymerisation
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.
- Odian G. Principles of Polymerization. 4th ed. Hoboken, NJ: Wiley-Interscience, 2004.
- Flory P.J. Principles of Polymer Chemistry. Ithaca, NY: Cornell University Press, 1953.