Chemistry · Ch 15 — Chemistry in Everyday Life
Types of Polymerisation
Types of Polymerisation
Polymerisation is simply the process of building one very large, high-molecular-mass polymer out of many small monomer units, and it happens by one of two fundamentally different routes: addition polymerisation (also called chain-growth polymerisation) or condensation polymerisation (also called step-growth polymerisation, covered from 15.4.3 onward).
In addition polymerisation, alkenes polymerise under the right conditions through a chain-growth mechanism -- the reactive end of an already-growing chain adds directly across the double bond of a fresh monomer, extending the chain by one unit each time. Depending on exactly what kind of reactive species drives this growth, the mechanism can be free-radical, cationic, or anionic; the unit develops the free-radical route in full detail, using styrene's polymerisation to polystyrene as the worked example. …
What this figure shows. A step-by-step mechanism diagram: benzoyl peroxide splitting into two phenyl radicals (plus CO2) in the initiation step; a phenyl radical adding across styrene's vinyl double bond to give a stabilised benzylic radical in the first propagation step; that radical attacking a further styrene monomer to extend the chain by one more unit, repeated indefinitely; and the resulting long polystyrene chain -(CH2-CH(C6H5))n- shown at the end, still carrying an unpaired radical …