Addition polymers
In addition polymerisation, many alkene monomers open their C=C π bonds and join into a long saturated chain; no other product forms. The repeat unit is drawn by turning C=C into C–C with extension bonds through brackets: ethene → poly(ethene), chloroethene → poly(chloroethene) (PVC), propene → poly(propene). Poly(alkenes) are unreactive (saturated, non-polar C–C and C–H bonds), so they're non-biodegradable. Disposal: landfill (lasts centuries), incineration (energy recovery but CO₂ and, for PVC, HCl and dioxins), recycling, or biodegradable alternatives.
≈ 35 minAlkenes and electrophilic addition
The C=C bond has a σ bond and a weaker π bond with high electron density, which attracts electrophiles. In electrophilic addition, the π electrons attack the electrophile (e.g. H in HBr, or an induced-dipole Br₂), forming a carbocation; the anion then bonds to it. Alkenes add H₂ (Ni, 150 °C), steam (H₃PO₄, 300 °C, 60 atm), HX and X₂. Markovnikov: with unsymmetrical alkenes, the more stable carbocation (tertiary > secondary > primary, because alkyl groups are electron-donating) forms the major product. Cold dilute acidified KMnO₄ gives a diol; hot concentrated KMnO₄ cleaves the C=C.
≈ 50 min