Q1(e). Natural rubber and polyethylene

Answer in brief

Natural rubber is cis-1,4-polyisoprene, derived from isoprene (2-methylbuta-1,3-diene); its residual double bonds distinguish it from saturated polyethylene.

Key points

• Isoprene is CH₂=C(CH₃)CH=CH₂; the rubber repeat is [–CH₂C(CH₃)=CHCH₂–]ₙ with cis chain geometry.
• Rubber shows C=C stretching near 1660 cm⁻¹, olefinic C–H near 3030 cm⁻¹ and a trisubstituted-alkene out-of-plane band near 830–840 cm⁻¹.
• Polyethylene, [–CH₂CH₂–]ₙ, lacks substantial alkene bands; it shows CH₂ stretches near 2920 and 2850, bending near 1460 and rocking near 720–730 cm⁻¹.
• Use the whole pattern; weak unsaturation from chain ends or defects does not make polyethylene a polyisoprene.