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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper II covers organic chemistry and spectroscopy.

UPSC Civil Services (Mains), Chemistry optional · 2019 · Paper II

Q8(c)(i). Monomers of PS, LDPE, PP, PMMA and Buna-S

  • 10 marks
  • Answered
Question

Name the monomers of the following polymers:
(1) PS (2) LDPE (3) PP (4) PMMA (5) Buna-S

Answer in brief

(1) Styrene; (2) ethylene; (3) propylene; (4) methyl methacrylate; (5) buta-1,3-diene and styrene (Buna-S = styrene–butadiene rubber; Bu = butadiene, Na = sodium initiator, S = styrene).

Key points

  • Introduction. All five are addition (chain-growth) polymers of alkenes: the C=C of each monomer opens and the units link head-to-tail. Buna-S is a copolymer of two monomers.
  • Application. SBR is the main tyre rubber; PS, LDPE and PP are the bulk packaging plastics, and PMMA replaces glass in lenses and signs.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

Introduction. All five are addition (chain-growth) polymers of alkenes: the C=C of each monomer opens and the units link head-to-tail. Buna-S is a copolymer of two monomers.

PolymerMonomerStructureNote
PS, polystyrenestyrene (vinylbenzene)-0.4q8cₛtyradical polymerisation; atactic, glassy
LDPEethylene (ethene)CH₂=CH₂radical polymerisation at high pressure; branched chains, low density
PP, polypropylenepropylene (propene)CH₂=CH-CH₃Ziegler–Natta catalyst gives isotactic PP
PMMAmethyl methacrylate-0.4q8cₘmaradical polymerisation; transparent “acrylic glass” (Perspex)
Buna-S (SBR)buta-1,3-diene + styreneCH₂=CH-CH=CH₂ + styreneemulsion copolymerisation (mostly butadiene); synthetic rubber

Conclusion. (1) Styrene; (2) ethylene; (3) propylene; (4) methyl methacrylate; (5) buta-1,3-diene and styrene (Buna-S = styrene–butadiene rubber; Bu = butadiene, Na = sodium initiator, S = styrene).

Application. SBR is the main tyre rubber; PS, LDPE and PP are the bulk packaging plastics, and PMMA replaces glass in lenses and signs.

Practising this answer? Write it in a Mains test and get it checked by VKC. Civil Services Mains test series · Telegram

Model answer to Q8(c)(i), 2019 Paper II: solution page 21
Solution page 21 of 23
Read the whole 2019 solution2019 question paper

All parts of Question 8