የኬሚስትሪ መግቢያ ፈተና፣ 2018 ዓ.ም. (2026)

A step-growth polymer builds up when two functional groups react and join, usually letting a small molecule such as water leave. A polyester forms when a diacid and a diol condense, so any two pieces that meet can link and the chain grows step by step. PTFE, polystyrene and PVC all come from monomers with a C=C bond that opens and adds onto a growing active end, which is chain growth. PTFE tempts because it is a famous plastic, but tetrafluoroethene polymerises through its double bond, not by condensation.

A thermoplastic softens every time it is heated, because its chains are separate and held together only by weak forces between them. Bakelite is made from phenol and formaldehyde into a rigid three dimensional network of covalent cross links, so heating cannot slide the chains past each other. It sets once and then chars instead of melting, which makes it a thermoset. Nylon looks like a candidate because it is also a condensation polymer, but its chains are linear, so it melts and can be reshaped.

An addition polymer keeps every atom of its monomer, because the monomer carries a C=C bond that opens and adds on. PVC, polypropylene and PMMA all come from vinyl monomers in that way. Dacron is poly(ethylene terephthalate), made when a diacid and a diol join and a water molecule leaves at each link, so it is a condensation polymer. PMMA can look out of place because of its ester side group, but that ester is already present in the monomer and the chain still grows across the C=C bond.

Poly(methyl methacrylate) is the hard, glass clear plastic sold as Lucite or Plexiglas, used for aircraft windows, safety screens and signs. It is transparent, tough and lighter than glass. PET is also clear, which is why option B tempts, but PET is the bottle and fibre polyester known as Dacron, not the rigid glass substitute.

Teflon is PTFE, a carbon chain wearing fluorine on every side. The C-F bonds are strong and the fluorine surface attracts other substances very weakly, so food and water do not stick and the coating survives cooking temperatures. That is why it lines frying pans and other cookware. Option A, aircraft windows, belongs to PMMA, the transparent plastic, while PTFE is waxy and opaque.

PVC stands for poly(vinyl chloride), so its monomer must be vinyl chloride, \( \mathrm{CH_2{=}CHCl} \). The double bond opens and the units join into a long chain carrying one chlorine on every second carbon. Option C, \( \mathrm{CH_2Cl-CH_2Cl} \), has the same atoms but no double bond, so there is nothing for the chain to add across. It is a saturated dichloroethane and cannot polymerise this way.