Polyethylene structure from ethylene monomer

Steps:

• Monomer CH2=CH2 (ethylene) undergoes addition polymerization by breaking the double bond.
• Forms repeating -CH2-CH2- units linked by C-C single bonds.
• Each carbon bonds to two chain carbons and two hydrogens.
• Side groups on each carbon are solely the two hydrogen atoms.

Why D is correct:

• Polyethylene's formula (C2H4)n confirms each chain carbon attaches only to hydrogen atoms as side groups, per standard addition polymer structure.

Why the others are wrong:

• A: -CH3 groups occur in polypropylene from CH2=CH-CH3, not ethylene.
• B: -CH2- units form the main chain, not side groups.
• C: No carbon has two -CH2- side groups; all have two hydrogens, not an "either/or" arrangement.

Final answer: D