Polymerization of symmetrical alkenes yields uniform substituents

Steps:

• Identify the repeating unit in polymer Z: -[CH(CH3)-CH(CH3)]-, where each chain carbon bonds to one H and one CH3.
• Recall addition polymerization opens the alkene double bond, forming the chain from the monomer's structure.
• Match the monomer to produce identical substituents on each carbon: requires CH3-CH=CH-CH3.
• Confirm but-2-ene polymerizes to -[CH(CH3)-CH(CH3)]-n, fitting the description.

Why B is correct:

• But-2-ene (CH3-CH=CH-CH3) undergoes addition polymerization to form -[CH(CH3)-CH(CH3)]-n, where each carbon has one H and one CH3 per the repeating unit formula.

Why the others are wrong:

• A: But-1-ene yields -[CH2-CH(CH2CH3)]-n; alternating carbons have two H or one H and one ethyl.
• C: Methylpropene yields -[CH2-C(CH3)2]-n; carbons have two H or no H and two CH3.
• D: Propene yields -[CH2-CH(CH3)]-n; alternating carbons have two H or one H and one CH3.

Final answer: B