Cis configuration in 1,4-addition polymerization

Steps:

• Buta-1,3-diene (CH2=CH-CH=CH2) undergoes 1,4-addition in polymerization.
• In cis-polybutadiene, the polymer chain forms with a double bond between carbons 2 and 3.
• The cis isomer has the chain substituents on the same side of the double bond.
• Repeat unit is -CH2-CH=CH-CH2- with cis geometry.

Why C is correct:

• C depicts the 1,4-repeat unit with cis double bond, as defined for cis-polybuta-1,3-diene.

Why the others are wrong:

• A: Shows 1,2-addition (pendant vinyl group), not 1,4-cis.
• B: Displays trans double bond configuration.
• D: Indicates saturated chain, lacking the required double bond.

Final answer: C