1,4-Addition Polymerization of Buta-1,3-Diene

Steps:

• Buta-1,3-diene monomer is CH2=CH-CH=CH2 with conjugated double bonds.
• In addition polymerization, 1,4-addition occurs when the chain bonds to carbon 1 of one monomer and carbon 4 of the next.
• This leaves a cis or trans double bond between carbons 2 and 3 in the backbone.
• The resulting repeat unit is -CH2-CH=CH-CH2-, with cis specifying the double bond geometry.

Why A is correct:

• It represents the standard 1,4-addition repeat unit for cis-polybuta-1,3-diene, as defined in polymer chemistry for retaining one double bond per monomer.

Why the others are wrong:

• B: Shows incorrect bonding (-CH2-CH=CH2-CH-), with a terminal =CH2 not matching butadiene's polymerization.
• C: Depicts a vinyl-like unit (=CH-CH2-CH=CH2-), typical of 1,2-addition but not the linear 1,4-backbone.
• D: Indicates fully conjugated -CH=CH-CH=CH-, which would require no single bonds added, unlike actual polymerization.

Final answer: A