Reversing addition polymerization to identify the alkene monomer

Steps:

• Examine the repeat unit's carbon skeleton, which forms by opening the monomer's C=C bond and linking units.
• Identify the two adjacent carbons in the repeat unit that were originally double-bonded, based on chain connectivity and substituents.
• Reconstruct the monomer by inserting a C=C bond between those carbons and adjusting valences.
• Determine the IUPHAC name, including stereochemistry from the unit's configuration.

Why D is correct:

• trans-4-methylhex-3-ene (CH₃CH₂CH=C(CH₃)CH₂CH₃, trans) opens at C3=C4 to yield the repeat unit -CH(CH₂CH₃)C(CH₃)(CH₂CH₃)-, matching the diagram's chain and branching.

Why the others are wrong:

• A: Invalid structure; 1,2-diethyl-1-methylethene violates alkene naming rules with overcrowded substitution at C1.
• B: Shorter pentene chain with ethyl/methyl at wrong positions, yielding mismatched repeat unit length.
• C: Dimethyl substitution on pentene creates symmetric branching not present in the diagram's unsymmetric skeleton.

Final answer: D