Reading a Free-Radical Copolymer: How the Chain Grows Head to Tail
Saran wrap is a copolymer of vinylidene chloride (1,1-dichloroethene, CH₂=CCl₂) and vinyl chloride (CH₂=CHCl). Questions on addition polymers give you the monomers and the first step, then ask for the repeat unit, so the whole skill is knowing which end of each monomer the growing radical attacks.
Repeat Unit of a Vinylidene Chloride and Vinyl Chloride Copolymer is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
The radical attacks the CH₂ end and ends up on the carbon that carries the chlorines
That end is the less hindered one, and the radical it leaves behind sits on the other carbon, where it is stabilised by the attached chlorine atoms (lone-pair donation) and by being more substituted.
This is head-to-tail growth: every new unit joins through its CH₂ end to the substituted carbon at the end of the chain.
Following the chain: CH₂ alternates with a halogen-bearing carbon
The chain-end radical on the CCl₂ carbon now attacks the CH₂= end of vinyl chloride, so the new bond is CCl₂-CH₂ and the radical moves onto the CHCl carbon.
Written as a repeat unit the backbone reads -CH₂-CCl₂-CH₂-CHCl⁻. In any head-to-tail copolymer of CH₂=CXY monomers the backbone alternates a CH₂ carbon with a substituted carbon: two CH₂ groups never sit side by side, and two halogen-bearing carbons never touch.
Use the pattern to reject the wrong drawings
To choose among drawn repeat units, walk along the chain and test each pair of neighbouring carbons. A correct head-to-tail unit alternates a CH₂ carbon with a halogen-bearing carbon all the way along. A unit containing -CH₂-CH₂⁻ (tail-to-tail) or two halogen-bearing carbons next to each other (head-to-head) breaks the rule and is wrong even if the atom count looks right. Also check that the unit contains both monomers, one CCl₂ carbon from vinylidene chloride and one CHCl carbon from vinyl chloride, and that the open bonds are drawn at the chain ends.
OlympIQ exam lens
- ✓Free-radical addition attacks the CH₂= end of a vinyl monomer and leaves the radical on the carbon that bears the halogen(s)
- ✓This gives head-to-tail growth: CH₂ alternates with a halogen-bearing carbon along the backbone
- ✓The Saran-type copolymer unit is -CH₂-CCl₂-CH₂-CHCl⁻
- ✓Reject repeat units with two adjacent CH₂ groups (tail-to-tail) or two adjacent halogen-bearing carbons (head-to-head)
- ✓A copolymer repeat unit must include both monomers; count the CCl₂ and CHCl carbons separately
Do not pick the drawing that merely has the right number of C, H and Cl atoms. The same atoms can be joined head-to-head, tail-to-tail or head-to-tail, and only head-to-tail follows from the radical always attacking the CH₂ end of the next monomer.
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