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Exercise · Q13

Q.Vinyl chloride, CH2=CHCl\text{CH}_2{=}\text{CHCl}, undergoes free-radical addition polymerization to form PVC. Write equations for the initiation, propagation and termination steps.

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Initiation. A radical from an initiator (e.g. benzoyl peroxide), R∙\text{R}^{\bullet}, adds across the double bond of one vinyl chloride molecule: R∙+CH2=CHCl→R–CH2–CHCl∙\text{R}^{\bullet}+\text{CH}_2{=}\text{CHCl} \rightarrow \text{R--CH}_2\text{--CHCl}^{\bullet}. Propagation. This radical adds to a further vinyl chloride molecule, extending the SAME chain and regenerating a radical at the new end: R–CH2–CHCl∙+CH2=CHCl→R–CH2–CHCl–CH2–CHCl∙\text{R--CH}_2\text{--CHCl}^{\bullet}+\text{CH}_2{=}\text{CHCl} \rightarrow \text{R--CH}_2\text{--CHCl--CH}_2\text{--CHCl}^{\bullet}; this repeats many times, building the −(CH2–CHCl)n−-(\text{CH}_2\text{--CHCl})_n- backbone of PVC. Termination. Two growing radical chain-ends meet and combine, ending chain growth: R1∙+R2∙→R1–R2\text{R}_1^{\bullet}+\text{R}_2^{\bullet}\rightarrow \text{R}_1\text{--R}_2.

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PVC forms by free-radical addition of vinyl chloride: initiation generates the first radical, propagation repeatedly extends the chain by adding more vinyl chloride, and termination combines two radical ends to give the −(CH2–CHCl)n−-(\text{CH}_2\text{--CHCl})_n- backbone.

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