Polymerisation and the addition mechanism · ⁨聚合和加成机理⁩

Tiny units, giant chains

• Many alkenes can join into one long polymer 聚合物.
• The addition mechanism goes through a carbocation 碳正离子.
• Its stability explains Markovnikov's rule 马氏规则.

Addition polymerisation 加成聚合

• In addition polymerisation, many alkene molecules open their double bonds and join into one long chain — with no other product.
• Ethene → poly(ethene); the long-chain product is a polymer.

Many ethene molecules join into poly(ethene)

The electrophilic addition mechanism

• The electron-rich C=C attracts the electrophile (e.g. $\text{Br}^{\delta+}$).
• A positive intermediate, a carbocation, forms.
• The negative part (e.g. $\text{Br}^-$) then attacks the carbocation.

Electrophilic addition of bromine to ethene via a carbocation

Carbocation stability and Markovnikov

• Alkyl groups push electrons towards the positive carbon — the inductive effect 诱导效应.
• So tertiary > secondary > primary in stability.
• When HBr adds to propene, the more stable carbocation forms — so H adds to the carbon that already has more H atoms. This is Markovnikov's rule.

Alkyl groups stabilise the positive carbon, so tertiary > secondary > primary — the basis of Markovnikov's rule

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