Carbocations attack pi bonds. Protonation of 2-methylpropene by hot aqueous sulfuric acid leads to the formation of two dimers:

The initial protonation produces a 1,1-dimethylethyl (tert-butyl) cation which then attacks the double bond of a second 2-methylpropene molecule. The cation addition proceeds according to the Markovnikov rule to generate the more stable carbocation. Deprotonation of the addition product from either adjacent carbon leads to a mixture of two products.

Repeated attack can lead to oligomerization and polymerization. When 2-methylpropene is treated with mineral acid under more stringent conditions, higher oligiomers can be obtained through repeated addition reactions:

At higher temperatures, polymers containing many subunits are formed.

Synthesis of Polymers Polymerization reactions can be categorized as cationic, radical, anionic, and metal catalyzed. Acid-catalyzed cationic polymerizations have already been covered. Initiators include H 2 SO 4, HF, and BF 3.

Radical polymerizations lead to commercially useful materials. The polymerization of ethene in the presence of an organic peroxide at high pressures and temperatures proceeds by a radical polymerization process.

Polyethene (polyethylene) polymerized in this way is actually a branched polymer. Branching occurs as a result of hydrogen abstraction along the growing chain by another radical center. The average molecular weight of polyethene is almost 1 million.

Polychloroethene (PVC or polyvinylchloride) is a polymer of chloroethene (vinyl chloride). The peroxide initiator and the intermediate radical chains add only to the unsubstituted end of the monomer (producing the most stable radical) which results in a very regular head-to-tail structure of molecular weight over 1.5 million. Pure PVC is fairly hard and brittle. It can be softened by the addition of carboxylic acid esters (plasticizers) for use in elastic materials such as vinyl leather, plastic covers, and garden hoses.

Anionic polymerizations require initiation by bases. Anionic polymerizations are initiated by strong bases such as alkyllithiums, amides, alkoxides, and hydroxide. The adhesive properties of “Super Glue” result from the hydroxide initiated polymerization of 2-cyanopropenoate. The electron withdrawing natures of the carbonyl and nitrile groups create a partially positive carbon center at which the hydroxide can initially attack. The negative charge on the resulting anion is then resonance stabilized by both the carbonyl and nitrile groups.

Ethene: An Important Industrial Feedstock Ethene is the basis for the production of polyethene (polyethylene). The major source of ethene is the pyrolysis of petroleum, or hydrocarbons derived from natural gas. Ethene is the starting material for the production of many other industrial chemicals:

Alkenes in Nature: Insect Pheromones Pheromones are chemical substances used for communication within a living species. Pheromones are used for sex, trail, alarm, and defense signaling, to name a few.

The sex attractant for the male silkworm moth is 10-trans-12-cis-hexadecadien-1- ol (bombykol). The natural pheromone is 10 billion times more active in eliciting a response than is the 10-cis-12- trans isomer, and 10 trillion times more active than the trans, trans isomer.