Name: how alkene monomers form polymers
Uploaded: 2025-05-22T09:05:34.000Z
Duration: 2 min 35 s
Channel: JoVE

The conversion of alkenes to the high-molecular-weight compounds, called &#8216;polymers&#8217;, is a commercially significant reaction. The smallest recurring structural fragment of the polymer is termed a repeat unit, and the subscript n denotes the number of such pieces along the chain.
For instance, propylene, in the presence of a radical initiator and heat or light, yields polypropylene by a free-radical mechanism.
The initiation step involves cleavage of an oxygen&#8211;oxygen bond in an initiator like benzoyl peroxide. The benzoyloxy radical further decomposes to a phenyl radical and carbon dioxide. An azo-radical initiator, such as azoisobutyronitrile, yields isobutyronitrile radicals and molecular nitrogen through an entropy-driven decomposition.
The initiator radical adds to the double bond of an alkene, like propylene, to form a new radical in the second step.
Once initiated, the chain propagates through the continuous addition of monomeric alkene units to the active sites on the polymer chain, and hence the process is called chain-growth polymerization.
The regiospecific addition of monomers to produce highly substituted radicals forms polymers with head-to-tail linkages.
The termination occurs when the two growing chain radicals combine head-to-head. The alternate termination process is the abstraction of the alpha hydrogen atom from the active radical by another chain radical, leading to the nonradical product chains.
The mechanism of ethylene polymerization features the formation of several butyl branches on the polymer chain.
The 1,5-hydrogen abstraction through a transition state adopting a cyclohexane chair-like conformation generates branching, and the active radical site is transferred from one chain to another.
Propagation through the new radical produces a branched polymer. Due to the massive branching, the polymer molecules are less closely packed, generating low-density polyethylene.
Polymers obtained by free-radical polymerization of alkenes can be found in various articles, like packaging materials made from transparent low-density polyethylene, carpet fibers and car tires produced from polypropylene, basketballs prepared using polyisobutylene, and others.

free-radical chain reaction and polymerization of alkenes

Free-Radical Chain Reaction and Polymerization of Alkenes

The conversion of alkenes to macromolecules called polymers is a reaction of high commercial importance. The structure of the polymer is defined by a repeating unit, while the terminal groups are considered insignificant. The average degree of polymerization represents the number of repeating units in the polymer molecule and is denoted by the subscript n.
<img alt="Polymer structure diagram; hydrocarbon chain with repeating units for chemistry education." src="/files/ftp_upload/11773/11773_PT_Figure_1.svg" style="text-align: center; height: 60px;" data-altupdatedat="1754904545000" data-alt="Polymer chain structure diagram; hydrocarbon backbone with side groups, chemistry, material science." />
Alkenes undergo polymerization via a free-radical mechanism involving three steps: initiation, propagation, and termination. 
Radicals are generated in the initiation step by heating a small quantity of free-radical initiators such as benzoyl peroxide. The benzoyloxy radical loses CO2 and forms a phenyl radical (Ph&#183;), which adds to the double bond of the alkene to initiate the polymerization process. One of the electrons of the alkene &#960; bond pairs with one electron from the phenyl radical to form a new C&#8211;C bond.
<img alt="Chemical reaction diagram showing benzoyl peroxide decomposition into radicals." src="/files/ftp_upload/11773/11773_PT_Figure_2.svg" style="text-align: center; height: 60px;" data-altupdatedat="1754904545000" data-alt="Radical reaction mechanism diagram, benzoyl peroxide decomposition into radicals, chemical process." />
<img alt="Organic reaction mechanism, chemical formula, free radical formation, diagram, benzoyl peroxide decomposition." src="/files/ftp_upload/11773/11773_PT_Figure_3.svg" style="text-align: center; height: 60px;" data-altupdatedat="1754904545000" data-alt="Chemical reaction diagram of phenyl cation formation via decarboxylation process with equations." />
<img alt="Aromatic electrophilic addition mechanism, C6H5&#8226; + C3H6 &#8594; C9H11&#8226;; reaction diagram." src="/files/ftp_upload/11773/11773_PT_Figure_4.svg" style="text-align: center; height: 80px;" data-altupdatedat="1754904545000" data-alt="Chemical reaction diagram showing radical polymerization steps with benzene and propylene." />
In the propagation step, the carbon radical generated in the initiation step adds to another molecule of alkene to generate a new radical. The continuous addition of alkene monomers at the radical site yields the polymer.
<img alt="Polymerization reaction diagram; formation of polystyrene via addition polymerization of styrene." src="/files/ftp_upload/11773/11773_PT_Figure_5.svg" style="text-align: center; height: 80px;" data-altupdatedat="1754904545000" data-alt="Allylic polymerization process, chemical reaction equation, polymer chain formation." />
The termination step of the free-radical mechanism occurs via recombination or disproportionation. In a recombination step, two growing chains form a bond at their radical sites.
<img alt="Polymerization process diagram; illustrates polystyrene chain reaction with benzene rings and C-H bonds." src="/files/ftp_upload/11773/11773_PT_Figure_6.svg" style="text-align: center; height: 60px;" data-altupdatedat="1754904545000" data-alt="Polymerization process diagram with structural formulas and reaction arrows." />
In termination by disproportionation, a &#946;-hydrogen atom is transferred from one radical to another radical center, resulting in two non-radical products.
<img alt="Radical-mediated hydrogen transfer reaction diagram; chemical structure transformation process." src="/files/ftp_upload/11773/11773_PT_Figure_7.svg" style="text-align: center; height: 300px;" data-altupdatedat="1754904545000" data-alt="Free radical reaction diagram; benzyl radical formation; organic chemistry mechanism." />
The free-radical polymerization of ethylene yields low-density polyethylene (LDPE), where the low density is a consequence of significant branching in the polymer chains. Articles made from polymers surround us in various forms, such as food packaging materials, plastic bags, bottles, stationery, and automotive parts.

The conversion of alkenes to macromolecules called polymers is a reaction of high commercial importance. The structure of the polymer is defined by a ...

free-radical-chain-reaction-and-polymerization-of-alkenes

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JoVE Core

Organic Chemistry

Unit 1

Reactions of Alkenes

KEY TERMS AND CONCEPTS

how alkene monomers form polymers

Alkene monomers can join together to form polymers, which are large macromolecules used in many commercial products. In a polymer, the repeating unit defines the structure, while the terminal groups are considered insignificant. The average degree of polymerization shows how many repeating units are in a polymer molecule and is written as the subscript n.

Alkenes form polymers through a free-radical chain reaction with three main stages: initiation, propagation, and termination. In the initiation step, a small amount of a free-radical initiator such as benzoyl peroxide is heated. The benzoyloxy radical then loses CO2 and becomes a phenyl radical. That radical adds to the alkene double bond and starts the reaction.

During initiation, one electron from the alkene pi bond pairs with one electron from the phenyl radical to make a new carbon-carbon bond. The carbon radical formed next keeps the chain going in the propagation step. It adds to another alkene molecule, forms a new radical, and repeats this process many times as more monomers join the growing polymer chain.

The reaction ends in the termination step. This can happen by recombination, where two growing chains join at their radical sites. It can also happen by disproportionation, where a beta-hydrogen atom moves from one radical to another radical center and forms two non-radical products. Free-radical polymerization of ethylene makes low-density polyethylene, or LDPE, and its low density comes from significant branching in the chains. Polymers made this way are found in food packaging materials, plastic bags, bottles, stationery, and automotive parts.

How Alkene Monomers Form Polymers

regioselectivity of electrophilic additions-peroxide effect

regioselectivity-of-electrophilic-additions-peroxide-effect

Peroxide Effect in HBr Alkene Addition

Hydrogen bromide can add to an alkene in a way that goes against Markovnikov’s rule when organic peroxides are present. For example, 2-methylpropene reacts with HBr and peroxide to give 1-bromo-2-methylpropane. This change in product comes from a free radical mechanism, which reverses the usual regioselectivity. The reaction has three stages: initiation, propagation, and termination. In initiation, an oxygen-oxygen bond in the radical initiator breaks by homolytic cleavage, meaning each atom...

Alkene monomers can join together to form polymers, which are large macromolecules used in many commercial products. In a polymer, the repeating unit defines the structure, while the terminal groups are considered insignificant. The average degree of polymerization shows how many repeating units are in a polymer molecule and is written as the subscript n. Alkenes form polymers through a free-radical chain reaction with three main stages: initiation, propagation, and termination. In the...

halogenation of alkenes

Anti Addition in Alkene Halogenation

Halogenation of alkenes adds chlorine or bromine across a carbon-carbon double bond to form a vicinal dihalide. This reaction is carried out in inert, non-nucleophilic solvents such as methylene chloride, chloroform, or carbon tetrachloride. The reaction starts when bromine is drawn toward the pi electrons of an alkene such as cyclopentene. One bromine atom acts as an electrophile and adds across the double bond. This forms a cyclic bromonium ion intermediate. The bromonium ion is more stable...

formation of halohydrin from alkenes

Bromohydrin Addition in Alkene Reactions

Bromohydrin addition shows how an alkene can react with bromine in water to form a halohydrin. In this reaction, the product has a halogen and a hydroxyl group on neighboring carbons. When the halogen is bromine, the product is called a bromohydrin. When the halogen is chlorine, it is called a chlorohydrin. The reaction begins when the pi electrons of the alkene react with electrophilic bromine. This step forms a bridged intermediate called a bromonium ion. Water then acts as a nucleophile. It...

acid-catalyzed hydration of alkenes

Hydrating Alkenes to Form Alcohols

Alkenes can be converted into alcohols by adding water in the presence of an acid. Without acid, this hydration happens too slowly to matter in practice. The acid is not used up in the reaction, so this is an acid-catalyzed process. Strong acids such as sulfuric acid dissociate completely in water. In that case, the reacting acid species is the hydronium ion. The first step is a slow protonation of the alkene at the less-substituted end. This forms the more-substituted carbocation, which is...

regioselectivity and stereochemistry of acid-catalyzed hydration

regioselectivity-and-stereochemistry-of-acid-catalyzed-hydration

Carbocation Stability in Alkene Hydration

Acid-catalyzed hydration of alkenes forms alcohols through a carbocation intermediate. The reaction starts when hydronium ion protonates the alkene. This step is slow and controls the overall rate. The alkene structure matters because alkyl substituents at the double bond can change how fast the reaction occurs. The kind of carbocation formed also affects the rate. A reaction that passes through a tertiary carbocation is faster than one that forms a secondary or primary carbocation. This is...

oxymercuration-reduction of alkenes

Markovnikov Hydration with Mercury

Oxymercuration-reduction is a way to turn an alkene into an alcohol. It hydrates the alkene using mercuric acetate in a mix of tetrahydrofuran and water. The reaction first makes an organomercury adduct and then reduces it to an alcohol with sodium borohydride. The solvent mix has two jobs. Tetrahydrofuran dissolves the alkene and the aqueous mercuric acetate solution. Water acts as both a reactant and a solvent for mercuric acetate. The mechanism can be seen in the conversion of...

hydroboration-oxidation of alkenes

Anti-Markovnikov Alcohols from Alkenes

Hydroboration-oxidation turns alkenes into alcohols with anti-Markovnikov orientation. It is a complementary route to oxymercuration-demercuration, which gives Markovnikov products. In this reaction, borane adds across the alkene first, and basic hydrogen peroxide finishes the conversion to an alcohol. Borane is very reactive because the boron atom has only six electrons in its valence shell. It also has an empty 2p orbital that sits perpendicular to the plane formed by boron and its three...

regioselectivity and stereochemistry of hydroboration

regioselectivity-and-stereochemistry-of-hydroboration

Hydroboration Reaction Paths and Product Control

Hydroboration shows how alkene reactions control both where atoms add and how the product is shaped. In hydroboration–oxidation, borane attacks the alkene π bond in a concerted way, which means the bond changes happen together. This forms a cyclic, four-centered transition state. In this transition state, the BH2 group attaches to the less substituted carbon, while H attaches to the more substituted carbon. The addition happens across the same face of the alkene, so the reaction gives syn...

oxidation of alkenes: syn dihydroxylation with osmium tetraoxide

oxidation-of-alkenes-syn-dihydroxylation-with-osmium-tetraoxide

Alkene Dihydroxylation with Osmium Tetroxide

Alkene dihydroxylation with osmium tetroxide adds two hydroxyl groups across a double bond to make 1,2-diols, also called glycols. The reaction can give either anti or syn products, and this method follows syn stereochemistry. That means both new O atoms add to the same face of the alkene. The reaction occurs in two steps. First, osmium tetroxide adds to the alkene double bond in a concerted step, where both bonds form at the same time. This creates a five-membered cyclic osmate ester...

oxidation of alkenes: syn dihydroxylation with potassium permanganate

oxidation-of-alkenes-syn-dihydroxylation-with-potassium-permanganate

Potassium Permanganate Tests for Alkene Bonds

Potassium permanganate can be used to test and change alkene double bonds. When an alkene reacts with cold, dilute potassium permanganate under basic conditions, it forms a cis-diol, which is a molecule with two hydroxyl groups on the same side. A brown precipitate of manganese dioxide also appears during the reaction. The reaction starts when the permanganate ion adds to the same side of the alkene pi bond. This syn addition forms a cyclic manganate ester intermediate. Water then hydrolyzes...

oxidation of alkenes: anti dihydroxylation with peroxy acids

oxidation-of-alkenes-anti-dihydroxylation-with-peroxy-acids

Epoxide Route to Trans Diols

Epoxides provide a route to trans diols from alkenes. In anti dihydroxylation, an alkene is treated with a peroxycarboxylic acid to first make an epoxide. A diol is a compound with two hydroxyl groups, and in this pathway the two OH groups end up on opposite sides. An epoxide is a highly strained three-membered ring. Oxygen and two carbon atoms sit at the corners of an equilateral triangle. The first step uses a peracid, which is a strong oxidizing agent related to a carboxylic acid but with...

oxidative cleavage of alkenes: ozonolysis

Alkene Bond Cutting by Ozonolysis

Ozonolysis is a reaction that cuts a carbon-carbon double bond in an alkene. Ozone attacks the double bond and breaks the molecule into smaller carbonyl products. Depending on the work-up, the products can be aldehydes, ketones, or carboxylic acids. Ozone is a symmetrical bent molecule with a resonance-stabilized structure. The reaction starts with electrophilic addition of ozone across the alkene double bond. This step forms an unstable molozonide, which then reacts further to give a carbonyl...

reduction of alkenes: catalytic hydrogenation

reduction-of-alkenes-catalytic-hydrogenation

Alkene Hydrogenation and Alkane Stability

Alkene hydrogenation turns an alkene into an alkane by adding molecular hydrogen. This reduction usually happens with a transition-metal catalyst, so the reaction is called catalytic hydrogenation. Palladium, platinum, and nickel are common catalysts in solid form as fine powder on an inert surface. These catalysts do not dissolve in the reaction mixture, so they are heterogeneous catalysts. The reaction takes place on the metal surface. First, hydrogen adsorbs onto the surface and the H-H...

reduction of alkenes: asymmetric catalytic hydrogenation

reduction-of-alkenes-asymmetric-catalytic-hydrogenation

Chiral Alkene Hydrogenation for Drug Synthesis

Catalytic hydrogenation adds molecular hydrogen to an alkene double bond and turns it into an alkane. A transition-metal catalyst drives the reaction, and the hydrogen atoms add in syn stereochemistry, meaning they enter from the same side. The catalyst can be heterogeneous or homogeneous. When hydrogenation creates a chiral center, the products are usually a pair of enantiomers, which are mirror-image molecules. An enantiomeric excess of one product can be obtained with an enantioselective...

key terms and concepts

Learn how alkene monomers turn into polymers through free-radical initiation, propagation, and termination, with LDPE and everyday plastic examples.

How Alkene Monomers Form Polymers | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

How Alkene Monomers Form Polymers

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