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Organic Chemistry
Table of Contents
Unit 8 Overview: Alkenes
8.1 Preparing Alkenes: A Preview of Elimination Reactions
8.2 Halogenation of Alkenes: Addition of X2
8.3 Halohydrins from Alkenes: Addition of HO-X
8.4 Hydration of Alkenes: Addition of H2O by Oxymercuration
8.5 Hydration of Alkenes: Addition of H2O by Hydroboration
8.6 Reduction of Alkenes: Hydrogenation
8.7 Oxidation of Alkenes: Epoxidation and Hydroxylation
8.8 Oxidation of Alkenes: Cleavage to Carbonyl Compounds
8.9 Addition of Carbenes to Alkenes: Cyclopropane Synthesis
8.10 Radical Additions to Alkenes: Chain-Growth Polymers
8.11 Biological Additions of Radicals to Alkenes
8.12 Reaction Stereochemistry: Addition of H2O to an Achiral Alkene
8.13 Reaction Stereochemistry: Addition of H2O to a Chiral Alkene

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8.10 Radical Additions to Alkenes: Chain-Growth Polymers

Citation:

Radical polymerization is a key process in creating many everyday plastics. It involves three main steps: initiation, propagation, and termination. Understanding these steps helps us grasp how small molecules join to form long chains.

Vinyl monomers, with their distinctive carbon-carbon double bonds, are the building blocks of many common polymers. As these monomers link up, they form regular patterns along the chain, giving plastics their unique properties. This process is crucial in manufacturing materials we use daily.

Radical Polymerization of Alkenes

Steps of radical polymerization

  • Initiation
    • Initiator (peroxide or azo compound) decomposes into free radicals when heated or exposed to light
    • Initiator radical adds to the alkene monomer (ethylene or propylene), forming a new radical on the monomer
  • Propagation
    • Monomer radical adds to another monomer, extending the chain by forming a new carbon-carbon single bond
    • Process repeats many times, forming a growing polymer chain with the radical always on the end
  • Termination
    • Occurs when the radical on the growing chain is destroyed, stopping further propagation
    • Two methods of termination:
      1. Combination: Two growing chains couple together, forming a single larger non-radical polymer
      2. Disproportionation: A hydrogen atom transfers from one growing chain to another, forming two separate non-radical polymers of different lengths

Vinyl monomers in polymer formation

  • Vinyl monomers have the general structure $\ce{CH2=CHR}$
    • $\ce{R}$ represents any substituent group (chloride, phenyl, etc.)
  • During polymerization, the carbon-carbon double bond breaks and forms single bonds with adjacent monomers
  • The $\ce{R}$ groups end up attached to every other carbon atom along the polymer chain backbone
  • This alternating pattern of substituents results in polymers with regularly spaced $\ce{R}$ groups (PVC, polystyrene)

Radical intermediates of vinyl monomers

  • Unsymmetrically substituted vinyl monomers have the general structure $\ce{CH2=CHR}$
  • Addition of the initiator radical can occur at either carbon atom of the double bond
  • Two possible radical intermediates can form:
    1. $\ce{•CH2-CHR-}$ (more stable)
      • Radical is stabilized by resonance with the adjacent $\ce{R}$ group
      • Favored intermediate due to increased stability from delocalization
    2. $\ce{-CH2-C•R-}$ (less stable)
      • Radical is located directly on the carbon bearing the $\ce{R}$ group
      • Less favored intermediate due to lack of resonance stabilization
  • The more stable $\ce{•CH2-CHR-}$ radical dominates propagation
    • Leads to formation of a polymer with a regular head-to-tail structure: $\ce{-[-CH2-CHR-]_n-}$

Chain-Growth Polymerization

  • Radical polymerization is a type of chain-growth polymerization
  • Process involves the addition of monomers to a growing chain
  • Free radicals initiate and propagate the chain reaction
  • Monomers are small molecules that can link together to form a polymer
  • The resulting polymer is a large molecule composed of repeating monomer units

Key Terms to Review (22)

Monomer: A monomer is a small molecule that can react with other monomer molecules to form a polymer through chemical reactions. In the context of alkenes, it often contains carbon-carbon double bonds that are reactive sites for polymerization.
Chain reaction: In organic chemistry, a chain reaction is a sequence of reactions where a reactive intermediate, often a free radical, initiates and propagates multiple steps that lead to the formation of products, with the process potentially continuing until terminated. This self-sustaining mechanism amplifies the effect of a single reactant through successive stages.
Propagation step: In the context of preparing alkenes through elimination reactions in organic chemistry, the propagation step is a phase within a chain reaction where intermediates react with stable molecules to produce new intermediates and propagate the reaction sequence. This step repeats itself, sustaining the chain reaction until terminated.
Azo compounds,: Azo compounds are organic compounds characterized by the presence of a nitrogen-nitrogen double bond (N=N), connecting two aryl groups. These are synthesized from arylamines and find extensive use in dyeing industries due to their vibrant colors.
Ethylene: Ethylene is a colorless, flammable gas with the chemical formula C₂H₄. It is the simplest alkene and is widely used in the chemical industry for the production of various organic compounds and polymers. Ethylene is a key term that connects to several important topics in organic chemistry, including the structure of alkenes, chemical bonding, and industrial applications.
Disproportionation: Disproportionation is a chemical reaction in which a single reactant is simultaneously oxidized and reduced, resulting in the formation of two or more different products. This process is an important concept in various areas of organic chemistry, including radical reactions, radical additions to alkenes, radical halogenation of alkanes, and chain-growth polymerization.
Combination: Combination is the process of two or more reactants coming together to form a new product. This term is particularly relevant in the context of radical reactions, radical additions to alkenes, radical halogenation of alkanes, and chain-growth polymerization.
Propylene: Propylene, also known as propene, is a colorless, flammable gas that is an important industrial chemical. It is a key building block for a variety of products and plays a crucial role in the production of many common materials and fuels.
Azo Compound: An azo compound is a class of organic compounds containing the azo functional group (-N=N-), which connects two aromatic rings. These compounds are known for their vibrant colors and are widely used as dyes, pigments, and in various industrial applications.
PVC: PVC, or polyvinyl chloride, is a widely used thermoplastic polymer that has become an essential material in a variety of applications due to its versatility, durability, and cost-effectiveness. PVC is a key player in the context of both radical additions to alkenes for chain-growth polymers and the structure and physical properties of polymers.
Propagation: Propagation refers to the process of continuing or extending a reaction or phenomenon, particularly in the context of radical reactions and chain-growth polymerization. It describes the steps that sustain and propagate a reaction once it has been initiated.
Radical Polymerization: Radical polymerization is a chain-growth polymerization process where the propagation of the polymer chain occurs through the addition of monomers to a reactive free radical site. This process is a key mechanism in the formation of many common polymers and is central to the topics of radical additions to alkenes and chain-growth polymers.
Head-to-Tail Structure: The head-to-tail structure refers to the arrangement of monomers within a polymer chain, where the monomers are connected in a specific orientation such that the head group of one monomer is joined to the tail group of the adjacent monomer. This structural feature is particularly important in the context of radical additions to alkenes and the formation of chain-growth polymers.
Free Radical: A free radical is a highly reactive chemical species that contains an unpaired electron in its outer shell. These unstable molecules are constantly seeking to pair up their unpaired electron, making them highly reactive and capable of initiating chain reactions in various chemical processes, including those involved in the formation of chain-growth polymers and biological additions to alkenes.
Peroxide: A peroxide is a chemical compound that contains an oxygen-oxygen single bond (O-O). Peroxides are commonly encountered in the context of electrophilic addition reactions of alkenes and radical additions to alkenes, which are important topics in organic chemistry.
Initiation: Initiation is the first and critical step in various processes, marking the beginning of a sequence of events or reactions. This term is particularly relevant in the contexts of radical additions to alkenes, radical halogenation of alkanes, transcription of DNA, and translation of RNA during protein biosynthesis.
Monomer: A monomer is a small, repeating molecular unit that can be chemically bonded to other identical molecules to form a larger polymer structure. Monomers are the fundamental building blocks of polymers, which are macromolecules composed of many repeating monomer units.
Vinyl Monomers: Vinyl monomers are unsaturated organic compounds containing a vinyl group, which is a carbon-carbon double bond attached to two hydrogen atoms. These monomers are the building blocks for the formation of chain-growth polymers through radical addition reactions.
Polymer: A polymer is a large molecule composed of repeating structural units called monomers, which are connected by covalent bonds. Polymers are the foundation of chain-growth polymerization reactions, where monomers link together to form long, complex macromolecules.
Polystyrene: Polystyrene is a synthetic, thermoplastic polymer made from the monomer styrene. It is a widely used material in the context of radical additions to alkenes and chain-growth polymerization.
Chain Reaction: A chain reaction is a sequence of reactions where the products of one reaction trigger additional reactions, leading to a self-sustaining and amplifying process. This concept is central to understanding radical additions to alkenes and the preparation of alkyl halides from alkanes.
Termination: Termination is the final step in a radical reaction mechanism where reactive radicals are converted into stable products, effectively stopping the chain reaction. This process is crucial in both synthetic organic chemistry and biological systems, as it ensures that the chain reactions do not continue indefinitely, leading to uncontrolled product formation or cellular damage.