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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.

Radical Chain-Growth Polymerization: Overview

Radical Chain-Growth Polymerization: Overview

Chain-growth or addition polymerization is successive addition reactions of monomers with a polymer chain. In radical chain-growth polymerization, the reaction proceeds via a free-radical intermediate. The free radical is formed from radical initiators, which spontaneously generate free radicals by homolytic fission. Organic peroxides (such as dibenzoyl peroxide, as shown in Figure 1) or azo compounds are popular radical initiators. A low concentration ratio of radical initiator to monomer is...

Radical Formation: Overview

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A bond can be broken either by heterolytic bond cleavage to form ions or homolytic bond cleavage to yield radicals. A fishhook arrow is used to represent the motion of a single electron in homolytic bond cleavage. There are two main sources from which radicals can be formed:
Radicals from spin-paired molecules:
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Radical Chain-Growth Polymerization: Mechanism

Radical Chain-Growth Polymerization: Mechanism

The radical chain-growth polymerization mechanism consists of three steps: initiation, propagation, and termination of polymerization. The polymerization initiates when a free radical generated from the radical initiator adds to the unsaturated bond in the monomer. The unpaired electron of the free radical and one π electron in the unsaturated bond creates a σ bond between the free radical and the monomer. As a result, the other π electron in the unsaturated bond converts this species into...

Radical Reactivity: Overview

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Radicals, the highly reactive species, gain stability by undergoing three different reactions. The first reaction involves a radical-radical coupling, in which a radical combines with another radical, forming a spin‐paired molecule. The second reaction is between a radical and a spin‐paired molecule, generating a new radical and a new spin‐paired molecule. The third reaction is radical decomposition in a unimolecular reaction, forming a new radical and a spin‐paired...

Radical Chain-Growth Polymerization: Chain Branching

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The skeletal structure of polymers synthesized via radical polymerization is always branched. For example, the polymerization of ethylene by radical polymerization results in a low-density grade of polyethylene with a heavily branched skeletal structure. Here, the radical site abstracts hydrogen from the growing chain, and the radical site shifts from the end (a primary carbon center) to anywhere within the growing chain (a secondary carbon center). Consequently, the part of the chain from the...

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Depolymerization of Free-Radical Polymers with Spin Migrations.

Tianrong Yu^1,2, Yang Gao^1,2, Bo Wang^1,2

¹Institute of Atomic and Molecular Physics, Jilin University, Changchun, 130012, P. R. China.

Chemphyschem : a European Journal of Chemical Physics and Physical Chemistry

|September 4, 2015

Summary

This summary is machine-generated.

Investigating free-radical depolymerization of poly(alpha-methylstyrene) tetramers reveals energy barriers for monomer dissociation. Reactions initiate at unsaturated chain ends, involving specific molecular orbitals and spin transfer.

Keywords:

density functional theory depolymerization polymers radicals reaction mechanisms

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Area of Science:

Chemical Engineering
Materials Science
Computational Chemistry

Background:

Depolymerization mechanisms are critical in polymer science and chemical engineering.
Understanding free-radical depolymerization is key to controlling polymer degradation and synthesis.

Purpose of the Study:

To investigate the depolymerization reactions of poly(alpha-methylstyrene) tetramers using computational methods.
To elucidate the reaction pathways, energy barriers, and electronic structure involved in free-radical depolymerization.

Main Methods:

First-principles density functional theory (DFT) calculations.
Analysis of reaction energy barriers, electronic structures, and spin populations.

Main Results:

Depolymerization reactions require energy barriers between 0.58 and 0.77 eV.
C-C bond scission at the chain end drives alpha-methylstyrene monomer dissociation.
Reactions are favored at unsaturated CR3 ends, with frontier molecular orbitals localized there.
Spin population analysis revealed a distinct net spin-transfer process.

Conclusions:

The study provides insights into the mechanism of free-radical depolymerization of poly(alpha-methylstyrene).
Findings contribute to the fundamental understanding of polymer degradation and can guide future experimental research.