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Cationic Chain-Growth Polymerization: Mechanism00:57

Cationic Chain-Growth Polymerization: Mechanism

The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization process, the π bond of a monomer gets protonated by the Lewis acid catalyst, which is formed from boron trifluoride and water. The protonation of the π bond generates a carbocation stabilized by the electron‐donating group. In the propagation step, the π bond of the second monomer acts as a nucleophile and attacks the generated carbocation,...
Anionic Chain-Growth Polymerization: Mechanism01:04

Anionic Chain-Growth Polymerization: Mechanism

The mechanism for anionic chain-growth polymerization involves initiation, propagation, and termination steps. In the initiation step, a nucleophilic anion, such as butyl lithium, initiates the polymerization process by attacking the π bond of the vinylic monomer. As a result, a carbanion, stabilized by the electron‐withdrawing group, is generated. The resulting carbanion acts as a Michael donor in the propagation step and attacks the second vinylic monomer, which acts as a Michael acceptor.
Radical Chain-Growth Polymerization: Mechanism01:09

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 the...
Radical Chain-Growth Polymerization: Overview01:10

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 Chain-Growth Polymerization: Chain Branching01:17

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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...
Polymers02:34

Polymers

The word polymer is derived from the Greek words “poly” which means “many” and “mer” which means “parts”. Polymers are long chains of molecules composed of repeating units of smaller molecules, known as monomers. They either occur naturally, such as DNA and proteins, or can be constructed synthetically, like plastics. They have varied structural characteristics, such as linear chains, branched chains, or complex networks, that contribute to the properties that they exhibit. Additionally,...

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Kinetics of loop formation in polymer chains.

Ngo Minh Toan1, Greg Morrison, Changbong Hyeon

  • 1Biophysics Program, Institute for Physical Science and Technology, University of Maryland at College Park, College Park, Maryland 20742, USA.

The Journal of Physical Chemistry. B
|February 14, 2008
PubMed
Summary
This summary is machine-generated.

We studied polymer loop formation kinetics, finding that solvent quality significantly impacts cyclization time. Poor solvents dramatically accelerate loop formation compared to good solvents, with implications for biological molecules.

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Area of Science:

  • Polymer Physics
  • Chemical Kinetics
  • Computational Biology

Background:

  • Understanding polymer chain dynamics is crucial for molecular biology.
  • Loop formation is a fundamental process in DNA, RNA, and protein folding.
  • Previous models often simplified solvent interactions and chain flexibility.

Purpose of the Study:

  • To investigate the kinetics of loop formation in ideal and solvated polymer chains.
  • To develop analytical models that accurately predict cyclization times.
  • To explore the influence of solvent quality and monomer interactions on loop formation.

Main Methods:

  • Modified Szabo, Schulten, and Schulten theory for Rouse model simulations.
  • Analysis of simulation data for varying chain lengths (N) and capture radii (a).
  • Development of a length-scale-dependent diffusion coefficient for a < b scenarios.

Main Results:

  • Cyclization time (tau_c) scales as N^2 in the Rouse model under specific conditions.
  • tau_c shows a power-law dependence on N (N^alpha) when capture radius is smaller than bead distance.
  • Solvent quality profoundly affects kinetics: tau_c decreases by over two orders of magnitude from good to poor solvents.
  • The exponent alpha ranges from ~2.4 in good solvents to ~1.0 in poor solvents, decreasing with attractive interactions.
  • Loop formation in poor solvents follows a reptation-like mechanism.

Conclusions:

  • Analytical models accurately predict simulation results for polymer loop formation.
  • Solvent quality is a critical factor, altering loop formation mechanisms and timescales.
  • Findings have implications for understanding loop formation in biological macromolecules like DNA, RNA, and polypeptides.