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Cationic Chain-Growth Polymerization: Mechanism00:57

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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...
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Step-growth or condensation polymerization is a stepwise reaction of bi or multifunctional monomers to form long-chain polymers. As all the monomers are reactive, most of the monomers are consumed at the early stages of the reaction to form small chains of reactive oligomers, which then combine to form long polymer chains in the late stages. Hence, the reaction has to proceed for a long time to achieve high molecular weight polymers.
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The polymerization process that involves carbanion as an intermediate is called anionic polymerization. It is also a type of addition or chain-growth polymerization. Anionic polymerization gets initiated by a strong nucleophile such as an organolithium or a Grignard reagent. The most commonly used initiator for anionic polymerization is butyl lithium. Monomers involved in anionic polymerization must possess a vinyl group bonded to one or two electron-withdrawing groups. For instance,...
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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...
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The rate of acid-catalyzed hydration of alkenes depends on the alkene's structure, as the presence of alkyl substituents at the double bond can significantly influence the rate.
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Tuning Aqueous Supramolecular Polymerization by an Acid-Responsive Conformational Switch.

Christina Rest1, Divya Susan Philips2, Torsten Dünnebacke2

  • 1Institut für Organische Chemie, Universität Würzburg am Hubland, 97078, Würzburg, Germany.

Chemistry (Weinheim an Der Bergstrasse, Germany)
|May 7, 2020
PubMed
Summary

This study demonstrates how 2,2'-bipyridines act as acid-responsive switches to control self-assembly. Acid addition alters molecular shape, transforming long fibers into shorter ones in supramolecular polymers.

Keywords:
acid-sensitiveamphiphilic systemsnoncovalent interactionsself-assemblyπ-conjugated systems

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Area of Science:

  • Supramolecular Chemistry
  • Materials Science
  • Organic Chemistry

Background:

  • 2,2 -bipyridines are known for cis-trans conformational changes triggered by metal ions and acids.
  • These conformational changes have been studied at the molecular level but not in self-assembly.
  • Exploiting these switches for tunable self-assembly remains an underexplored area.

Purpose of the Study:

  • To demonstrate the use of 2,2 -bipyridines as acid-responsive conformational switches.
  • To tune supramolecular polymerization processes using these switches.
  • To investigate the impact of acid-induced conformational changes on self-assembled structures.

Main Methods:

  • Design and synthesis of a bipyridine-based linear bolaamphiphile.
  • Investigation of self-assembly in aqueous media.
  • Induction of conformational changes using trifluoroacetic acid (TFA).
  • Characterization of structural transformations using microscopy and spectroscopy.

Main Results:

  • The designed bolaamphiphile self-assembles into ordered supramolecular polymers via aromatic and hydrophobic interactions.
  • Monoprotonation of the 2,2 -bipyridine moiety by TFA induces a switch from a linear (trans) to a V-shaped (cis) conformation.
  • This conformational change and electrostatic repulsion lead to fiber shortening and thinning.

Conclusions:

  • 2,2 -bipyridines can serve as effective acid-responsive conformational switches in self-assembly.
  • This work introduces a novel method for controlling supramolecular polymerization.
  • Findings open new avenues for developing molecular switches and stimuli-responsive materials.