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Related Concept Videos

Radical Chain-Growth Polymerization: Chain Branching01:17

Radical Chain-Growth Polymerization: Chain Branching

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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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.
Polymer Classification: Architecture01:14

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Polymers are classified as linear or branched on the basis of their chain architecture. The polymer chains in linear polymers have a long chain-like structure with minimal to no branching at all. Even if a polymer features large substituent groups on the monomer, which appear as branches to the skeleton, it is not considered a branched polymer. A branched polymer contains secondary polymer chains that arise from the main polymer chain. The branching occurs when the polymer growth shifts from...
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A comparison of the enthalpies of hydrogenation of dienes reveals that conjugated dienes release less heat on hydrogenation, rendering them more stable than their nonconjugated analogs.
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 generated carbocation,...
Anionic Chain-Growth Polymerization: Mechanism01:04

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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 acceptor.

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Synthesis of Cyclic Polymers and Characterization of Their Diffusive Motion in the Melt State at the Single Molecule Level
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Published on: September 26, 2016

Structural transitions in polydiacetylene Langmuir films.

Yevgeniy Lifshitz1, Yuval Golan, Oleg Konovalov

  • 1Department of Materials Engineering, Ilse Katz Institute of Nanoscience and Nanotechnology, Beer-Sheva, Israel.

Langmuir : the ACS Journal of Surfaces and Colloids
|April 16, 2009
PubMed
Summary

Polydiacetylene (PDA) Langmuir films exhibit distinct structural changes during polymerization. These transformations, from monomer to blue to red phases, alter chain packing and film morphology.

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Area of Science:

  • Materials Science
  • Polymer Chemistry
  • Surface Science

Background:

  • Polydiacetylene (PDA) Langmuir films (LFs) are versatile materials with tunable properties.
  • Understanding their structural evolution at the air/water interface is crucial for controlling their characteristics.

Purpose of the Study:

  • To investigate the in situ structural transformations of PDA Langmuir films during polymerization.
  • To correlate chromatic phase transitions with changes in crystallographic structure and chain packing.

Main Methods:

  • In situ synchrotron grazing incidence X-ray diffraction (GIXD) at the air/water interface.
  • Ex situ transmission electron microscopy (TEM) and diffraction.
  • Compression and polymerization of PDA LFs on pure water.

Main Results:

  • A crystallographic model was developed describing monomer, blue (metastable), and red (stable) phases.
  • Chromatic transitions correlate with altered in-plane crystal structure and pendant chain packing.
  • Transition from arced to near-vertical chain packing observed from monomer/blue to red phases.

Conclusions:

  • The study elucidates the structural basis for PDA Langmuir film phase transitions.
  • The linear strand morphology of red-phase PDA films is attributed to reduced inter-chain spacing.
  • In situ GIXD provides critical insights into dynamic structural changes at the air/water interface.