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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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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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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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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.
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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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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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Fabrication of Large-area Free-standing Ultrathin Polymer Films
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Free Space Makes the Polymer "Dead Layer" Alive.

Zhichao Jiang1,2, Bin Cheng1,2, Jingfa Yang1,2

  • 1Beijing National Research Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.

The Journal of Physical Chemistry. B
|December 8, 2022
PubMed
Summary
This summary is machine-generated.

Free space in polymer nanolayers, introduced by bulky side groups, significantly enhances molecular motion. This finding impacts understanding of surface-bound polymer dynamics and glass transition temperatures.

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Area of Science:

  • Polymer Science
  • Surface Chemistry
  • Materials Science

Background:

  • Polymer nanolayers and "dead layers" near surfaces exhibit unique properties distinct from bulk polymers.
  • Understanding molecular motion and glass transition temperature (Tg) in these confined systems is crucial for material design.

Purpose of the Study:

  • To investigate the influence of "free space" on molecular motion within polymer nanolayers.
  • To explore how polymer structure and interfaces affect dynamics and Tg in confined polymer films.

Main Methods:

  • Utilized poly(n-butyl methacrylate) (PnBMA), a polymer with large side groups, to introduce free space.
  • Employed polarization-resolved single-molecule fluorescence microscopy to monitor probe rotational motion.
  • Studied nanolayer evolution, molecular dynamics, and Tg during prolonged annealing and with top layers.

Main Results:

  • Significantly enhanced rotational motion of fluorescent probes was observed at moderate temperatures.
  • Nanolayers exhibited lower Tg than bulk, but Tg increased when covered by a polymer top layer.
  • Free space from side groups and the air-polymer interface promoted molecular motion despite surface attraction.

Conclusions:

  • Free space is a key factor in enhancing molecular mobility within polymer nanolayers.
  • Surface and interface effects, alongside polymer architecture, critically modulate dynamics and Tg in confined systems.
  • Findings offer insights into controlling polymer behavior at interfaces for advanced material applications.