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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...
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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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Ziegler–Natta polymerization is another form of addition or chain‐growth polymerization used for synthesizing linear polymers over branched polymers. The catalyst used for polymerization is the Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, who developed it in 1953. This catalyst is an organometallic complex of titanium tetrachloride and triethyl aluminum, with the active form of the catalyst being an alkyl titanium compound. Using the Ziegler–Natta...
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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 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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Ethylene Polymerizations Using Parallel Pressure Reactors and a Kinetic Analysis of Chain Transfer Polymerization
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Cross Nucleation in Polyethylene with Precisely Spaced Ethyl Branches.

Yoshinobu Nozue1, Shuichiro Seno1, Tatsuhiro Nagamatsu1

  • 1Petrochemicals Research Laboratory, Sumitomo Chemical Co., Ltd., Kitasode, Chiba, Japan.

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|May 24, 2022
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Summary

This study demonstrates cross nucleation in crystalline polymers, where one crystal type initiates the formation of another distinct crystal type. This phenomenon was observed in branched polyethylene, showing faster growth rates for the newly formed crystals.

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Area of Science:

  • Polymer Science
  • Materials Science
  • Crystallization Phenomena

Background:

  • Cross nucleation involves one crystalline polymorph (A) inducing the nucleation of a different polymorph (B) without transformation.
  • While observed in small molecules, evidence for cross nucleation in crystalline polymers has been limited.
  • Understanding polymer crystallization is crucial for tailoring material properties.

Purpose of the Study:

  • To investigate and confirm the occurrence of cross nucleation in a crystalline polymer system.
  • To characterize the crystallographic and kinetic aspects of this phenomenon in polymers.

Main Methods:

  • Isothermal crystallization of polyethylene with precisely spaced ethyl branches.
  • Polarized optical microscopy to observe spherulite growth and measure radial growth rates.
  • Scanning microbeam wide-angle X-ray scattering (WAXS) to determine crystalline polymorphs.

Main Results:

  • New spherulites nucleated at the growth front of initially formed spherulites in branched polyethylene.
  • The newly grown spherulites exhibited a faster radial growth rate (1.01 μm/min) than the initial ones (0.76 μm/min).
  • WAXS confirmed that the initial and newly formed spherulites possessed different crystalline polymorphs.

Conclusions:

  • This study provides the first definitive evidence of cross nucleation in a crystalline polymer.
  • The observed faster growth rate of the secondary polymorph supports the criteria for cross nucleation.
  • This finding opens new avenues for controlling polymer morphology and properties through crystallization engineering.