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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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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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Step-Growth Polymerization: Overview01:03

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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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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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Cyclopropenimine Superbases: Competitive Initiation Processes in Lactide Polymerization.

Tyler S Stukenbroeker1, Jeff S Bandar2, Xiangyi Zhang1

  • 1Department of Chemistry, Stanford University, Stanford CA 94305.

ACS Macro Letters
|February 26, 2016
PubMed
Summary

Cyclopropenimine superbases efficiently catalyze lactide polymerization. A competitive deprotonation mechanism was observed, generating enolates and leading to specific endgroups in the resulting poly(lactides).

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Area of Science:

  • Polymer Chemistry
  • Organic Catalysis

Background:

  • Ring-opening polymerization (ROP) is a crucial method for synthesizing polyesters like poly(lactide).
  • Superbases are potent catalysts for various organic transformations, including polymerization.
  • Understanding initiation mechanisms is key to controlling polymer properties.

Purpose of the Study:

  • To investigate the efficacy of cyclopropenimine superbases in catalyzing the ring-opening polymerization of lactide.
  • To elucidate the initiation mechanism, particularly in the absence of alcohol initiators.
  • To compare the catalytic behavior of cyclopropenimines with other superbase families.

Main Methods:

  • Ring-opening polymerization of lactide using cyclopropenimine superbases.
  • Nuclear Magnetic Resonance (NMR) spectroscopy for polymer characterization.
  • Matrix-Assisted Laser Desorption/Ionization Time-Of-Flight (MALDI-TOF) mass spectrometry for endgroup analysis.
  • Model studies and comparative experiments with guanidine and phosphazene catalysts.

Main Results:

  • Cyclopropenimine superbases effectively catalyzed lactide polymerization with and without alcohol initiators.
  • In the absence of alcohol initiators, a competitive initiation pathway involving lactide deprotonation by the superbase was identified.
  • NMR and MALDI-TOF analyses revealed acylated lactide and hydroxyl endgroups in poly(lactides) formed via this competitive pathway.
  • Comparative studies highlighted the influence of superbase structure on the initiation mechanism.

Conclusions:

  • Cyclopropenimines are effective catalysts for lactide ROP, exhibiting a unique competitive initiation mechanism.
  • This mechanism leads to specific endgroup functionalities in the synthesized poly(lactides).
  • The choice of superbase significantly impacts the polymerization pathway and resulting polymer structure.