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  1. Abstract

    Block polyethers comprised of poly(propylene oxide) (PPO) and poly(ethylene oxide) (PEG or PEO) segments form the basis of ABA‐type PEO‐b‐PPO‐b‐PEO poloxamer materials. The inverse architecture with an internal hydrophilic PEO segment flanked by hydrophobic blocks can be difficult to prepare with control of architecture by use of traditional anionic polymerization. These oxyanionic polymerizations are plagued by chain‐transfer‐to‐monomer side reactions that occur with substituted epoxides such as propylene oxide (PO). Herein, we report a new method for the preparation of block polymers through a controlled polymerization involving a N‐Al Lewis adduct catalyst and an aluminum alkoxide macroinitiator. The Lewis pair catalyst was able to chain‐extend commercial PEO macroinitiators to prepare di‐, tri‐, and pentablock polyethers with low dispersity and reasonable monomer tolerance. Chain extension was confirmed using size exclusion chromatography and diffusion ordered nuclear magnetic resonance spectroscopy. The resulting block polymers were additionally analyzed with small‐angle X‐ray scattering to correlate the morphology to molecular architecture.

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    Free, publicly-accessible full text available March 17, 2025
  2. Cyclohexene oxide (CHO) is a useful building block for the synthesis of novel materials and is a model substrate for polymerization catalyst development. The driving force for CHO polymerization is derived from its bicyclic structure, which combines the release of the enthalpy from epoxide ring-opening (ca. −15 kcal/mol) and a twist-chair-to-chair conformation shift in the cyclohexane ring (ca. −5 kcal/mol) upon enchainment. The lack of regio-defined functional handles attached to the CHO monomer limits the ability to both pre- and post-functionalize the resultant materials and establish structure–property relationships, which reduces the versatility of currently accessible materials. We report the synthesis of two series of CHO derivatives with butyl, allyl, and halogen substituents in the α and β positions relative to the epoxide ring. Adding substituents to the CHO ring was found to affect polymerization kinetics, with 4-substituted (β) CHO being more reactive than 3-substituted (α) CHO analogs when initiated with a mono(μ-alkoxo)bis(alkylaluminum) pre-catalyst. Polymer thermal properties depended on substituent location and identity. Halogenated CHO rings were most reactive and produced the highest glass transition temperatures in the resultant polymers (up to 105 °C). Density functional theory revealed a possible mechanistic explanation consistent with the observed differences in polymerization rate for the 3- and 4-substituted CHOs derived from a combination of steric and thermodynamic considerations. 
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    Free, publicly-accessible full text available July 11, 2024
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    Fluorescent linear dendronized polyols (LDPs) were prepared in two steps involving a ring-opening metathesis polymerization (ROMP) followed by acid-catalyzed deprotection. The resulting water-soluble fluorophores are compact in size (<6 nm) and show similar photostability compared to previously reported crosslinked dendronized polyols (CDPs) and significantly improved photostability compared to the free fluorophores. In contrast to the synthesis of CDPs, the production of LDPs requires less preparation time, synthetic effort, and significantly less Grubbs catalyst. The photophysical properties, including the photostability and emission wavelength of LDPs, can be further fine-tuned by incorporating different combinations of dendronized monomers and fluorophores. Interestingly, fluorescence resonance energy transfer (FRET) was observed when two different kinds of fluorophores were incorporated into the LDPs. This provides a new type of fluorophore with a large Stokes shift allowing fluorescence detection with reduced background overlap. Cytotoxicity and fluorescence imaging studies confirmed the biocompatibility of these LDPs, which make them potential candidates for biological applications. 
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