Enhanced Site Isolation in Cross-Link-Functionalized Polystyrene Networks:  Mobility Studies Using Steady-State Fluorescence and ESR Techniques

Abstract

Enhanced site isolation and reduced site mobility have been demonstrated in polystyrene networks having trityl functionality at the cross-links through steady-state fluorescence measurements of intraresin excimer formation by 4-(1-pyrenyl)butanol labels and through ESR nitroxide spin-label studies. Suspension copolymerization of R,R-bis(4-vinylphenyl)benzyl alcohol with styrene afforded beads having trityl groups at the cross-links (C‚Tr). Pendent-functionalized beads (P‚Tr), prepared by copolymerization of R,R-diphenyl-4-vinylbenzyl alcohol with styrene and divinylbenzene served as a reference. Comparison of (Ie/Im) as a function of polymer loading and solvent demonstrated that crosslink functionalization reduced site interactions, providing 30% higher effective dilution of labels. ESR of polymers labeled with 2,2,6,6-tetramethyl-4-oxopiperidin-1-oxyl oxime demonstrated C‚Tr are less mobile than P‚Tr. Possible origins of enhanced site-separation and reduced mobility are discussed and explored through various experiments.

Comparison of Solid-Phase and Solution-Phase Chiral Auxiliaries in the Alkylation/Iodolactonization Sequence to γ-Butyrolactones

“…This latter result is remarkable because there is no obvious reason based upon the local environment about C α alone to have expected any significant difference in behavior in allylation between 7a and 10 , let alone a 3-fold selectivity enhancement. We demonstrated earlier that bulky functional groups at cross-link s experience a relatively constrained environment with respect to molecular motion, e.g., freedom of rotation …”

Investigating Resins for Solid Phase Organic Synthesis: The Relationship between Swelling and Microenvironment As Probed by EPR and Fluorescence Spectroscopy

“…Dansyl probes are, however, known to photobleach over long periods of time, and care must be taken to limit exposure to light. 42 N-Dansyl-4-vinylbenzylamine 43 was selected as the fluorescence probe. Emulsion polymerization of N-dansyl-4vinylbenzylamine together with stryrene and divinyl benzene (MR) or styrene and 1,4-bis(4-vinylphenoxy)butane (JJ) was performed to afford polymer beads having 2, 5, and 10% cross-linking, respectively.…”

Resin Dynamics Contributes to the NMR Line Broadening of Organic Molecules Grafted onto a Polystyrene Resin

“…Moreover, contrary to EPR studies where the concentration of free radicals has to be kept low in order to avoid large spin-spin couplings or Heisenberg exchange terms (44), NMR has no such limitations, and the full loading can be used. A major advantage of this fact is a diminished risk for the probe going only into the more accessible regions that mostly correspond to the lesser degree of cross-linking.…”