Scott C. Radzinski

S-aroylthiooximes: a facile route to hydrogen sulfide releasing compounds with structure-dependent release kinetics.

We report the facile preparation of a family of S-aroylthiooxime (SATO) H2S donors, which are synthesized via a click reaction analogous to oxime formation between S-aroylthiohydroxylamines (SATHAs) and aldehydes or ketones. Analysis of cysteine-triggered H2S release revealed structure-dependent release kinetics with half-lives from 8-82 min by substitution of the SATHA ring. The pseudo-first-order rate constants of substituted SATOs fit standard linear free energy relationships (ρ = 1.05), demonstrating a significant sensitivity to electronic effects.

Bottlebrush Polymer Synthesis by Ring-Opening Metathesis Polymerization: The Significance of the Anchor Group.

Control over bottlebrush polymer synthesis by ring-opening metathesis polymerization (ROMP) of macromonomers (MMs) is highly dependent on the competition between the kinetics of the polymerization and the lifetime of the catalyst. We evaluated the effect of anchor group chemistry-the configuration of atoms linking the polymer to a polymerizable norbornene-on the kinetics of ROMP of polystyrene and poly(lactic acid) MMs initiated by (H2IMes)(pyr)2(Cl)2Ru═CHPh (Grubbs third generation catalyst). We observed a variance in the rate of propagation of >4-fold between similar MMs with different anchor groups. This phenomenon was conserved across all MMs tested, regardless of solvent, molecular weight (MW), or repeat unit identity. The observed >4-fold difference in propagation rate had a dramatic effect on the maximum obtainable backbone degree of polymerization, with slower propagating MMs reducing the maximum bottlebrush MW by an order of magnitude (from ∼10(6) to ∼10(5) Da). A chelation mechanism was initially proposed to explain the observed anchor group effect, but experimental and computational studies indicated that the rate differences likely resulted from a combination of varying steric demands and electronic structure among the different anchor groups. The addition of trifluoroacetic acid to the ROMP reaction substantially increased the propagation rate for all anchor groups tested, likely due to scavenging of the pyridine ligands. Based on these data, rational selection of the anchor group is critical to achieve high MM conversion and to prepare pure, high MW bottlebrush polymers by ROMP grafting-through.

Synthesis of bottlebrush polymers via transfer-to and grafting-through approaches using a RAFT chain transfer agent with a ROMP-active Z-group

A novel dithiocarbamate chain transfer agent (CTA1) with a directly polymerizable Z-group was synthesized for use in reversible addition–fragmentation chain transfer polymerization (RAFT). This CTA effectively mediated RAFT polymerization of styrenic and acrylic monomers with dispersities (Đ) 90%). Bottlebrush polymers made by a transfer-to strategy were also synthesized from CTA1. In this case, ROMP was first carried out to produce poly(CTA1) (PCTA1), then RAFT was performed from the PCTA1 backbone. This technique allows for the preparation of high molecular weight bottlebrush polymers without radical coupling between bottlebrush polymers. Lastly, regardless of the synthetic method, all bottlebrush polymers produced using CTA1 are composed of polymeric side chains that are attached to the bottlebrush backbone through a labile dithiocarbamate linkage that can be cleaved in the presence of nucleophiles such as amines. The unique combination of these capabilities allows for the study of bottlebrush polymer formation by both transfer-to and grafting-through strategies using a single agent.

H2S-Releasing Polymer Micelles for Studying Selective Cell Toxicity

We report the preparation of S-aroylthiooxime (SATO) functionalized amphiphilic block copolymer micelles that release hydrogen sulfide (H2S), a gaseous signaling molecule of relevance to various physiological and pathological conditions. The micelles release H2S in response to cysteine with a half-life of 3.3 h, which is substantially slower than a related small molecule SATO. Exogenous administration of H2S impacts growth and proliferation of cancer cells; however, the limited control over H2S generation from inorganic sulfide sources results in conflicting reports. Therefore, we compare the cellular cytotoxicity of SATO-functionalized micelles, which release H2S in a sustained manner, to Na2S, which releases H2S in a single dose. Our results show that H2S-releasing micelles significantly reduce the survival of HCT116 colon cancer cells relative to Na2S, GYY4137, and a small molecule SATO, indicating that release kinetics may play an important role in determining toxicity of H2S toward cancer cells. Furthermore, H2S-releasing micelles are well tolerated by immortalized fibroblasts (NIH/3T3 cells), suggesting a selective toxicity of H2S toward cancer cells.

Preparation of Bottlebrush Polymers via a One‐Pot Ring‐Opening Polymerization (ROP) and Ring‐Opening Metathesis Polymerization (ROMP) Grafting‐Through Strategy

Bottlebrush polymers are synthesized using a tandem ring-opening polymerization (ROP) and ring-opening metathesis polymerization (ROMP) strategy. For the first time, ROP and ROMP are conducted sequentially in the same pot to yield well-defined bottlebrush polymers with molecular weights in excess of 10(6) Da. The first step of this process involves the synthesis of a polylactide macromonomer (MM) via ROP of d,l-lactide initiated by an alcohol-functionalized norbornene. ROMP grafting-through is then carried out in the same pot to produce the bottlebrush polymer. The applicability of this methodology is evaluated for different MM molecular weights and bottlebrush backbone degrees of polymerization. Size-exclusion chromatographic and (1)H NMR spectroscopic analyses confirm excellent control over both polymerization steps. In addition, bottlebrush polymers are imaged using atomic force microscopy and stain-free transmission electron microscopy on graphene oxide.

Factors affecting bottlebrush polymer synthesis by the transfer-to method using reversible addition–fragmentation chain transfer (RAFT) polymerization

The transfer-to method is a unique way to prepare bottlebrush polymers by reversible addition–fragmentation chain transfer (RAFT) polymerization. This little-studied bottlebrush polymer synthesis strategy is distinct from the grafting-from, grafting-to, and grafting-through strategies and therefore may have specific advantages over these other synthetic approaches. Herein, we study the factors affecting the composition of bottlebrush polymer samples prepared by RAFT transfer-to, with particular emphasis on bottlebrush polymer molecular weight (MW) and dispersity (Đ) and the percentage of “dead” linear polymer as a function of initiator concentration, [M]/[CTA] ratio, backbone length, and monomer type. The lowest quantities of dead polymer were obtained under conditions that limited the MW of the bottlebrush polymer side-chains and that discouraged termination reactions. Under optimized conditions, high MW bottlebrush polymers were prepared with low dispersities and few dead polymer impurities.

Reversibly Cross-linkable Bottlebrush Polymers as Pressure-Sensitive Adhesives

Dynamically cross-linkable bottlebrush polymer adhesives were synthesized by the grafting-from strategy through a combination of ring-opening metathesis polymerization (ROMP) and photoiniferter polymerization. A norbornene-containing trithiocarbonate was first polymerized by ROMP to form the bottlebrush polymer backbone; this was followed by blue-light-mediated photoiniferter polymerization of butyl acrylate initiated by the poly(trithiocarbonate) to form the bottlebrush polymer. This strategy afforded well-defined bottlebrush polymers with molar masses in excess of 11 000 kg/mol. For un-cross-linked bottlebrush polymers, 180° peel tests revealed a cohesive failure mode and showed similar peel strengths (∼30 g/mm) regardless of the backbone polymer degree of polymerization (DP). The bottlebrush polymers were then treated with butylamine to remove the trithiocarbonate, liberating thiols on each side-chain terminus. In the presence of oxygen, these thiols readily cross-linked via disulfide bond formation. The cross-linked bottlebrush polymers with a backbone DP of 400 showed a greater than sixfold improvement in peel strength, whereas those with a backbone DP of 100 exhibited a twofold enhancement compared with un-cross-linked samples along with a change to adhesive failure. Triphenylphosphine readily reduced the disulfide bonds, effectively removing all cross-links in the bottlebrush network and allowing for recasting of the adhesive, which showed similar adhesive and rheological properties to the original un-cross-linked samples.

Correction to “Bottlebrush Polymer Synthesis by Ring-Opening Metathesis Polymerization: The Significance of the Anchor Group”

Figure 1. (A) Structures of propagating alkylidenes for various MMs highlighting (i) the potential for chelation between the carbonyl oxygen of the anchor group and the Ru center, and (ii) regioisomers that are unlikely to form chelates. (B) Kinetic analyses of ROMP of MMs with different anchor groups: representative kinetic plot of in situ NMR experiments using polystyrene MMs ofMn≈ 5000 Da with an [MM]/[G3] ratio of 100 at 50 mg/mL (green circles = 1S5k, blue circles = 2S5k, red circles = 3S5k). Solid lines represent fits of each data set generated using experimentally determined kp values based on the equation p = 1− e(−kpt). (B, inset) Representative log plot for in situ NMR kinetic analysis ofMM 1S5k