Cyclic polymers were produced by end-to-end coupling of telechelic linear polymers under dilute conditions in THF, using intramolecular atom transfer radical coupling or click chemistry. In addition to the expected shift to longer elution times on gel permeation chromatography (GPC) indicative of the formation of cyclic product, lower molecular weight species were consistently observed upon analysis of the unpurified cyclization reaction mixture. By systematically removing or altering single reaction components in the highly dilute cyclization reaction, it was found that THF itself was responsible for the low-molecular-weight material, forming oligomeric chains of poly(THF) regardless of the other reaction components. When the reactions were performed at higher concentrations and for shorter time intervals, conducive to intermolecular chain-end-joining reactions, the low-molecular-weight peaks were absent. Isolation of the material and analysis by 1 H NMR confirmed that poly(THF) was being formed in the highly dilute conditions required for cyclization by end-to-end coupling. When a series of mock cyclization reactions were performed with molar ratios of the reactants held constant, but concentrations changed, it was found that lower concentrations of reactants led to higher amounts of poly(THF) side product.
Monobrominated polystyrene (PSBr) was prepared by ATRP, and the resulting chain ends were activated in the presence of radical traps to induce chain end-coupling. In atom transfer radical coupling (ATRC) with radical trap assistance, to achieve significant coupling requires excess metal catalyst, ligand, and a reducing agent that is often additional metal. In this work, activators generated by electron transfer (AGET) and radical trap assistance are used in the ATRC sequence to successfully lead to chain-end coupling without the need for the oxidatively unstable copper (I) and with environmentally friendlier agents in place of copper metal. High extents of coupling (Xc) were achieved using ascorbic acid (AA) as the reducing agent and copper(II) bromide as the oxidized version of the catalyst, and when combined with AGET ATRP to prepare the PSBr precursor, only a fraction of the total metal was required compared to traditional atom transfer reactions, while still retaining similar Xc values.
Atom transfer radical coupling (ATRC), performed with or without radical traps, has allowed for high extents of coupling (Xc) for a variety of brominated polymers, yet structurally different polymeric chain ends require unique reagents and reaction conditions. Inspired by a similar study that focused on universal conditions for the controlled polymerization of different monomers using atom transfer radical polymerization (ATRP), this work focuses on developing a single set of conditions (or conditions with as little variation as possible) that will achieve extents of coupling greater than 80% or end-brominated chains of polystyrene (PSBr), poly(methyl methacrylate) (PMMABr), and poly(methyl acrylate) (PMABr). The radical traps α-phenyl-tert-butylnitrone (PBN), 2-methyl-2-nitrosopropane (MNP), and nitrosobenzene (NBz) were chosen in this study, along with copper catalysts, reducing agents, and nitrogen-based ligands. Ultimately, a single set of effective reaction conditions was identified with the only difference being the radical trap used: MNP was effective for coupling PSBr and PMABr while NBz was necessary to achieve similarly high extents of coupling for PMMABr.
Monobrominated versions of poly(methyl methacrylate) (PMMABr) and poly(methyl acrylate) (PMABr) are prepared by atom transfer radical polymerization and subjected to a variety of chain end‐coupling reactions, with the goal of achieving high extents of coupling with minimal metal content. By using atom transfer radical coupling (ATRC) with radical trap assistance in conjunction with activators generated by electron transfer, high extents of coupling (Xc) could be achieved for both polymer classes without requiring copper(I) as an initial reactant, and replacing elemental copper with environmentally benign reducing agents (Xc > 0.85). Specifically, the highest extents of coupling are observed with ascorbic acid as the reducing agent along with copper(II) bromide as the pre‐catalyst. Overall, similar extents of coupling are observed while using ≈1% of the total metal of a traditional ATRC reaction for PMABr and ≈3% for PMMABr.
Introduction The number of people dying while unhoused is increasing nationally. In Santa Clara County (SCC), deaths of unhoused people have almost tripled in 9 years. This is a retrospective cohort study examining mortality trends among unhoused people in SCC. The objective of the study is to characterize mortality outcomes in the unhoused population, and compare these to the SCC general population. Materials and methods We obtained data from the SCC Medical Examiner-Coroner’s Office on unhoused people’s deaths that occurred between 2011–2019. We analyzed demographic trends and cause of death, compared to mortality data on the SCC general population obtained from CDC databases. We also compared rates of deaths of despair. Results There were a total of 974 unhoused deaths in the SCC cohort. The unadjusted mortality rate among unhoused people is higher than the general population, and unhoused mortality has increased over time. The standardized mortality ratio for unhoused people is 3.8, compared to the general population in SCC. The most frequent age of death among unhoused people was between 55–64 years old (31.3%), followed by 45–54 (27.5%), compared to 85+ in the general population (38.3%). Over ninety percent of deaths in the general population were due to illness. In contrast, 38.2% of unhoused deaths were due to substance use, 32.0% illness, 19.0% injury, 4.2% homicide, and 4.1% suicide. The proportion of deaths of despair was 9-fold higher in the unhoused cohort compared to the housed cohort. Discussion Homelessness has profound impacts on health, as people who are unhoused are dying 20 years younger, with higher rates of injurious, treatable, and preventable causes, than people in the general population. System-level, inter-agency interventions are needed. Local governments need to systematically collect housing status at death to monitor mortality patterns among unhoused people, and adapt public health systems to prevent rising unhoused deaths.
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