Experimental and computational investigations of the zwitterionic ring-opening polymerization (ZROP) of δ-valerolactone (VL) catalyzed by the N-heterocyclic carbenes (NHC) 1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene (1) and 1,3,4,5-tetramethyl-imidazol-2-ylidene (2) were carried out. The ZROP of δ-valerolactone generates cyclic poly(valerolactone)s whose molecular weights are higher than predicted from [VL]0/[NHC]0. Kinetic studies reveal the rate of polymerization is first order in [VL] and first order in [NHC]. Density functional theory (DFT) calculations were carried out to elucidate the key steps involved in the ring-opening of δ-valerolactone and its subsequent oligomerization. These studies have established that the initial steps of the mechanism involve nucleophilic attack of the NHC on δ-valerolactone to form a zwitterionic tetrahedral intermediate. DFT calculations indicate that the highest activation barrier of the entire mechanism is associated with the ring-opening of the tetrahedral intermediate formed from the NHC and δ-valerolactone, a result consistent with inefficient initiation to generate reactive zwitterions. The large barrier in this step is due to the fact that ring-opening requires a partial positive charge to develop next to the directly attached NHC moiety which already bears a delocalized positive charge.

Computational investigations with density functional theory (DFT) have been performed on the N-heterocyclic carbene (NHC) catalyzed ring-opening polymerization of ε-caprolactone in the presence and in the absence of a methanol initiator. Much like the zwitterionic ring opening (ZROP) of δ-valerolactone which was previously reported, calculations predict that the mechanism of the ZROP of caprolactone that occurs without an alcohol present involves a high-barrier step involving ring opening of the zwitterionic tetrahedral intermediate formed after the initial nucleophilic attack of NHC on caprolactone. However, the operative mechanism by which caprolactone is polymerized in the presence of an alcohol initiator does not involve the analogous mechanism involving initial nucleophilic attack by the organocatalytic NHC. Instead, the NHC activates the alcohol through hydrogen bonding and promotes nucleophilic attack and the subsequent ring-opening steps that occur during polymerization. The largest free energy barrier for the hydrogen-bonding mechanism in alcohol involves nucleophilic attack, while that for both ZROP processes involves ring opening of the initially formed zwitterionic tetrahedral intermediate. The DFT calculations predict that the rate of polymerization in the presence of alcohol is faster than the reaction performed without an alcohol initiator; this prediction has been validated by experimental kinetic studies.

Progress in artificial intelligence has led to growing concern about the capabilities of AI-powered surveillance systems. This data brief uses bibliometric analysis to chart recent trends in visual surveillance research — what share of overall computer vision research it comprises, which countries are leading the way, and how things have varied over time.

Problems of AI safety are the subject of increasing interest for engineers and policymakers alike. This brief uses the CSET Map of Science to investigate how research into three areas of AI safety — robustness, interpretability and reward learning — is progressing. It identifies eight research clusters that contain a significant amount of research relating to these three areas and describes trends and key papers for each of them.

Devices based on superconductor electronics can achieve much higher energy efficiency than standard electronics. Research in superconductor electronics could advance a range of commercial and defense priorities, with potential applications for supercomputing, artificial intelligence, sensors, signal processing, and quantum computing. This brief identifies the countries most actively contributing to superconductor electronics research and assesses their relative competitiveness in terms of both research output and funding.