Radical Migration Research Articles

Evidence for O2- Radical Stabilization at Surface Oxygen Vacancies on Polycrystalline TiO2

The characterization and stability of superoxide radicals (O2-) over polycrystalline TiO2 (Degussa P25) was investigated using electron paramagnetic resonance (EPR) spectroscopy. The adsorbed oxygen molecules act as efficient electron scavengers and were therefore used to indirectly probe the sites of electron transfer at the surface of the anatase component of the mixed phase P25 material. A distribution of various stabilization sites on the surface was identified by analysis of the g values and further confirmed by identification of several well-resolved 17O hyperfine patterns. For the first time, on a polycrystalline TiO2 surface evidence for stabilization of superoxide radicals specifically at anion vacancy sites is presented by EPR. These radicals, labeled [Vac...O2-], are characterized by the spin Hamiltonian parameters of gxx = 2.005, gyy = 2.011, gzz = 2.019, and 17OAxx = 7.64 mT (Ayy = Azz > 1 mT). The [Vac...O2-] radicals exhibit pronounced reactivity under the influence of thermal, photochemical, and chemical treatment compared to the remaining surface O2- anions bound at nonvacancy sites. The extent of site occupancy was found to be sensitive to the oxygen adsorption temperature and the extent of O2- radical migration on the surface. Thus, the stability and lifetime of the surface O2- anions are directly correlated to the structure of the adsorption site itself at the anatase surface of P25.

The Structures and Electronic Configuration of Compound I Intermediates of Helicobacter pylori and Penicillium vitale Catalases Determined by X-ray Crystallography and QM/MM Density Functional Theory Calculations

The structures of Helicobacter pylori (HPC) and Penicillium vitale (PVC) catalases, each with two subunits in the crystal asymmetric unit, oxidized with peroxoacetic acid are reported at 1.8 and 1.7 A resolution, respectively. Despite the similar oxidation conditions employed, the iron-oxygen coordination length is 1.72 A for PVC, close to what is expected for a Fe=O double bond, and 1.80 and 1.85 A for HPC, suggestive of a Fe-O single bond. The structure and electronic configuration of the oxoferryl heme and immediate protein environment is investigated further by QM/MM density functional theory calculations. Four different active site electronic configurations are considered, Por*+-FeIV=O, Por*+-FeIV=O...HisH+, Por*+-FeIV-OH+ and Por-FeIV-OH (a protein radical is assumed in the latter configuration). The electronic structure of the primary oxidized species, Por*+-FeIV=O, differs qualitatively between HPC and PVC with an A2u-like porphyrin radical delocalized on the porphyrin in HPC and a mixed A1u-like "fluctuating" radical partially delocalized over the essential distal histidine, the porphyrin, and, to a lesser extent, the proximal tyrosine residue. This difference is rationalized in terms of HPC containing heme b and PVC containing heme d. It is concluded that compound I of PVC contains an oxoferryl Por*+-FeIV=O species with partial protonation of the distal histidine and compound I of HPC contains a hydroxoferryl Por-FeIV-OH with the second oxidation equivalent delocalized as a protein radical. The findings support the idea that there is a relation between radical migration to the protein and protonation of the oxoferryl bond in catalase.

Side-Chain Losses in Electron Capture Dissociation To Improve Peptide Identification

Analysis of a database of some 20 000 conventional electron-capture dissociation (ECD) mass spectra of doubly charged ions belonging to tryptic peptides revealed widespread appearance of w ions and related u ions that are due to partial side chain losses from radical z* ions. Half of all z* ions that begin with Leu or Ile produce w ions in conventional one-scan ECD mass spectra, which differentiates these isomeric residues with >97% reliability. Other residues exhibiting equally frequent side chain losses are Gln, Glu, Asp, and Met (cysteine was not included in this work). Unexpectedly, Asp lost not a radical group like other amino acids but a molecule CO2, thus giving rise to a radical w* ion with the possibility of a radical cascade. Losses from amino acids as distant as seven residues away from the cleavage site were detected. The mechanism of such losses seems to be related to radical migration from the original site at the alphaCn atom in a zn* ion to other alphaC and betaC atoms. The side chain losses confirm sequence assignment, improve the database matching score, and can be useful in de novo sequencing.

Tryptophan 334 oxidation in bovine cytochrome c oxidase subunit I involves free radical migration

A single tryptophan (W 334(I)) within the mitochondrial-encoded core subunits of cytochrome c oxidase (CcO) is selectively oxidized when hydrogen peroxide reacts with the binuclear center. W 334(I) is converted to hydroxytryptophan as identified by reversed-phase HPLC-electrospray ionization tandem mass spectrometry analysis of peptides derived from the three SDS–PAGE purified subunits. Total sequence coverage of subunits I, II and III was limited to 84%, 66% and 54%, respectively. W 334(I) is located on the surface of CcO at the membrane interface. Two other surface tryptophans within nuclear-encoded subunits, W 48(IV) and W 19(VIIc), are also oxidized when hydrogen peroxide reacts with the binuclear center (Musatov et al. (2004) Biochemistry 43, 1003–1009). Two aromatic-rich networks of amino acids were identified that link the binuclear center to the three oxidized tryptophans. We propose the following mechanism to explain these results. Electron transfer through the aromatic networks moves the free radicals generated at the binuclear center to the surface-exposed tryptophans, where they produce hydroxytryptophan.

Controlling energy and charge transfer in linear chlorophyll dimers

A series of linkers constructed from combinations of phenyl and ethynyl groups is shown to permit ultrafast energy transfer between two chlorophylls, while allowing control over radical cation migration between them.

Surface smoothness of plasma-deposited amorphous silicon thin films: Surface diffusion of radical precursors and mechanism of Si incorporation

We present a detailed analysis of the fundamental atomic-scale processes that determine the surface smoothness of hydrogenated amorphous silicon (a-Si:H) thin films. The analysis is based on a synergistic combination of molecular-dynamics (MD) simulations of radical precursor migration on surfaces of a-Si:H films that are deposited computationally using MD simulation with first-principles density functional theory (DFT) calculations on the hydrogen-terminated $\mathrm{Si}(001)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ surface. The surfaces of the MD-grown a-Si:H films are remarkably smooth, as the mobile precursor, the $\mathrm{Si}{\mathrm{H}}_{3}$ radical, diffuses fast and incorporates in surface valleys. Analysis of the MD simulations of $\mathrm{Si}{\mathrm{H}}_{3}$ radical migration on a-Si:H surfaces yields an effective diffusion barrier of $0.16\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The low diffusion barrier on the a-Si:H surface is attributed to $\mathrm{Si}{\mathrm{H}}_{3}$ migration through overcoordinated surface Si atoms, where the radical remains weakly bonded to the surface at all times and does not break any strong Si-Si bonds along its migration pathway. Furthermore, the diffusing $\mathrm{Si}{\mathrm{H}}_{3}$ radical incorporates into the a-Si:H film only when it transfers an H atom and forms a second Si-Si backbond. On rough a-Si:H films, such H transfer from diffusing $\mathrm{Si}{\mathrm{H}}_{3}$ radicals is more likely to occur in surface valleys, even when the dangling bond (DB) density is low and DBs are not present in surface valleys. In addition, this H-transfer process is thermally activated with activation energy barriers $({E}_{a})$ over the range $0.29--0.65\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$; ${E}_{a}$ is determined by the Si-Si interatomic distance between the Si of the $\mathrm{Si}{\mathrm{H}}_{3}$ radical and the surface Si atom to which the H is transferred. The preferential incorporation in valleys is explained by both the increased residence time of the migrating precursor in valleys and the decreased activation barrier for incorporation reactions occurring in valleys. The mechanism and activation barrier for the H-transfer reaction on the a-Si:H surface were validated by performing first-principles DFT calculations on the crystalline Si surface.

First-principles theoretical analysis of silyl radical diffusion on silicon surfaces

We report results from a detailed analysis of the fundamental radical precursor diffusion processes on silicon surfaces and discuss their implications for the surface smoothness of hydrogenated amorphous silicon (a-Si:H) thin films. The analysis is based on a synergistic combination of first-principles density functional theory (DFT) calculations of SiH(3) radical migration on the hydrogen-terminated Si(001)-(2 x 1) surface with molecular-dynamics (MD) simulations of SiH(3) radical precursor migration on surfaces of a-Si:H films. Our DFT calculations yield activation energies for SiH(3) migration that range from 0.18 to 0.89 eV depending on the local electronic environment on the Si(001)-(2 x 1):H surface. In particular, when no substantial surface relaxation (Si-Si bond breaking or formation) accompanies the hopping of the SiH(3) radical the activation barriers are highest, whereas hopping between nearest-neighbor overcoordinated surface Si atoms results in the lowest radical diffusion barrier of 0.18 eV; this low barrier is consistent with the activation barrier for SiH(3) migration through overcoordinated sites on the a-Si:H surface. Specifically, the analysis of the MD simulations of SiH(3) radical migration on a-Si:H surfaces yields an effective diffusion barrier of 0.16 eV, allowing for the rapid migration of the SiH(3) radical prior to its incorporation in surface valleys; rapid migration and subsequent incorporation constitute the two-step mechanism responsible for the smoothness of plasma deposited a-Si:H thin films.

The Effect of Radical Trap Moieties on Electron Capture Dissociation Spectra of Substance P

To further test the hypothesis that electron capture dissociation (ECD) involves long-lived radical intermediates and radical migration occurs within these intermediates before fragmentation, radical trap moieties were attached to peptides with the assumption that they would reduce fragmentation by decreasing the mobility of the radical. Coumarin labels were chosen for the radical traps, and unlabeled, singly-labeled, and doubly-labeled Substance P were analyzed by ECD. The results demonstrated a correlation between the number and position of tags on the peptide and the intensity of side-chain cleavages observed, as well as an inverse correlation between the number of tags on the peptide and the intensity of backbone cleavages. Addition of radical traps to the peptide inhibits backbone cleavages, suggesting that either radical mobility is required for these cleavages, or new noncovalent interactions prevent separation of backbone cleavage fragments. The enhancement of side-chain cleavages and the observation of new side-chain cleavages associated with aromatic groups suggest that the gas-phase conformation of this peptide is substantially distorted from untagged Substance P and involves previously unobserved interactions between the coumarin tags and the phenylalanine residues. Furthermore, the use of a double resonance (DR)-ECD experiment showed that these side-chain losses are all products of long-lived radical intermediate species, which suggests that steric hindrance prevents the coumarin-localized radical from interacting with the backbone while simultaneously increasing the radical rearrangements with the side chains.

Long-Lived Electron Capture Dissociation Product Ions Experience Radical Migration via Hydrogen Abstraction

To explore the mechanism of electron capture dissociation (ECD) of linear peptides, a set of 16-mer peptides were synthesized with deuterium labeled on the alpha-carbon position of four glycines. The ECD spectra of these peptides showed that such peptides exhibit a preference for the radical to migrate to the alpha-carbon position on glycine via hydrogen (or deuterium) abstraction before the final cleavage and generation of the detected product ions. The data show c-type fragment ions, ions corresponding to the radical cation of the c-type fragments, c*, and they also show c*-1 peaks in the deuterated peptides only. The presence of the c*-1 peaks is best explained by radical-mediated scrambling of the deuterium atoms in the long-lived, metastable, radical intermediate complex formed by initial electron capture, followed by dissociation of the complex. These data suggest the presence of at least two mechanisms, one slow, one fast. The abundance of H* and -CO losses from the precursor ion changed upon deuterium labeling indicating the presence of a kinetic isotope effect, which suggests that the values reported here represent an underestimation of radical migration and H/D scrambling in the observed fragments.

Role of Tyr348 in Tyr385 Radical Dynamics and Cyclooxygenase Inhibitor Interactions in Prostaglandin H Synthase-2

Both prostaglandin H synthase (PGHS) isoforms utilize a radical at Tyr385 to abstract a hydrogen atom from arachidonic acid, initializing prostaglandin synthesis. A Tyr348-Tyr385 hydrogen bond appears to be conserved in both isoforms; this hydrogen bonding has the potential to modulate the positioning and reactivity of the Tyr385 side chain. The EPR signal from the Tyr385 radical undergoes a time-dependent transition from a wide doublet to a wide singlet species in both isoforms. In PGHS-2, this transition results from radical migration from Tyr385 to Tyr504. Localization of the radical to Tyr385 in the recombinant human PGHS-2 Y504F mutant was exploited in examining the effects of blocking Tyr385 hydrogen bonding by introduction of a further Y348F mutation. Cyclooxygenase and peroxidase activities were found to be maintained in the Y348F/Y504F mutant, but the Tyr385 radical was formed more slowly and had greater rotational freedom, as evidenced by observation of a transition from an initial wide doublet species to a narrow singlet species, a transition not seen in the parent Y504F mutant. The effect of disrupting Tyr385 hydrogen bonding on the cyclooxygenase active site structure was probed by examination of cyclooxygenase inhibitor kinetics. Aspirin treatment eliminated all oxygenase activity in the Y348F/Y504F double mutant, with no indication of the lipoxygenase activity observed in aspirin-treated wild-type PGHS-2. Introduction of the Y348F mutation also strengthened the time-dependent inhibitory action of nimesulide. These results suggest that removal of Tyr348-Tyr385 hydrogen bonding in PGHS-2 allows greater conformational flexibility in the cyclooxygenase active site, resulting in altered interactions with inhibitors and altered Tyr385 radical behavior.

Visualizing the evolution of surface morphology and surface bond strain during plasma deposition of amorphous silicon thin films

Fundamental understanding of atomic-scale processes that determine the surface morphology of hydrogenated amorphous silicon (a-Si:H) thin films during plasma deposition is essential to develop systematic strategies for depositing smooth device-quality a-Si:H films. We have developed visualization tools for monitoring the evolution of surface morphology, atomic coordination, and bond strain distribution during radical precursor migration on a-Si:H surfaces; these tools are used here to study the mechanisms of SiH/sub 3/ diffusion on the a-Si:H surface and elucidate valley-filling phenomena leading to smooth a-Si:H films. We present surface characterization results during a radical migration trajectory representative of the early stage of plasma deposition: the SiH/sub 3/ precursor is impinged on a hill and migrates until it is incorporated into a nearby valley on the a-Si:H surface.

Active Site Reactant Center Geometry in the CoII−Product Radical Pair State of Coenzyme B12-Dependent Ethanolamine Deaminase Determined by Using Orientation-Selection Electron Spin−Echo Envelope Modulation Spectroscopy

The distances and orientations among reactant centers in the active site of coenzyme B12-dependent ethanolamine deaminase from Salmonella typhimurium have been characterized in the Co(II)-product radical pair state by using X-band electron paramagnetic resonance (EPR) and two-pulse electron spin-echo envelope modulation (ESEEM) spectroscopies in the disordered solid state. The unpaired electron spin in the product radical is localized on C2. Our approach is based on the orientation-selection created in the EPR spectrum of the biradical by the axial electron-electron dipolar interaction. Simulation of the EPR line shape yielded a best-fit Co(II)-C2 distance of 9.3 A. ESEEM spectroscopy performed at four magnetic field values addressed the hyperfine coupling of the unpaired electron spin on C2 with 2H in the C5' methyl group of 5'-deoxyadenosine and in the beta-2H position at C1 of the radical. Global ESEEM simulations (over the four magnetic fields) were weighted by the orientation dependence of the EPR line shape. A Nelder-Mead direct search fitting algorithm was used to optimize the simulations. The results lead to a partial model of the active site, in which C5' is located a perpendicular distance of 1.6 A from the Co(II)-C2 axis, at distances of 6.3 and 3.5 A from Co(II) and C2, respectively. The van der Waals contact of the C5'-methyl group and C2 indicates that C5' remains close to the radical species during the rearrangement step. The C2-Hs-C5' angle including the strongly coupled hydrogen, Hs, and the C5'-Hs orientation relative to the C1-C2 axis are consistent with a linear hydrogen atom transfer coordinate and an in-line acceptor p-orbital orientation. The trigonal plane of the C2 atom defines sub-spaces within the active site for C5' radical migration and hydrogen atom transfers (side of the plane facing Co(II)) and amine migration (side of the plane facing away from Co(II)).

Neck dissections: radical to conservative

BackgroundNeck dissection is an important surgical procedure for the management of metastatic nodal disease in the neck. The gold standard of neck nodal management has been the radical neck dissection. Any modification in the neck dissection is always compared with this standard. Over the last few decades, in order to alleviate the morbidity of radical neck dissection, several modifications and conservative procedures have been advocated. These procedures retain certain lymphatic or non-lymphatic structures and have been shown not to compromise oncological safety.MethodsA literature search of the Medline was carried out for all articles on neck dissections. The articles were systematically reviewed to analyze and trace the evolution of neck dissection. These were then categorized to address the nomenclature, management of node positive and node negative neck including those who had received chemoradiation.ResultsThe present article discusses the neck nodal nomenclature, the radical neck dissection, its modifications and migration to more conservative procedures and possible advances in the near future.ConclusionRadical neck dissection is now replaced with modified radical neck dissections in most situations. Attempts are being made to replace modified radical neck dissections with selective neck dissections for early node positivity. Sentinel node biopsy is being studied to address the issue of node negative neck. More conservative surgeries are likely to replace the 'radical' surgeries of bygone era. This process is facilitated by earlier detection of the disease and better understanding of cancer biology.

Atomic-scale analysis of fundamental mechanisms of surface valley filling during plasma deposition of amorphous silicon thin films

We present a detailed atomic-scale analysis of the fundamental processes that determine the surface smoothness of hydrogenated amorphous silicon (a-Si:H) thin films deposited computationally using molecular-dynamics (MD) simulation. The surfaces of these MD-grown films are remarkably smooth due to a valley-filling mechanism where the mobile precursor, SiH 3, diffuses and passivates dangling bonds present in surface valleys or at the valley edges. The mechanisms of SiH 3 precursor migration on the a-Si:H surface are studied placing special emphasis on elucidating the role of the surface bond strain in mediating the valley-filling phenomena. Surface transport of the SiH 3 precursor is driven by the Si–Si bond strain distribution on the surface, which is strongly coupled with the surface morphology and reactivity. There is a strong driving force for SiH 3 to migrate from a hill toward a valley, but not such a strong driving force for migration out of a valley. As a result, SiH 3 radicals impinged on a surface hill eventually migrate to a nearby valley, regardless of the presence of dangling bonds at the hill, while radicals impinged in a valley remain confined within the valley’s morphological well. Analysis of the MD trajectories for numerous SiH 3 radical migration paths revealed the development of tensile strain bands along these paths, which typically lead to dangling bonds. Adsorbed SiH 3 radicals follow these tensile strain bands, relieve strain along their migration paths, and passivate dangling bonds in valleys or at valley edges, thus leading to surface valley filling. During diffusion on the a-Si:H surface, SiH 3 is observed to insert frequently into strained Si–Si bonds and relieve strain through an athermal and exothermic reaction.

Reductive Electron Injection into Duplex DNA by Aromatic Amines

An assay based on photoinduced reaction and subsequent cleavage of duplex DNA containing a bromodeoxyuridine ((Br)U) residue and an abasic site was developed to screen aromatic amines for their ability to initiate charge transfer by reductive electron donation. Two candidates, N,N,N',N'-tetramethyl-1,5-diaminonaphthalene (TMDN) and 1,5-diaminonaphthalene (DAN), expressed the desired activity, and an oligodeoxynucleotide-TMDN conjugate was subsequently prepared to identify additional variables affecting the efficiency of electron injection and transfer into DNA. This system demonstrated only mild sensitivity to molecular oxygen but was strongly inhibited by high concentrations of 2-mercaptoethanol. The nucleobase counter to the attached TMDN strongly modulated charge transfer as evident by a 60-fold decrease in reduction of the distal (Br)U when the counterbase A was substituted for C. An inverse relationship between this reduction and quenching of TMDN fluorescence by the counterbase was also discovered and is consistent with a competition between radical recombination and electron migration away from the initial site of its injection into DNA.

Cycloreversion and other gas-phase reactions of neutral and cationic pyrrolidine-fused chlorins and isobacteriochlorins under ion bombardment and electrospray.

Neutral and cationic pyrrolidine-fused chlorins and isobacteriochlorins derived from meso-tetrakis(pentafluorophenyl)porphyrin undergo cycloreversion reactions in the gas phase, either when desorbed from a liquid matrix by ion bombardment or when electrosprayed. Cycloreversion occurs through loss of either neutral or charged moieties, with and without hydrogen and methyl radical migration, and both as high- and low-energy collision processes. For the doubly charged isobacteriochlorin, one-electron reduction with methyl loss occurs under ion bombardment and electrospray, through hypervalent pyrrolidinium radical formation.

Identification of Tyr504 as an alternative tyrosyl radical site in human prostaglandin H synthase-2.

Hydroperoxides induce formation of a tyrosyl radical on Tyr385 in prostaglandin H synthase (PGHS). The Tyr385 radical initiates hydrogen abstraction from arachidonic acid, thereby mechanistically connecting the peroxidase and cyclooxygenase activities. In both PGHS isoforms the tyrosyl radical undergoes a time-dependent transition from a wide doublet to a wide singlet species; pretreatment with cyclooxygenase inhibitors results in a third type of signal, a narrow singlet [Tsai, A.-L.; Kulmacz, R. J. (2000) Prost. Lipid Med. 62, 231-254]. These transitions have been interpreted as resulting from Tyr385 ring rotation, but could also be due to radical migration from Tyr385 to another tyrosine residue. PATHWAYS analysis of PGHS crystal structures identified four tyrosine residues with favorable predicted electronic coupling: residues 148, 348, 404, and 504 (ovine PGHS-1 numbering). We expressed recombinant PGHS-2 proteins containing single Tyr --> Phe mutations at the target residues, a quadruple mutant with all four tyrosines mutated, and a quintuple mutant, which also contains a Y385F mutation. All mutants bind heme and display appreciable peroxidase activity, and with the exception of the quintuple mutant, all retain cyclooxygenase activity, indicating that neither of the active sites is significantly perturbed. Reaction of the Y148F, Y348F, and Y404F mutants with EtOOH generates a wide singlet EPR signal similar to that of native PGHS-2. However, reaction of the Y504F and the quadruple mutants with peroxide yields persistent wide doublets, and the quintuple mutant is EPR silent. Nimesulide pretreatment of Y504F and the quadruple mutant results in an abnormally small amount of wide doublet signal, with no narrow singlet being formed. Therefore, the formation of an alternative tyrosine radical on Tyr504 probably accounts for the transition from a wide doublet to a wide singlet in native PGHS-2 and for formation of a narrow singlet in complexes of PGHS-2 with cyclooxygenase inhibitors.

Radiolytic unsaturation decay in polyethylene. Part III—the effect of certain chain transfer agents

Small amounts of certain halogenated compounds are found to have, at most, only a slight enhancing effect on the radiolytic decay rates of added poly-unsaturated compounds in polyethylene, but significantly increase the elastic modulus at 433 K (melt modulus) obtained thereby. Experiments with model chlorine-containing additives suggest that this increase is due to a more random distribution of polymer and monomer mediated crosslinks in the polymer, that it does not result from a significant increase in crosslinking and that it is mediated by chlorine atoms, in a similar manner to radiolytic hydrogen atoms, through facilitation of long range free radical migration. Although low molecular weight chloro-paraffins inhibit radiolytically induced growth of melt modulus in monomer containing polyethylenes, even very small additions of chlorinated polyethylenes, which form a separate phase, increase the melt modulus. This again indicates that the active species is the chlorine radical.

A study of the radiation degradation of styrene/alkane and α‐methylstyrene/alkane copolymers

AbstractThe effects of copolymer composition and microstructure on the radiation chemistry of styrene/alkane and α‐methylstyrene/alkane copolymers have been studied. The primary radical species formed on radiolysis of the copolymers at 77 K, and identified by ESR spectroscopy, are the same as those formed during radiolysis of the homopolymers. The yields of radicals for the copolymer are as predicted assuming that the cross‐section is proportional to the electron density of each component; however, there is some evidence of radical migration to aromatic groups at 77 K. Changes in molecular structure on irradiation were detected by using 13C NMR spectroscopy. Evidence of the consumption of terminal double bonds, and chain scission in α‐methylstyrene/alkane copolymers was found. Measurements of viscosity supported the mechanism of cross‐linking predominating in styrene/alkane copolymers, while in α‐methylstyrene/alkane copolymers chain scission was the major result of irradiation. Copyright © 2003 Society of Chemical Industry

Design and synthesis of highly photopolymerizable styrenyl compounds in solid‐state photoinitiated polymerization

AbstractWe designed a new type of styrenyl compound applicable to conventional photopolymerization systems, aiming at the production of polymers with improved mechanical properties, resistance to chemicals, and elevated glass‐transition temperatures (Tg's). A series of styrenyl monomers bearing 2,5‐dithio‐1,3,4‐thiadiazole groups were prepared, and their reactivity was studied in solid‐state photopolymerization initiated by 2‐(4′‐methoxystyryl)‐4,6‐bis(trichloromethyl)‐1,3,5‐triazine. These monomers exhibited much higher polymerization rates than usual, and the final conversion nearly reached completion, despite the relatively high Tg of the solid‐state photopolymerization system. Even at temperatures below Tg, the polymerization proceeded without a ceiling phenomenon. These features were explained by intermolecular interactions between the monomers that induced monomer alignments effective for solid‐state polymerization, large excess free volumes arising from rotation around the methylthio groups, and intramatrix radical migration leading to encounters with the remaining monomers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3227–3242, 2003