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. 2025 Apr 5;64(15):7716–7725. doi: 10.1021/acs.inorgchem.5c00760

Dependence of O2 Depletion on Transition Metal Catalyst in Radical Polymerization of Cross-Linking Alkene Resins

Hugo den Besten 1, Yanrong Zhang 1, Linda E Eijsink 1, Andy S Sardjan 1, Anouk Volker 1, Wesley R Browne 1,*
PMCID: PMC12015813  PMID: 40186563

Abstract

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Cobalt(II) carboxylates show broad reactivity with peroxides and O2 and are the industry standard catalyst for the activation of peroxide initiators for the radical polymerization of alkenes under ambient conditions. Curing alkene-based resins containing cross-linking units, i.e., monomers containing two or more alkene units, is important in forming hard protective coatings and materials. The activation of peroxide initiators produces the propagating chain end radicals needed for polymerization. Since polymerization progress depends on the rate of initiator activation and the concentration of propagating radicals, interception of radicals by O2 can inhibit curing. Cobalt(II) carboxylates are used due to their reactivity in the presence of oxygen, even in resin coatings. Alternative catalysts based on manganese and iron are desirable. Hence, the impact of O2 on their performance in resin curing is of interest. Here, we use NIR emission and time-resolved spectroscopy, employing the O2-sensitive probe [Ru(ph2phen)3]2+, to determine the concentration of dissolved [O2] in alkene resins during curing with three representative catalysts, Co(II)(2-ethylhexanoate)2, Fe(II)-bispidine, and Mn(II)(neodecanoate)2. The rate of depletion of O2 is highly dependent on the catalyst used, but in all cases, it is well before the onset of the autoacceleration of polymerization in cross-linking resins.

Short abstract

In this study, we use NIR emission and time-resolved spectroscopy, employing the O2-sensitive probe [Ru(ph2phen)3]2+, to determine the concentration of dissolved O2 in alkene resins during curing with three representative catalysts, Co(II)(2-ethylhexanoate)2, Fe(II)-bispidine, and Mn(II)(neodecanoate)2. The rate at depletion of O2 is highly dependent on the catalyst used, but in all cases, it is well before the onset of the autoacceleration of polymerization in cross-linking resins.

Introduction

The in situ formation of a solid material by polymerization of liquid precursors (e.g., a resin) is central to many applications in materials science, from 3D-printing and resin-molding to dental fillers and protective coatings, and typically uses initiated polymerization of monomers (alkenes, epoxides, etc.) together with cross-linking agents.1 Free radical polymerization of alkenes is controlled typically using initiators, which are frequently alkyl (hydro)peroxides.2 Radicals are generated by thermal decomposition (heating to 80–100 °C), at ambient temperatures by photolysis (UV light), or metal-catalyzed decomposition. The choice of the initiation method depends on the specific application, e.g., photolysis is used in dental applications.3 Cross-linked alkene-based resins are used widely as protective coatings and in molding. Initiation by metal-catalyzed decomposition of alkyl (hydro)peroxides is used under ambient conditions.2,4,5

Cobalt(II) carboxylates are the current industry standard catalysts in room-temperature alkene-based resin curing, catalyzing the decomposition of cumene hydroperoxide to radical species, which then initiate polymerization. In recent years, efforts have been directed toward replacing cobalt(II) carboxylates with environmentally benign catalysts, specifically those based on iron(II) and manganese(II) (Figure 1).6,7 While these catalysts can activate alkyl hydroperoxide initiators, the impact of dissolved O2 is of substantial interest.8 Dissolved O2 is a known inhibitor in the free radical polymerization of alkenes.9−14 The inhibition is attributed to the formation of less reactive ROO· radicals from the reaction of R· with O2.9,15,16 Therefore, dissolved O2 in resins can impact polymerization (curing) kinetics due to both interference with radical propagation reactions8 and, in some cases, interaction with catalysts, as shown recently by Anastasaki and co-workers in the case of copper-catalyzed ATRP reactions.17 Purging to remove O2 avoids such interference but is unfeasible on a large scale and when resins are applied to form protective coatings under ambient conditions.

Figure 1.

Figure 1

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Structures of the catalysts, Co(II)(2-ethylhexanoate)2, Mn(II)(neodecanoate)2, and Fe(II)-bispidine, discussed in the text.

Additives, e.g., tertiary amines, are used in commercial formulations to scavenge O2, and the oxygen radicals formed and they have been optimized to work with cobalt(II) carboxylate catalysts over the last half century. This codevelopment presents challenges in efforts toward Co(II) replacement (e.g., interference or deactivation of Fe(II) and Mn(II) catalysts by additives) since both Co(II) and Co(III) salts can react with alkyl-oxy and -peroxy radicals in a manner that is unique among transition metals.18,19 Furthermore, curing alkene-based resins using alkyl hydroperoxides (typically cumene hydroperoxide) is characterized by a significant lag period before the onset of polymerization, i.e., autoacceleration (see Scheme 1 and the SI for further details).20 The lag phase needs to be long enough to allow for the application of coatings to surfaces but cannot be too long due to the loss of reactive diluents (e.g., styrene) over time by evaporation. These characteristics place considerable restrictions on the development of catalyst replacements. The relation between [O2] and the lag phase observed during alkene resin curing with cobalt(II) carboxylates and their prospective replacements is therefore of interest.

Scheme 1. Transition Metal Catalysts Initiate Polymerization (i) by Decomposing Cumene Hydroperoxide into Radicals that (ii) Initiate the Polymerization of Alkenes.

Scheme 1

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The polymerization is interrupted by (iii) growing chains reacting with each other and (iv) with O2. For a detailed discussion of the relative rates, see the SI.

In the present contribution, interference by O2 in the curing of a representative additive-free alkene resin, comprising styrene and the cross-linker BADGE-MA (Figure 2), with two potential Co(II) carboxylate replacements, Mn(II)(neodecanoate)2 and Fe(II)-bispidine (Figure 1), is investigated.21 The cross-linker BADGE-MA is a representative example of the bifunctional cross-linkers used in commercial alkene-based resin mixtures. It forms cross-linked polymers together with a reactive diluent such as styrene; Figure 2. The role of O2 in the delay to the onset of autoacceleration following the addition of an initiator to alkene-based resins is explored. Inline spectroscopy is used to determine the change in [O2] over time in both BADGE-MA/alkene resins and in model non-cross-linking alkene mixtures (Figure 2).

Figure 2.

Figure 2

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Left: BADGE-MA/styrene resin mixture; right: styrene/MMA/MeOPrOH mixture used as a model, consisting of styrene, methyl methacrylate, and 1-methoxy-2-propanol (1:1:1 vol. ratio).

Changes in [O2] are determined through dynamic quenching of the emission of the complex [Ru(ph2phen)3]2+ (where ph2phen is 4,7-diphenyl-1,10-phenanthroline, Figure 3). Interference by emission quenchers, e.g., the catalysts and species formed during the curing, is determined using the sensitized NIR phosphorescence of 1O222 together with the observed emission decay rate of the sensitizer [Ru(ph2phen)3]2+.23 Additionally, the exchange of O2 between the resin and air is determined by headspace Raman spectroscopy. Together, the data allows for the relation between the presence of O2 and the lag period before the onset of autoacceleration (rapid polymerization) to be established under a wide range of conditions, with the reference cobalt(II) catalyst and the potential replacement Mn(II) and Fe(II) catalysts (Figure 1).

Figure 3.

Figure 3

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Structure of the luminescent probe [Ru(ph2phen)3]2+.

Results and Discussion

The resin mixture studied here is composed of the reactive diluent styrene and the cross-linker BADGE-MA (Figure 2). The preparation and characterization of BADGE-MA have been reported earlier24 and are used here to ensure a known chemical composition and residual acid content. Although the mixture is a simplification of commercial resins, the curing profile (Figure 4) exhibits the expected lag period, i.e., slow consumption of alkene, followed by a period of autoacceleration (Trommsdorff effect)20 in which the rate of alkene polymerization accelerates until the glass point is reached and curing is halted. A model solvent system comprising styrene, methyl methacrylate, and 1-methoxy-2-propanol (styrene/MMA/MeOPrOH) was selected to match closely the chemical composition and solvent properties of BADGE-MA/styrene, with 1-methoxypropan-2-ol both to provide the alcohol functional groups present in BADGE-MA and to ensure solubility of the catalysts used (Figure 2).

Figure 4.

Figure 4

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Integrated area of the νC=C,str (normalized to the initial area) Raman band at 1630–1637 cm–1 over time following addition of cumene hydroperoxide to a mixture of (Co(II)(2-ethylhexanoate)2) and (red) BADGE-MA/styrene (1.0 g/0.34 g), and (blue) styrene/MMA/MeOPrOH (styrene, methyl methacrylate, and 1-methoxy-2-propanol, 1:1:1 vol. ratio) at 19 °C. Significant alkene conversion in the model mixture is not observed within 24 h. Data are normalized to the area of the Raman band corresponding to the alkene C=C stretching vibration (1630–1637 cm–1).

Although comparable with regard to functionality, the viscosity of this model mixture is lower than that of BADGE-MA/styrene, which impacts diffusion-controlled processes, i.e., radical–radical combinations, bimolecular quenching, etc., vide infra. The alkene polymerization in the model mixture is not observed over the first few hours after addition of the initiator, and indeed, even after 24 h, the extent of conversion is negligible. Therefore, the viscosity of the mixture remains relatively constant over the period of interest to the present study, i.e., the first few hours after addition of the initiator (Figure 4), and hence, molecular diffusion coefficients can be assumed to be constant over time. The only variable of concern in the photophysical studies described below is the concentration of the interacting species, namely, [Ru(ph2phen)3]2+ and quenchers, such as O2.

Dependence of O2 Uptake/Release on the Catalytic Decomposition of Cumene Hydroperoxide

The transition metal-catalyzed decomposition of alkyl peroxides, such as cumene hydroperoxide, to release O2 was studied earlier by Spier et al. in cyclohexane.5 The rapid equilibration of gases between the solution and the headspace above it allows for the release of O2 to be monitored using headspace Raman spectroscopy.25,26 The release of O2 into the headspace, following addition of cumene hydroperoxide to Co(II)(2-ethylhexanoate)2 in cyclohexane, proceeded as expected,5 corresponding to ca. 10 μmol of O2 (i.e., 10% with respect to the cumene hydroperoxide added) over 5 min (Figure 5).

Figure 5.

Figure 5

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Time dependence of P(O2) in the headspace above (a) cyclohexane and (b) above styrene/methyl methacrylate (1:1 vol/vol) during the decomposition of cumene hydroperoxide by Co(II)(2-ethylhexanoate)2 monitored by Raman spectroscopy (λexc 785 nm). Orange/Blue indicate independent experiments.

The decomposition of cumene hydroperoxide by Co(II)(2-ethylhexanoate)2 in a mixture of styrene and methyl methacrylate is relatively fast (determined iodometrically earlier24), and hence, the release of O2 into the headspace would be expected. However, the opposite is observed: O2 is removed from the headspace over time, indicating that O2 is consumed in solution (Figure 5). Indeed, in contrast to in cyclohexane, in styrene/methyl methacrylate, ca. 8 μmol of O2 was consumed from the headspace at a steady rate over 20 min (Figure 5), corresponding to the period of rapid decrease in the concentration of cumene hydroperoxide (Figure S1). Together with the estimated initial [O2] in the mixture of 3.9 mM (see Table S1), this change corresponds to a stoichiometry of ca. 0.1:1 with respect to the cumene hydroperoxide added. Surprisingly, although neither Fe(II)-bispidine nor Mn(II)(neodecanoate)2 show substantial decomposition of cumene hydroperoxide over the first hour (Figure S1),6 the observed rate and extent of consumption of O2 are only marginally lower than with Co(II)(2-ethylhexanoate)2 (Figure 6). Therefore, the reaction with O2 is not directly correlated with the extent and rate at which the catalysts decompose cumene hydroperoxide.

Figure 6.

Figure 6

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Time dependence of pO2 in the headspace of a sealed vial after addition of cumene hydroperoxide (CumOOH) to styrene/MMA/MeOPrOH without a catalyst, and with Co(II)(2-ethylhexanoate)2, Fe(II)-bispidine, or Mn(II)(neodecanoate)2 and with Co(II)(2-ethylhexanoate)2 only. CumOOH, cumene hydroperoxide.

Hence, depletion of dissolved O2 in the resins would be expected in the first minutes also. However, although equilibration of O2 between solution and headspace is relatively rapid for cyclohexane and mixtures of styrene and methyl methacrylate, such equilibration is slow in the more viscous BADGE-MA/styrene resin, with only the first millimeters of solution in contact with the headspace showing significant exchange (vide infra). Hence, the in situ determination of dissolved [O2] is required.

In Situ Determination of O2 Using [Ru(II)(ph2phen)3](PF6)2

In situ determination of [O2] in alkene mixtures, which do not contain cross-linking monomers, is possible, e.g., using a Clark probe or optical oxygen probe sensors.27 In resins containing cross-linkers that undergo a large increase in viscosity and form hard solids, noncontact optical detection using a dispersed O2-sensitive luminescent compound is advantageous.28−30 The quenching of emission by O2 is dynamic and hence dependent on diffusion rate constants, as well as concentrations, and several studies have focused on accounting for these variables.31−33

The change in [O2] in the resin and model mixtures following addition of cumene hydroperoxide was determined indirectly through the emission decay lifetime (τobs) of [Ru(II)(ph2phen)3](PF6)2.34,35 Quenching of the phosphorescence of [Ru(II)(ph2phen)3](PF6)2 by dissolved O2 results in a reduction in the former’s emission decay lifetime and a corresponding NIR emission from the 1O2 generated in the process (vide infra). The [O2] is directly related to the observed rate of decay (kobs) of the emission (dynamic quenching) through the Stern–Volmer equation, eq 1.

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where τobs is the observed emission lifetime, kobs is the observed rate of emission decay, kr is the radiative decay rate, kq is the rate of quenching, and knr is the rate of other nonradiative decay paths.

However, as it is a diffusion-controlled process, it is also dependent on solvent viscosity (Inline graphic, where η is viscosity and T is temperature, in the Einstein–Stokes model), which differs considerably between the solvent mixture styrene/MMA/MeOPrOH and the BADGE-MA/styrene resin, with the latter much more viscous. Furthermore, the high viscosity of cross-linking alkene resins, even before curing has been initiated, poses an additional challenge due to the potential irreversible decrease in the concentration of O2 in the confocal volume of the spectrometer during the measurement. Hence, initial studies have focused on the chemically equivalent styrene/MMA/MeOPrOH mixture.

The [O2] in the model mixture is estimated at ca. 3.6 mM (Table S1),25,36 which is consistent with the emission lifetime of [Ru(II)(ph2phen)3](PF6)2 (315 ns) in the mixture. The emission lifetime increases to 4.8 μs after purging with N2 or argon, similar to that in other solvents at room temperature in the absence of O2.37 The addition of cumene hydroperoxide or any of the catalysts, Fe(II)-bispidine, Co(II)(2-ethylhexanoate)2, or Mn(II)(neodecanoate)2, does not affect the emission lifetime in an air-equilibrated solvent (Figure S2) even 1 h after addition. In contrast, a substantial increase in emission lifetime (τobs) is observed after the addition of cumene hydroperoxide to mixtures of styrene/MMA/MeOPrOH containing any of the three catalysts, indicating that dissolved O2 is consumed, vide supra.

With Co(II)(2-ethylhexanoate)2 (Figure S2a), τobs increases to 4 μs within 1 min of addition of the cumene hydroperoxide due to a decrease in [O2] by at least 90% to <0.4 mM. After 1 h, however, τobs decreases again to ca. 2.5 μs. The subsequent decrease in τobs is not due to re-equilibration of O2 between the solution and the headspace (vide supra) since the same changes in τobs were observed in completely filled and sealed sample vials (i.e., with no headspace).

With Fe(II)-bispidine, τobs increases less rapidly (over 10 min) to almost 5 μs (Figure S2b). In the first minute, the emission decay is multiexponential due to the lifetime increasing during the time taken to acquire the data (ca. 2 min). At 5 min and 1 h, the emission decay is monoexponential. With Mn(II)(neodecanoate)2, τobs increases much more slowly, reaching the maximum emission lifetime only after 20 min (Figure S2c).

The change in τobs over time for the three catalysts is shown in Figure S3. Since the decay rate (kobs, eq 1), and not the emission lifetime, is linearly proportional to [O2], Figure 7 provides for a clearer overview of the change in [O2] over time.

Figure 7.

Figure 7

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kobs (squares) and integrated area of the νC=C,str (normalized to the initial area) Raman band at 1630–1637 cm–1 (line) over time after addition of cumene hydroperoxide (92 mM) with Fe(II)bispidine, Co(II)(2-ethylhexanoate)2, or Mn(II)(neodecanoate)2 (a) in styrene/MMA/MeOPrOH and (b) in BADGE-MA/styrene resin. See Figure S4 for a change over 200 min.

The data together show that with Co(II)(2-ethylhexanoate)2 and Fe(II)-bispidine, the solution is depleted of O2 within a few minutes of addition of cumene hydroperoxide. The increase in the emission lifetime with Mn(II)(neodecanoate)2 is more gradual. Indeed, even after 5 min, the [O2] is essentially unchanged, and it takes up to 1 h after addition for the maximum lifetime to be reached.

The higher viscosity of the resin, compared to the model mixture, reduces the extent of quenching by O2, and hence, initially, the emission lifetime (τobs) is longer, i.e., ca. 1.1 μs in the resin. Nevertheless, essentially the same changes in τobs, and hence [O2], over time were observed with each of the catalysts and cumene hydroperoxide in the BADGE-MA/styrene resin. The increase in τobs occurred more rapidly in the resin with Fe(II)-bispidine and Co(II)(2-ethylhexanoate)2 than in the model mixture, and in both cases, the final emission lifetimes reached were longer. With Mn(II)(neodecanoate)2, a gradual increase in τobs was observed after the addition of cumene hydroperoxide with a rapid increase in τobs after ca. 100 min after.

The changes in emission decay rates (kobs) can be related directly with [O2]. It is apparent therefore (Figure 7) that with Co(II)(2-ethylhexanoate)2, Fe(II)-bispidine, and eventually Mn(II)(neodecanoate)2 in BADGE-MA/styrene, all O2 is consumed following addition of cumene hydroperoxide, in both cases, well before the onset of autoacceleration (manifested in a rapid decrease in the intensity of the Raman band (νC=C) at ca. 1630 cm–1, Figure 7).

The decrease in τobs observed with Co(II)(2-ethylhexanoate)2 in the model mixture is likely not observed in more viscous resin since diffusion, and hence bimolecular quenching, has much less impact (vide infra). With Co(II)(2-ethylhexanoate)2 in styrene/MMA/MeOPrOH, the decrease in emission lifetime several minutes after it had reached a maximum indicates that a quencher forms in situ in the mixture over time. Given that O2 is liberated by the reaction of Co(II)(2-ethylhexanoate)2 with cumene hydroperoxide in cyclohexane (vide infra), it is possible that a steady state is reached where oxygen consumption and production balance. The lifetime reached (2.5 μs) would require the [O2] to reach 0.55 mM. However, the formation of another quencher should be considered too, i.e., Co(III) species (vide infra).

Singlet Oxygen Emission

Quenching of the excited state of ruthenium(II) polypyridyl complexes by energy transfer to (3O2) results in the generation of singlet oxygen (1O2), which relaxes to the ground state (3O2) primarily by radiationless deactivation, but with some radiative decay resulting in phosphorescence at 1268 nm. Hence, the observation of changes in the intensity of the characteristic phosphorescence of 1O2 in the NIR region can confirm that the increase in the emission lifetime of [Ru(ph2phen)3]2+ is due to a decrease in [3O2]. These data supplement the emission lifetime data discussed above, and the data sets should be consistent if O2 is the only quencher involved.

The NIR emission spectrum obtained upon excitation of [Ru(ph2phen)3]2+ in styrene/MMA/MeOPrOH shows the expected narrow emission band of 1O2 at 1268 nm, on top of the broad tail of the emission from [Ru(ph2phen)3]2+, allowing for changes in emission intensity for both species to be monitored simultaneously. Addition of cumene hydroperoxide resulted in a minor decrease in the intensity of 1O2 emission and a small increase in emission from [Ru(ph2phen)3]2+ (Figure 8a). The emission spectrum remained essentially unchanged for over 1 h thereafter, consistent with the absence of change in the emission lifetime of [Ru(ph2phen)3]2+ (Figure S2d).

Figure 8.

Figure 8

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NIR emission spectrum of [Ru(ph2phen)3](PF6)2 in styrene/MMA/MeOPrOH (a) without a catalyst, and with (b) Co(II)(2-ethylhexanoate)2 (2 mM), (c) Fe(II)-bispidine (0.2 mM), or (d) Mn(II)(neodecanoate)2 (2 mM), before and at selected times after addition of cumene hydroperoxide (92 mM), (λexc 355 nm).

Addition of either Co(II)(2-ethylhexanoate)2 or Mn(II)(neodecanoate)2 to styrene/MMA/MeOPrOH with [Ru(ph2phen)3]2+ resulted in a decrease in intensity of the emission of 1O2. The decrease in the case of Co(II)(2-ethylhexanoate)2 and Mn(II)(neodecanoate)2 is due to primary inner filter effects, i.e., absorbance by the solutions at the excitation wavelength used (355 nm). In contrast, addition of Fe(II)-bispidine, which does not absorb significantly at 355 nm, does not affect the emission spectrum. UV/vis absorption spectroscopy shows a further increase in absorption at 355 nm over time following addition of cumene hydroperoxide for solutions containing any of the three catalysts (Figures S5–S7).38

With Co(II)(2-ethylhexanoate)2 present, a complete loss in emission due to 1O2 was observed within 1 min of the addition of cumene hydroperoxide (Figure 8b). The tailing emission from [Ru(ph2phen)3]2+ increased and then decreased partly again, consistent with the decrease in the emission lifetime of [Ru(ph2phen)3]2+ between 5 min and 1 h after addition of cumene hydroperoxide (Figure S2a,b). Notably, the absence of emission at 1268 nm from 1O2 between 5 min and 1 h confirms that oxygen is not responsible for quenching of the emission of [Ru(ph2phen)3]2+ and therefore is no longer present (<10%; see the Experimental Section for details). Furthermore, purging with argon (to remove any remaining O2) 1 h after the addition of cumene hydroperoxide has no effect on the emission lifetime.

With Fe(II)-bispidine, the decrease in emission intensity was slower (Figure 8c), with weak emission persisting for up to 5 min, consistent with the changes seen in the emission lifetime (Figure S2b).

With Mn(II)(neodecanoate)2, τobs increases slowly over time after addition of cumene hydroperoxide, corresponding well to the decrease in 1O2 emission observed (Figures 9 and S3). After 15 min, the rate of change in τobs and 1O2 emission increases rapidly, with eventually a complete loss in 1O2 emission.

Figure 9.

Figure 9

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(a) NIR emission spectrum with [Ru(ph2phen)3](PF6)2 in styrene/MMA/MeOPrOH containing Mn(II)(neodecanoate)2 (2 mM), before (blue) and after addition of cumene hydroperoxide (λexc 450 nm), and (b) integrated area of emission at 1268 nm (of 1O2, blue) and τobs (of [Ru(ph2phen)3](PF6)2, red) over time.

Quenching of Emission with Cumene Hydroperoxide and Co(II)(2-Ethylhexanoate)2

The emission lifetime of [Ru(ph2phen)3](PF6)2 in the presence of Co(II)(2-ethylhexanoate)2 and cumene hydroperoxide changes in a more complex manner than with Fe(II)-bispidine or Mn(II)(neodecanoate)2. Specifically, the emission lifetime increases from 300 ns to 4.5 μs within 20–30 s of the addition of cumene hydroperoxide (vide supra, Figure S3), after which the emission lifetime decreases to 2.5 μs over 15–20 min. The latter decrease in emission lifetime indicates an increase in the concentration of a quencher in the mixture over time. The absence of emission from 1O2 (Figure S9) confirms that the [O2] does not increase again (vide supra). Furthermore, the emission decay lifetime of [Ru(ph2phen)3](PF6)2 was not affected by the presence of Co(II) in the model solvent mixture. Hence, the formation of Co(III) species in the reaction mixture over time was considered the most likely cause since quenching of the excited states of ruthenium(II) polypyridyl complexes by Co(III) salts (via oxidative electron transfer) is well-known.39,40

Indeed, a linear dependence of kobs, 15 min after addition of cumene hydroperoxide, on the initial concentration of Co(II)(2-ethylhexanoate)2 was observed (Figures 10 and S10),41 consistent with the oxidative electron transfer quenching of [Ru(ph2phen)3]2+ by Co(III) formed in situ. The quenching is consistent with changes in the UV/vis absorption spectrum of the reaction mixture (Figure S5), which indicates conversion from the Co(II) to Co(III) oxidation state also.42,43

Figure 10.

Figure 10

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Dependence of 1/τobs for [Ru(ph2phen)3](PF6)2 in tertbutylstyrene/MMA/MeOPrOH containing increasing initial concentrations of Co(II)(2-ethylhexanoate)2. τobs was determined 15 min after the addition of cumene hydroperoxide (92 mM). Note that the lifetime is unaffected by the presence of Co(II)(2-ethylhexanoate)2 alone over this range of concentrations (Figure S10).

NIR Emission Spectroscopy in BADGE-MA/Styrene Resins

The higher viscosity of the cross-linker containing resin (BADGE-MA/styrene) reduces the diffusivity of O2 and [Ru(ph2phen)3](PF6)2 and hence the intensity of the sensitized 1O2 emission. Nevertheless, the NIR emission of 1O2 was observed in the resin mixture. However, a rapid decrease in emission intensity at 1268 nm (1O2) and a corresponding increase in the emission intensity of [Ru(ph2phen)3](PF6)2 (within 2–3 s) are observed with laser excitation due to rapid depletion of the O2 in the confocal volume of the spectrometer (Figure S11).44 The high viscosity of the resin prevents rapid replenishment of O2 in the confocal volume. Continuous movement of the sample during acquisition of NIR emission spectra is nevertheless sufficient to allow for spectra to be recorded with minimum impact on intensity from the photoinduced consumption of O2.

The NIR emission of 1O2 in the resin is unaffected by addition of Mn(II)(neodecanoate)2 and shows a modest initial decrease with the addition of Co(II)(2-ethylhexanoate)2, with no further change in either case over 2 h (Figure 11). Notably, the addition of cumene hydroperoxide to BADGE-MA/styrene reduces the emission intensity slowly over time, which indicates that O2 is depleted slowly even in the absence of a catalyst. Raman spectra recorded 2 h after addition of any of the components alone show that alkene polymerization has not occurred to a significant extent (Figure S12).

Figure 11.

Figure 11

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NIR emission spectra (λexc 450 nm) with [Ru(ph2phen)3](PF6)2 in BADGE-MA/styrene alone (blue) and with Mn(II)(neodecanoate)2 (purple), Co(II)(2-ethylhexanoate)2 (gray), and cumene hydroperoxide (initially (green) and after 30 min (black)). The spectra are offset-corrected at 1370 nm and normalized at 1200 nm. CumOOH, cumene hydroperoxide.

As observed in the model mixture (vide supra), the emission from 1O2 decreases within minutes of addition of cumene hydroperoxide to BADGE-MA/styrene with Co(II)(2-ethylhexanoate)2, and the expected6 eventual extent of alkene polymerization (at 19 °C) was observed (Figure S12).

With Mn(II)(neodecanoate)2 and cumene hydroperoxide, the NIR emission decreased slowly over 2 h, similar to that observed with cumene hydroperoxide alone (Figure 12). Alkene polymerization was not observed over this time period although the extent of polymerization after 24 h was the same as that with Co(II)(2-ethylhexanoate)2 (Figure S12). Notably, the emission intensity did not decrease near the top of the sample (i.e., the resin in contact with the headspace), consistent with replenishment of O2 by diffusion from the headspace.

Figure 12.

Figure 12

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NIR emission spectra (λexc 450 nm) with [Ru(ph2phen)3](PF6)2 in BADGE-MA/styrene alone (blue) and with cumene hydroperoxide (green), with Co(II)(2-ethylhexanoate)2 (gray), with Co(II)(2-ethylhexanoate)2 and cumene hydroperoxide (red), with Mn(II)(neodecanoate)2 (purple), and with Mn(II)(neodecanoate)2 and cumene hydroperoxide (black). The spectra are offset-corrected at 1370 nm and normalized at 1200 nm and were recorded 24 h after the addition of cumene hydroperoxide. CumOOH, cumene hydroperoxide.

The data indicate that although some differences are observed between the model mixture styrene/MMA/MeOPrOH and the resin BADGE-MA/styrene, the reactivity with respect to the depletion of the O2 observed is equivalent in both mixtures.

Conclusions

The replacement of Co(II) carboxylates with Fe(II) and Mn(II) catalysts is desirable, and in the present study, the catalysts were selected due to their use already as driers in alkyd-based paints.50 Of particular interest are the differences in the reactivity of O2 in the presence of these catalysts compared to that with Co(II)(2-ethylhexanoate)2. Knowledge of the rate of depletion of O2 during alkene-based resin curing is important due to the role that O2 can play in inhibiting polymerization. In the present study, we show that the luminescent probe [Ru(ph2phen)3]2+ can be used to track the depletion of dissolved O2 in real time during resin curing and that interferences by other quenchers (e.g., Co(III) species) can be accounted for by the simultaneous determination of emission from the 1O2 generated during the measurement, as a positive control. The data also show that chemically equivalent low-viscosity models for alkene resins can serve as accurate surrogates for more viscous cross-linking alkene resins, provided that the impact of differences in viscosity on bimolecular processes is taken into account.

O2 is consumed from the headspace of alkene mixtures through the reaction of cumene hydroperoxide with all three catalysts to a greater or lesser extent, with the most rapid uptake observed with Co(II)(2-ethylhexanoate)2. The high viscosity of the BADGE-MA/styrene resin limits the exchange of O2 with the headspace to the first microns of the resin surface, and hence, more rapid depletion and eventually a complete loss of dissolved O2 can be expected.

Importantly, with regard to the replacement of Co(II)-based catalysts, the depletion of dissolved O2 is shown to be dependent on the catalyst replacement used. However, in viscous cross-linking resins, depletion of O2 occurs in all cases well before the onset of autoacceleration. In applications of such resins as thin-film coatings, O2 diffusion can maintain a steady state of [O2] in the outer layer of a coating, as observed here in studies of a resin with Mn(II)(neodecanoate)2, where the rate of oxygen uptake from the headspace matches the rate of depletion due to the action of the catalyst. Hence, although inhibition of polymerization by O2 is not of relevance for systems such as those studied here under bulk conditions, the competition between diffusion of O2 from the atmosphere and consumption of O2 by the action of the catalyst on the initiator is likely to be relevant in determining the properties of such resins when used as coatings. This latter aspect is the focus of ongoing studies in our group.

Experimental Section

Bisphenol-A-based bismethacrylate was provided by AkzoNobel, Sassenheim, for which the synthesis and characterization have been reported earlier.24 Styrene (≥99.0%), Co(II)(2-ethylhexanoate)2 solution (65 wt %), and cumene hydroperoxide (80%) were obtained from SigmaAldrich. The Al2O3 90 active 70–230 mesh was obtained from Merck. All monomers were filtered over Al2O3 before use to remove stabilizers. Mn(II)(neodecanoate)2 (8 wt % on a metal basis) in mineral spirits and Fe(II)-bispidine (BorchiOXY-Coat 1410) were provided by Borchers. Resins were prepared by mixing BADGE-MA (bisphenol A-based diglycidyl ether dimethacrylate) and styrene. BADGE-MA was warmed in an oven at 80 °C for ca. 30 min. 10 g of warm BADGE-MA was poured into a disposable 20 mL glass vial with a screw cap, to which 3.5 g (i.e., 0.35 g/g BADGE-MA resin) of styrene was added, and a vortex mixer (Scientific Industries, Vortex Genie 2) or a SpeedMixer (FlackTek, DAC 330-100 SE) was used to homogenize the mixture. The mixture was allowed to cool to room temperature before use.

Headspace Raman Spectroscopy

The composition of the gases in the closed headspace of quartz cuvette or glass vials was determined by Raman spectroscopy at λ785 as described earlier using the Raman bands of O2 and N2 at 1550 and 2320 cm–1.25,26 The concentration of dissolved oxygen in the alkenes used in the present study and in particular in the model mixture of styrene/methyl methacrylate/methoxy-2-propanol (1:1:1 by volume) and in BADGE-MA/styrene was estimated using calculated Henry’s law constant using the COSMO-RS method45,46 as implemented in AMS2024.47 The default settings were used to generate the .coskf files, BP86/TZP + ZORA, and “good” numerical quality. From the Raman measurement, oxygen partial pressure can be measured, and thus, the dissolved oxygen concentration can be calculated using

graphic file with name ic5c00760_m003.jpg

Emission Spectroscopy and Phosphorescence Decay Lifetimes

A stock solution of 2 mg/mL [Ru(ph2phen)3](PF6)2 was prepared in acetonitrile for addition to the model mixture and the resin. Solutions of catalysts in mixtures of styrene/methyl methacrylate/methoxy-2-propanol (1:1:1 by volume) were prepared, and 1 mL (1.8 mL to avoid a headspace in the vial) was transferred into a 2 mL GC vial. 5 μL of the stock solution of [Ru(ph2phen)3](PF6)2 was added to the sample. BADGE-MA/styrene resin was prepared as described above, and the same concentration of [Ru(ph2phen)3](PF6)2, as used in the mixture of styrene/MMA/MeOPrOH, was added to the resin in a stock solution in acetonitrile. The vial was sealed with a GC-vial cap with a PTFE septum.

Cumene hydroperoxide (17 μL/ml) was added through the septum using a 50 μL Hamilton microliter syringe. Emission decay lifetimes were recorded using an FS-5 spectrofluorimeter by MCS (multichannel scaling) with a 450 nm (EPL-450) pulsed laser diode (Edinburgh Instruments).

NIR Emission Spectroscopy

NIR emission spectra were recorded with a 355 nm CW laser (Cobolt lasers, 2 mW at sample) or 450 nm (45 mW, PowerTechnology) directed into the optical path of the spectrometer with a 45 ° long-pass dichroic beam splitter and focused onto the sample with a 25 mm diameter (f = 35 mm) planoconvex lens. Emission was collected by the same lens, passed through the dichroic beam splitter and a long-pass filter (1064 nm, Semrock) to remove visible light, and focused with a 25 mm diameter (f = 40 mm) planoconvex lens into a Shamrock 193i spectrograph equipped with an idus-InGaAs diode array (Andor Technology) with a 600 l/mm grating blazed at 860 nm. Spectra were recorded using Andor Solis and processed using Spectragryph 1.2.17.

Relation between the τobs of [Ru(ph2phen)3](PF6)2 and the Intensity of Emission from 1O2

The emission lifetime of [Ru(ph2phen)3](PF6)2 under air-equilibrated conditions in styrene/MMA/MeOPrOH is 315 ns, as expected considering that although the concentration of dissolved O2 (ca. 3.6 mM)25,36,48 is similar to CH3CN (τ = 169 ns), it is more viscous and hence molecular diffusivity is reduced. However, purging with argon gas increases the emission lifetime to 6.1 μs in CH3CN, which is consistent with values reported in the literature (namely, 6.3 μs in acetonitrile at 25 °C, degassed by four freeze–pump thaw cycles),48 and the lifetime only increased to 4.86 μs in the model mixture due to excited state deactivation by solvent, O–H oscillators.49 As expected, emission from 1O2 was not observed after argon purging either in CH3CN or the model solvent mixture. The ratio of Raman bands of O2 to N2 in the Raman spectrum recorded in the headspace above the model alkene solvent mixture was used, together with the Henry constant, to estimate the concentration of dissolved O2. As expected, a linear correlation between the concentration of dissolved O2 and the emission decay rate was obtained.

The limit of detection for emission from 1O2 was determined by increasing the concentration of 3O2 incrementally and determining both the emission lifetime of [Ru(ph2phen)3](PF6)2 and the intensity of the NIR emission from 1O2 (Figure S13); emission from 1O2 was above the limit of detection in samples where the emission lifetime of [Ru(ph2phen)3](PF6)2 was between 2.7 and 2.9 μs, and even at 3.9 μs, a distinct emission at 1268 nm was observable under the conditions employed here (Figure S8), which corresponds to less than 10% of the original [O2].

Acknowledgments

Financial support was provided by the University of Groningen (A.S.S), The Netherlands Organisation for Scientific Research, Advanced Research Center Chemical Building Blocks Consortium (2021.038.C.RUG.8, HdB, LEE, WRB) and the Chinese Scholarship Council (YRZ). J. Flapper, K. van den Berg, and R. Hage are thanked for the discussion.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.5c00760.

  • Additional discussion of polymerization kinetics, time-dependent UV/vis absorption, emission, emission decay data, alkene, and oxidant conversion over time, and calibration curves (PDF)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ic5c00760_si_001.pdf (6.9MB, pdf)

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