N-Heterocyclic carbene-borane radicals as efficient initiating species of photopolymerization reactions under air

Abstract

Eight N-heterocyclic carbene-boranes (NHC-boranes) are proposed as new efficient co-initiators for acrylate photopolymerization reactions. They are particularly interesting in aerated conditions, where they help overcome the classical oxygen inhibition. The carbene boryl radicals that are the initiating species have been characterized by their transient absorption spectra obtained in laser flash photolysis (LFP) experiments. Rate constants for the generation of the carbene boryl radicals by hydrogen abstraction with t-butoxyl radical and triplet benzophenone as well as the reactions with oxygen, electron rich and electron poor alkenes, two alkyl halides (CHCl₃ and iodopropane) and diphenyliodonium hexafluorophosphate have been measured. The reactivity of N-heterocyclic carbene borane radicals is clearly affected by the NHC substituent.

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Introduction

In photopolymerization reactions that are involved in the radiation curing and laser imaging areas,¹ the photoinitiating system (PIS) plays a pivotal role. Among PISs, Type II PISs are based on a hydrogen transfer process between a suitable photoinitiator (PI) (e.g.benzophenones, thioxanthones, ketocoumarins, camphorquinone or dyes…) and a co-initiator DH (e.g.amine, thiol, silane…).¹ The quest for new and efficient co-initiators remains a big challenge in this area and the discovery of radicals exhibiting an enhanced reactivity should provide new opportunities for the development of new materials.

In recent work, we have been especially interested in the photopolymerization of low viscosity monomers (LVMs) under air. In that case, the ratio between the rate of polymerization R_p in air and in laminate can drop down to 10% when considering usual Type II systems. Some novel co-initiator structures such as the silanes^1s–1t provide a noticeably better ratio. Recently, we checked the polymerization initiating ability of newly developed^2–4N-heterocyclic carbene boranes (NHC-boranes)⁵ as well as classical amine-boranes⁶ in laminate. To extend this study, we selected eight new NHC-borane derivatives (Scheme 1) as suitable co-initiators for free radical polymerization (FRP) of LVMs under air. The co-initiator ability and the NHC substituent effect of these structures will be presented. Little is known about the kinetics of the NHC boryl radicals, so we measured rate constants for their formation and interaction with alkenes, oxygen, CHCl₃, iodopropane and diphenyliodonium hexafluorophosphate by laser flash photolysis (LFP) experiments. The results will be discussed within the context of the small molecule chemistry^6a,7 and will help to suggest structure/reactivity relationships for these N-heterocyclic carbene boranes as co-initiators of photopolymerization.

Scheme 1 Investigated compounds.

Experimental

(i) Investigated compounds

The borane complexes investigated are depicted in Scheme 1. NHC-boranes can be easily prepared according to established procedures.⁸ All NHCBs were prepared with analytical purity up to accepted standards for new organic compounds (>98%) which was checked by high field NMR analysis.⁸ The benzophenone model photoinitiator (BP), methyl acrylate (MA), ethyl vinyl ether (EVE), diphenyliodonium hexafluorophosphate (Ph₂I⁺) were obtained from Aldrich. Ethyl dimethylaminobenzoate (EDB; Esacure EDB from Lamberti) was chosen as a reference amine co-initiator. The alkene stabilizers were removed by column purification (Aldrich AL-154) before the LFP experiments. 2,2′-di(ortho-chlorophenyl)-4,4′,5,5′-tetraphenylbiimidazole (o-Cl-HABI) was obtained from Tokyo Chemical Industry (TCI-Europe).

(ii) Laser flash photolysis experiments (LFP)

Nanosecond laser flash photolysis (LFP) experiments were carried out with a Q-switched nanosecond Nd/YAG laser (λ_exc = 355 nm, 9 ns pulses; energy reduced down to 10 mJ, from Powerlite 9010 Continuum) and an analyzing system consisting of a pulsed xenon lamp, a monochromator, a fast photomultiplier and a transient digitizer.⁹ The ketyl radical quantum yield (Φ_K.) in the ³BP/NHC borane reaction was determined by a classical procedure.^9,10 The t-BuO˙ radicals were generated at 355 nm by the direct cleavage of di-tert-butylperoxide as in ref. 6a and 9b.

(iii) ESR spin trapping experiments

This ESR technique (ESR-ST) is powerful for the identification of the radical centers.¹¹ The radicals were generated under the polychromatic light exposure of a Xe–Hg lamp (Hamamatsu, L8252, 150 W) of NHC-boranes in di-tert-butylperoxide/tert-butylbenzene (50/50); 6 mm quarts cylindrical ESR tubes were used and the samples were argon purged 15 min prior to measurements. The irradiation was carried out inside the cavity (TE102) of the spectrometer through a filter to cut off the light below 310 nm. The generated radicals were trapped by phenyl-N-tbutylnitrone (PBN). The ESR spectra simulations were carried out with the WINSIM software.¹²

(iv) DFT calculations

All the calculations were performed by using the hybrid functional B3LYP from the Gaussian 03 suite of program.¹³ Reactants and products were fully optimized at the B3LYP/6-31+G* level (checked for imaginary frequencies). The electronic absorption spectra were calculated with the time-dependent density functional theory at MPW1PW91/6-311++G** level on the relaxed geometry determined at UB3LYP/6-31+G* level.

(v) Photopolymerization experiments

For film polymerization experiments, a given PI was dissolved into a bulk formulation based on ethoxylated pentaerythritol tetraacrylate (EPT from Cray Valey; viscosity = 150 cP).¹⁴ The films (20 μm thick) were deposited on a BaF₂ pellet either under air or in laminate as in^14c and irradiated with a polychromatic light (Xe–Hg lamp, Hamamatsu, L8252, 150 W). The evolution of the double bond content was followed by real-time FTIR spectroscopy (Nexus 870, Nicolet) at RT.¹⁴ In this paper, the R_p quantities refer to the maximum rates of the polymerization reaction and were determined from the maximum of the first derivative of the conversion vs. time curves.

Results and discussion

(1) Co-initiator photopolymerization ability of NHC-borane

The results for the acrylate FRP kinetics of ethoxylated pentaerythritol tetraacrylate (EPT) using benzophenone as the photoinitiator in both the absence and presence of the NHC-borane co-initiators are shown in Fig. 1 and Table 1; o-Cl-HABI was also studied as a PI (Fig. 2).

Fig. 1 (A) Radical photopolymerization ability of various BP/co-initiator couples (1%/1% w/w; in EPT, in laminate; λ > 300 nm): EDB (2); A (3); B (4); C (5); D (6); without co-initiator (1); without BP and B alone (7). (B) Radical photopolymerization ability of various BP/co-initiator couples (1%/1% w/w; in EPT, under air; λ > 300 nm): EDB (2); E (3); F (4); G (5); without co-initiator (1). (C) Radical photopolymerization ability of various BP/co-initiator couples (1%/1% w/w; in EPT, under air; λ > 300 nm): EDB (2); A (3); B (4); C (5); D (6); without co-initiator (1). (D) Radical photopolymerization ability of various BP/Ph₂I⁺/co-initiator initiating systems (1%/1%/1% w/w; in EPT, under air; λ > 300 nm): EDB (2); C (3); D (4); without co-initiator (1).

Table 1 Radical photopolymerization. Polymerization rates of EPT using the BP/NHC-borane photoinitiating system (1%/1%, w/w) under UV light irradiation (Xe–Hg lamp)

	Laminated conditions	Under air
Co–I	R p/[M0] × 100^a	R p/[M0] × 100^a
a R p/[M0] (s^−1) where [M0] is the initial monomer concentration. b Conversion reached at t = 120 s.
—	5.9 (56^b)	0.06 (7^b)
EDB	9.5 (60^b)	0.5 (16^b)
A	0.6 (56^b)	0.3 (20^b)
B	3 (60^b)	0.8 (16^b)
C	1.1 (59^b)	0.5 (22^b)
D	1.5 (58^b)	0.2 (10^b)
E	1.4 (61^b)	1.0 (31^b)
F	1.2 (58^b)	0.5 (16^b)
G	1.4 (61^b)	0.9 (26^b)
H	0.8 (53^b)	0.1 (9^b)

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Fig. 2 (A) Radical photopolymerization ability of various o-Cl-HABI/co-initiator couples (1%/1% w/w; in EPT, in laminate; λ > 300 nm): without co-initiator (1); without o-Cl-HABI and G alone (2); o-Cl-HABI/G (3). (B) Radical photopolymerization ability of various o-Cl-HABI/co-initiator couples (1%/1% w/w; in EPT, under air; λ > 300 nm): without co-initiator (1); without o-Cl-HABI and G alone (2); o-Cl-HABI/G (3).

In laminate (Fig. 1A, black line or line 1), the polymerization using BP alone is not very efficient and leads to an inhibition period of about 5 s. As previously,⁵ this is ascribed to the abstraction of a labile hydrogen from the monomer by the BP triplet state (³BP). In contrast, the addition of the NHC-borane co-initiators (1% w/w) suppresses the inhibition period; the polymerization process starts immediately under light irradiation (Fig. 1A). Because the inhibition period corresponds to the required time for the consumption of the oxygen dissolved in the formulation, this suggests that the NHC-boranes are good candidates to overcome the oxygen inhibition of the FRP. A noticeable structure effect on the polymerization profiles in laminate is found: the co-initiators ability decreases in the order B > C, D, E, F, G > H > A. Even though an inhibition period is avoided with NHC-boranes, the maximum R_ps are usually lower than for BP alone. Only BP/B can be considered as competitive with the BP/EDB reference system.

In contrast, a completely different behavior for the NHC-borane based systems is observed under air (Fig. 1B–C show the different BP/co-initiator couples). Indeed, the R_ps associated with all BP/NHC-borane systems are drastically improved compared to BP alone (where the polymerization does not occur at all). The co-initiator ability decreases in the order E > G > B > C, F > A > D > H and is better for B, E and G (and similar for C and F) than for the EDB. In addition to the decrease of the inhibition period previously observed in laminate, this shows the better ability of these compounds compared to EDB to suppress the oxygen inhibition. The use of o-Cl-HABI as a photoinitiator leads to similar trends (Fig. 2).

Some typical polymerization profiles of EPT upon addition of diphenyliodonium hexafluorophosphate (Ph₂I⁺PF₆⁻) to BP/co-initiator under air are shown in Fig. 1D. The R_ps are strongly increased when using the BP/NHC-borane/Ph₂I⁺PF₆⁻ three-component photoinitiating system (Fig. 1D, lines 3,4) instead of BP/EDB/Ph₂I⁺PF₆⁻ (Fig. 1D, curve 2). This unambiguously shows the high reactivity of the NHC-boranes in combination with Ph₂I⁺PF₆⁻ for a photopolymerization under air.

(2) Rate constants for reactions of boryl radical with small molecules

(a) Kinetic data on the formation of the NHC-boryl radicals. The formation of the NHC-boryl radicals is well evidenced by ESR-spin trapping experiments (Fig. 3). For example, the spin adduct of the boryl radical derived from C is characterized by a_N = 15.4 a_H = 2.1 and a_B = 4.4 G in excellent agreement with previous data on boryl radicals.^3a,15

Fig. 3 ESR spectra observed in spin trapping experiments for the adduct of the NHC-boryl radical derived from C (using PBN) in tert-butylbenzene/di-tert-butylperoxide (the ESR spectrum simulation includes ¹⁰B as mentioned in ref. 15): experimental (1) and simulated (2) spectra.

NHC-boryl radicals can be formed in LFP through the quenching of the tert-butoxyl radical t-BuO˙ (reaction (1)) or the benzophenone triplet state ³BP (reaction (2)); t-BuO˙ is easily obtained by the direct cleavage of di-tert-butylperoxide at 355 nm^6,9b and ³BP is directly observed at 525 nm (intersystem crossing quantum yield: 1.0 ¹⁰). Following the rising time of the NHC-boryls (Fig. 4 and 5) and the ³BP lifetime provides a direct access to the rate constant k_H¹ and k_H², respectively (Table 2). The ketyl radical quantum yields (Φ_K˙) were also measured in the usual way¹⁰ (Φ_K˙s are equal to the boryl radical quantum yields).

t-BuO˙ + NHC–BH₂R → t-BuOH + NHC–B˙HR (k_H¹) (1)

³BP + NHC– BH₂R → BPH˙ + NHC– B˙HR (k_H²) with R = H or Ph (2)

Fig. 4 (A) The kinetics of the formation of the NHC-boryl derived from A at λ = 400 nm in acetonitrile/di-tert-butylperoxide; the AA concentrations are 0; 0.002 and 0.0057 M. (B) The kinetics at 550 nm corresponding to the NHC-boryl derived from D for different CHCl₃ concentrations from: [CHCl₃] = 0 M to [CHCl₃] = 0.084 M.

Fig. 5 (A) Transient absorption spectra in acetonitrile/di-tert-butylperoxide for the formation of the NHC-boryl derived from H (recorded at different times after the laser excitation (from 0.5 μs to 32 μs by steps of 3.5 μs). The rise of the boryl radical occurs within 500 ns. (B) Transient absorption spectrum in acetonitrile/di-tert-butylperoxide at t = 0.5 μs for the formation of the NHC-boryl derived from F (1) and E (2). The rise of the NHC-boryl radical occurs within 500 ns.

Table 2 Rate constants (k¹_H, k²_H) for the formation of the NHC-boryl radicals

	k ^1 H ^c (tBu-O˙) 10^7M^−1s^−1	k ^2 H (^3BP) 10^7M^−1s^−1	BDE (B–H)^b kcal mol^−1
a Ketyl radical quantum yield (ΦK˙) in acetonitrile. b UB3LYP/6-31+G* level. c Acetonitrile/di-tert-butylperoxide.
AA	24	120 (0.7^a)	83.3
BB	11	250 (<0.15^a)	80.7
CC	26	45.5 (1^a)	82.6
DD	17	68.7	84.1
EE	26	96	81.9
FF	14	91.1	80.2
GG	21	100 (1^a)	82.8
HH	26.2		79.0

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The transient absorption spectra for the NHC-boryl derived from A, B, C, E and G exhibit a maximum absorption at about 360 nm while those derived from D, F and H exhibit a visible light absorption at about 550 nm. This shows the influence of the phenyl Ph group on the NHC– B˙HR absorption spectra, which leads to a new red-shifted transition (Fig. 5B). The spectral features predicted by time-dependent density functional theory modeling (TD-DFT) fit quite well this experimental finding. A red shift of 129 nm is calculated between the maximum absorption spectra of the boryl derived from C (376 nm) and D (505 nm), respectively (MPW1PW91/6-311++G** level).

From this work, good to excellent hydrogen donating properties of N-heterocyclic carbene-borane complexes is noted as revealed by the high k_H¹ and k_H² rate constants (>10⁷ M⁻¹s⁻¹). Interestingly, for the phenyl substitution, a slight decrease of k_H¹ is observed i.e. k_H¹(D) < k_H¹(C) and k_H¹(F) < k_H¹(E). This can be ascribed to a steric hindrance for the hydrogen abstraction process. The Φ_K˙s are also strongly affected by the NHC structure (< 0.2 for B to 1 for C). ³BP being quite bulky, the change of Φ_K˙ can probably be partly correlated with the steric hindrance in the NHC borane. ³BP reacts like an alkoxy radical. A similar behavior was recently found for other NHC-boranes.⁵

(b) Structure of the NHC-boryl radicals. The different bond dissociation energy BDE(B–H) evaluated at UB3LYP/6-31+G* level for NHC-boranes are gathered in Table 2. Compared to amine (or phosphine)-boranes, the BDEs are strongly reduced (i.e. for triethylamine-borane, BDE(B–H) = 101 kcal mol^−1 6a). This is in line with the excellent hydrogen donating properties of the NHC-boranes found here. Interestingly, BDEs(B–H) are quite similar for NHC–BH₃ and NHC–BH₂Ph showing a weak influence of the phenyl substitution. This can be rationalized by molecular orbital calculations. In previous work, a planar π-type structure for various NHC-boryl radicals was found.⁴ This was ascribed to a significant spin delocalization from the boron atom into the NHC ligand due to the empty p-orbital on the central carbene carbon atom. This holds true here in A, B, C, E and G. In the NHC–B˙HAr structure (D, F and H), however, the aryl is almost orthogonal to the NHC plane ruling out an important delocalization on the aryl ring (Fig. 6). This behavior is also exemplified by the spin density on the boron atom which is quite similar for NHC–B˙HAr and NHC–B˙H₂ (Table 3).

Fig. 6 Structure of the NHC-boryl radical derived from D (geometry optimized at UB3LYP/6-31+G* level).

Table 3 Interaction rate constants of the NHC-boryl radicals with various additives at RT in acetonitrile/di-tert-butylperoxide. MA for methylacrylate

	k add (R˙/MA) 10^6 M^−1s^−1	k′add (R˙/O2) 10^8 M^−1s^−1	k ox (R˙/Ph2I^+) 10^9 M^−1s^−1	k Cl (R˙/CHCl3) 10^7 M^−1s^−1	k I (R˙/C3H7I) 10^7 M^−1s^−1	Spin B^a
a UB3LYP/6-31+G*. b For the addition to ethyl vinylether, a rate constant < 10^5 M^−1s^−1 is found.
AA	35	>5	1.4			0.57
BB	9.5	>5	1.1			0.51
CC	31^b	>6	0.7	1.3	8.3	0.56
DD	1.9	>6	0.6	0.5	2.3	0.53
EE	38	>6	0.8	0.6	18	0.55
FF	11	>6	0.4	0.15	2.8	0.50
GG	21	>6				0.56
HH	10.5	>6				0.48

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(c) Reactivity of the NHC-boryl radicals. Through the direct observation of the NHC-boryl radicals offered by LFP, important absolute radical/molecule and radical/ion reaction rate constants are accessible. All these kinetic data are compiled in Table 3. Because preliminary experiments showed that the NHC-boryl radicals react very rapidly with oxygen (k_O2 > 5 × 10⁸ M⁻¹s⁻¹, Table 3), all the kinetic experiments were carried out under argon.

(c-1) Addition of NHC–boryl radicals to double bonds. The addition rate constants (k_add) to methyl acrylate (MA) and ethyl vinyl ether (EVE) are compiled in column 2 of Table 3. The addition to an electron deficient alkene (MA) is very fast (>1.9 × 10⁶ M⁻¹s⁻¹). The back reaction can be probably ruled out (or is slow) because no fragmentation (with a concomitant boryl radical re-formation) is observed on a 200 μs time scale. Interestingly, NHC–B˙HPh radicals are characterized by lower k_add than NHC–B˙H₂ (i.e. 31 × 10⁶vs. 1.9 × 10⁶ M⁻¹s⁻¹ for the boryl radicals derived from C and D, respectively). This can be ascribed to steric hindrance by the phenyl group. The addition rate constant to the electron rich alkene (EVE) is lower (< 10⁵ M⁻¹s⁻¹ for the boryl radical derived from C which is one of the most reactive for the addition to MA). This combination of a slow addition to an electron rich alkene (EVE) and a fast addition to an electron poor alkene (MA) suggests a nucleophilic behavior for the boryl radicals investigated here. This is similar to that noted for other recently studied NHC-boryls as well as for classical amine-boryls.⁵

(c-2) Reactivity of NHC-boryl radicals towards alkyl halides. The reaction of boryl radicals with alkyl halides is an important process in small molecule radical chemistry. The rate constants for the reactions of the NHC boryls with an alkylchloride (CHCl₃) (k_Cl) and iodopropane (k_I), gathered in columns 5–6 of Table 3 (Fig. 4B), are quite high (1.5 × 10⁶ to 1.3 × 10⁷ M⁻¹s⁻¹ for the reaction with CHCl₃ and 2.3 × 10⁷ to 8.3 × 10⁷ M⁻¹s⁻¹ for iodopropane). Interestingly, as for the addition process (see above), NHC–B˙HPh radicals have a lower reactivity than NHC–B˙H₂. This can be still ascribed to the crowded character of the NHC–B˙HPh radical center. NHC-boryl radicals exhibit low ionization potentials IP.⁵ Therefore, the charge transfer from the NHC-boryl radical to the alkyl halide might explain in part their high reactivity towards CHCl₃ and iodopropane.

(c-3) Oxidation of NHC-boryl radicals by Ph₂I⁺PF₆⁻. Due to its very low reduction potential (∼−0.2 V), diphenyliodonium hexafluorophosphate is a classical compound used to oxidize radicals.¹⁶ The rate constants for the chemical oxidation of NHC-boryl radicals by Ph₂I⁺PF₆⁻ were found to be quite close to the diffusion limit (k > 4 × 10⁸ M⁻¹s⁻¹, Table 3), in line with the low IPs (previously reported⁵) and the high nucleophilic character (see above) of the NHC-boryls.

(3) The boryl radical reactivity and the polymerization initiation mechanism

The high intrinsic reactivity of the NHC-boryls towards the acrylate unit found above is in agreement with their good polymerization initiating ability. However, the R_ps (Fig. 1–2) do not correlate with the radical reactivity towards MA (k_add), i.e. the boryl radical derived from E which is characterized by the highest k_add to MA does not lead to the highest R_p in laminate. This can be ascribed to the large difference of Φ_K˙ in the NHC-borane series (from <0.15 for B to 1 for G). It was found previously that E exhibits the highest reactivity for radical processes.²

Interestingly, the high reactivity of NHC-boryl radicals with O₂ is in full agreement with the ability of these structures to overcome the oxygen inhibition as previously described in amine-boranes in.^6b They consume oxygen and re-generate new boryls (reactions (3a) and (3b)). The reactivity of ligated-boranes with the peroxyls generated by the addition of initiating or propagating radicals with O₂ (which converts stable peroxyls into new initiating structures, reaction 3c) can also probably be invoked to explain the good ability of NHC-boranes to initiate the polymerization under air.

NHC–B(˙)HR + O₂ → NHC–BOO(˙)HR (3a)

NHC–BOO(˙)HR + NHC–BH₂R → NHC–B(OOH)HR + NHC–B(˙)HR (3b)

ROO˙ + NHC–BH₂R → ROO–H + NHC–B(˙)HR (3c)

For the three-component photoinitiating systems BP/NHC-BH₂R/Ph₂I⁺PF₆⁻, the drastic increase of the polymerization rates compared to BP/NHC-BH₂R systems stems from the high rate constants for the oxidation of the boryl radicals to borenium cations (4).^3b,5 The release of a phenyl radical (Ph˙), which exhibits a higher polymerization initiating property than NHC-boryls, is also expected to increase R_p. Indeed, Ph˙ has a higher k_add (MA) [1.9 × 10⁸ M⁻¹s⁻¹].¹⁷

NHC–B˙HR + Ph₂I⁺ → NHC–B⁺HR + PhI + Ph˙ (4)

Conclusions

NHC-boranes appear to be an excellent class of co-initiators for FRP. A remarkable behavior in the acrylate photopolymerization in aerated conditions is noted i.e.polymerization profiles better than for the reference system are obtained. The characterization of the derived NHC-boryl radicals by LFP has allowed the determination of the key rate constants for the elementary events. The NHC substitution affects the different processes. More interestingly, the phenyl substitution on the radical center is found to decrease both the formation and reaction rate constants of the boryls. The present work allows a better knowledge of the structure/reactivity relationship for the NHC-boryl radicals. The design of optimized co-initiators with better hydrogen donating properties and higher reaction rate constants with monomers, O₂ and ROO˙ should be investigated in forthcoming works to further improve the photopolymerization process in aerated conditions.

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