The 1,3-bis(dicyanomethylidene)indane skeleton as a (photo) initiator in thermal ring opening polymerization at RT and radical or cationic photopolymerization

Abstract

1,3-Bis(dicyanomethylidene)indane is presented as a new initiator for ring opening polymerization of epoxides at RT. This compound behaves as a strong acid (AH) with an associated basic form (A⁻) that does not inhibit the propagation of the cationic polymerization. Remarkably, A⁻ is characterized by a strong visible light absorption and can also photosensitize iodonium salt decomposition. A new iodonium salt based on A⁻ as a counter-anion is proposed. This latter compound exhibits unusual properties: (i) excellent absorption in the 300–700 nm wavelength range and (ii) a free radical initiating ability for λ > 300 nm. The chemical mechanisms are investigated by ESR, fluorescence and steady state photolysis experiments.

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Introduction

The design of photoinitiators (PIs) and photoinitiating systems (PISs) has been the subject of huge efforts; a recent review of available systems is given in ref. 1 and many examples for light induced radical and/or cationic polymerizations can also be found in ref. 2. In the present paper, we explore the design of a molecule playing the role of a strong acid generator AH and possessing a highly delocalized and stabilized anion structure A⁻. This outstanding compound should be able to (i) initiate a ring opening polymerization ROP at RT (the search for initiating systems for such ROP remains the subject of intense works), (ii) react under visible lights (through the excitation of A⁻) with an iodonium salt (I⁺) to form a radical species for free radical polymerization FRP and subsequently a cationic species (through reactions with the medium) for ROP and (iii) allow the synthesis of a long wavelength absorbing A⁻–I⁺ ion-pair (this ion-pair should start a radical photopolymerization). Such an approach might be interesting in two directions. First, a neutral mono-component system AH is useful in the field of thermal ROP using low activation energy initiating systems. Indeed, in this area, the redox chemistry has already led to successful results employing e.g. the copper-catalyzed reduction of diaryliodonium salts with benzoins (for the cationic polymerization of cyclic ethers and esters³) or the Pt catalyzed decomposition of the silane (or borane)/iodonium salt couple (for the polymerization of epoxides^4–7). Two possible drawbacks, however, have been already mentioned: (i) redox initiators are usually applied as two-part systems: the oxidizing and reducing agents are stored separately and then combined at the time of use and (ii) the synthesized polymers incorporate residual metals and potentially extractible organic compounds. Secondly, there is a place for new proposals of photoinitiators based on ion-pairs (that can be isolated and should not be confused with usual two-component systems) even if few examples are known (e.g. for cationic dye-borate salt, dye-iodonium salt, dye-ferrocenium salt, positively charged polymeric chain containing thioxanthonemethyl carboxylate⁸).

The novel PIs proposed here are 1,3-bis(dicyanomethylidene)indane and 1-(dicyanomethylene)-3-indanone (CN_1 and CN_2 in Scheme 1). The presence of the dicyanomethylidene electron acceptor groups are expected to weakness the C–H bond and increase the electron delocalization of the anion. 2,2′-(2-(Anthracen-9-ylmethylene)-1H-indene-1,3(2H)-diylidene)dimalononitrile (CN_3) is considered as a reference compound where no labile hydrogen exits. CN_1 and CN_2 will be used as mono-component initiators (AH) for the ROP of an epoxide a RT. The cationic polymerization using the A⁻–iodonium salt combination will also be checked. Finally, a new A⁻–iodonium salt ion-pair will be proposed for the free radical polymerization of an acrylate under visible and red lights. The chemical mechanisms are investigated by ESR, steady state photolysis, cyclic voltammetry and fluorescence experiments.

Scheme 1

Experimental section

i Synthesis of the compounds

All reagents and solvents were purchased from Aldrich or Alfa Aesar and used as received without further purification. Anthracene-9-carbaldehyde and 1,3-indanedione were used as supplied. Mass spectroscopy was performed by the Spectropole of Aix-Marseille University. ESI mass spectral analyses were recorded with a 3200 QTRAP (Applied Biosystems SCIEX) mass spectrometer. The HRMS mass spectral analysis was performed with a QStar Elite (Applied Biosystems SCIEX) mass spectrometer. Elemental analyses were recorded with a Thermo Finnigan EA 1112 elemental analysis apparatus driven by the Eager 300 software. ¹H and ¹³C NMR spectra were determined at room temperature in 5 mm o.d. tubes on a Bruker Avance 400 spectrometer of the Spectropole: ¹H (400 MHz) and ¹³C (100 MHz). The ¹H chemical shifts were referenced to the solvent peak DMSO-d₆ (2.49 ppm), CDCl₃ (7.26 ppm) and the ¹³C chemical shifts were referenced to the solvent peak DMSO-d₆ (39.5 ppm), CDCl₃ (77 ppm). 1-(Dicyanomethylene)-3-indanone CN_2 and 1,3-bis(dicyanomethylidene)indane CN_1 were synthesized as previously reported.^9,10 CN_3 was synthesized by a Knoevenagel condensation described in detail in ESI.†

ii Other chemical compounds

The (3,4-epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate (EPOX or UVACURE 1500) and Ebecryl 605 (E605) were provided by Cytec. Ebecryl 605 is a representative acrylate matrix: a bisphenol-A epoxy diacrylate oligomer diluted with 25% of tripropyleneglycol diacrylate monomer. (Epoxycyclohexylethyl)methylsiloxane-dimethylsiloxane copolymer (EPOX-Si) was provided by Bluestar Silicones – France (Silcolease UV POLY 200). Epoxidized soybean oil was obtained from Arkema (ESO) – France. These monomers are well known in the photopolymerization field and represent excellent structures to evaluate the initiating ability of new systems (Scheme 2). For the photopolymerization processes of these multifunctional monomers, the formation of polymer networks is expected preventing a full conversion. The diphenyliodonium hexafluorophosphate (Iod) was used as the iodonium salt was obtained from Aldrich.

Scheme 2

iii Photopolymerization procedures

Ebecryl 605 (E605) was irradiated in laminate (the 20–25 μm thick formulation is sandwiched between two polypropylene films). The evolution of the acrylate content is continuously followed by real time FTIR spectroscopy (FTIR NEXUS 870) at ∼1640 cm⁻¹. The formulations based on the cationic monomer (EPOX) were deposited on a BaF₂ pellet (25 μm thick) and irradiated (if necessary) under air inside the IR spectrometer cavity.¹¹ The evolution of the epoxy content is continuously followed by real time FTIR spectroscopy (FTIR NEXUS 870). A Xe–Hg lamp (Hamamatsu, L8252, 150 W, filtered for λ > 340 nm) and a laser diode (λ = 635 nm) were used as irradiation sources.

iv Redox potentials

The redox potentials were measured in acetonitrile by cyclic voltammetry with tetrabutylammonium hexafluorophosphate 0.1 M as a supporting electrolyte (Voltalab 06-Radiometer; the working electrode was a platinum disk and the reference a saturated calomel electrode-SCE). Ferrocene was used as a standard and the potentials determined from the half peak potential were referred to the reversible formal potential of this compound. The free energy change ΔG_et for an electron transfer reaction is calculated from the classical Rehm–Weller equation (eqn (1))¹² where E_ox, E_red, E_S and C are the oxidation potential of the donor, the reduction potential of the acceptor, the excited state energy and the coulombic term for the initially formed ion pair, respectively. C is neglected as usually done in polar solvents.¹³

ΔG_et = E_ox − E_red − E_S + C (1)

v ESR experiments

ESR spin-trapping (ESR-ST) experiments were carried out using a X-Band ESR spectrometer (EMX-plus from Bruker Biospin or MS400 from Magnettech). The radicals were produced at RT under a Xe–Hg lamp exposure and trapped by phenyl-N-t-butylnitrone (PBN) according to a procedure described in detail in ref. 14.

vi Fluorescence experiments

The fluorescence properties of the different compounds were studied using a JASCO FP-750 spectrometer.

vii Computational procedure

Molecular orbital calculations were done with the Gaussian 03 suite of programs.¹⁵ The electronic absorption spectra for the different compounds were calculated with the time-dependent density functional theory (B3LYP/6-31G* level) on the relaxed geometries calculated at UB3LYP/6-31G* level (the geometries were frequency checked).

Results and discussion

i CN_1 as a thermal initiator of cationic polymerization

a Polymerization at RT. Interestingly, using CN_1 alone in EPOX led to a fast curing at RT. A final conversion of ∼45% is obtained (Fig. 1A): the formation of the polyether network (as characterized by its absorption band at 1080 cm⁻¹ i.e. Fig. 1B) is associated with the consumption of the epoxy function followed at ∼790 cm⁻¹. The addition of Iod (1% w/w) to CN_1 leads to a similar polymerization profile. The polymerization of thick samples (from 1 mm to 1 cm) of EPOX can also be carried out in 12 h (tack free samples were obtained). CN_1 is also able to initiate efficiently the polymerization of EPOX-Si and ESO at RT. On the opposite, CN_3 as expected but also CN_2 are very stable in EPOX, EPOX-Si or ESO and didn't lead to any polymerization process.

Fig. 1 (A) Polymerization profile of EPOX at RT using CN_1 (2% w/w); (B) IR spectra recorded in the course of the polymerization process.

b Chemical mechanisms. For a better understanding of the ROP initiating ability of CN_1, a NMR investigation was carried out. Remarkably, it is found that CN_1 (also noted A–H in Scheme 3) is a strong acid. Indeed, only the basic form (also noted A⁻ in Scheme 3) is observed in protic solvents as evidenced by ¹H NMR spectroscopy (Fig. 2). The acidity of A–H is issued from the presence of two electron-withdrawing groups at the α-position of the CH₂ group, thereby stabilizing the anionic form. In ¹H NMR spectroscopy, only a one-proton signal is found at 5.71 ppm and assigned to the CH⁻ group of the CN_1 anion. The broad 3H-signal located at 6.35 ppm also confirms the presence of the anionic form of CN_1 by the detection of H–H or H–D exchange with water molecules. In the ¹³C spectrum, the resonance at 50.3 ppm, which is specific to the carbon attached to the acidic proton, supports the proposed structure. These NMR observations are fully confirmed by acidity measurements i.e. for a solution of [CN_1] = 4 × 10⁻³ M, a pH of 2.5 (±0.1) has been determined in full agreement with the expected pH of a strong acid (pH = −log[AH] = 2.4). This behavior is also underlined by the respective UV-visible absorption spectra of A–H and A⁻. In non-polar and aprotic media (toluene or tert-butylbenzene), only A–H exists and this compound exhibits a UV absorption (λ_max = 350 nm; ε_{350 nm} ∼ 30 000 M⁻¹ cm⁻¹; Fig. 3a). In protic media (acetonitrile–water), the solution of CN_1 turns deep blue i.e. only A⁻ is observed and characterized by a strong absorption in the visible range (λ_max = 580 nm; Fig. 3b). Such a behavior was reported recently in ref. 16.

Scheme 3

Fig. 2 (A) ¹H and (B) ¹³C NMR spectra of CN_1 anion in DMSO-d₆.

Fig. 3 UV-visible absorption spectra associated with CN_1 (2 × 10⁻⁵ M): (a) in tert-butylbenzene and (b) in acetonitrile–water (0.5 M of water).

The released proton is able to initiate a ROP according to reactions (2)–(4) where A–H and M stands for CN_1 and EPOX, respectively.

A–H → A⁻ + H⁺ (2)

H⁺ + M → H–M⁺ (3)

H–M⁺ + M → polyether (4)

The presence of two dicyanovinyl functions strongly stabilizes the negative charge in A⁻ leading to a favorable process of proton release. Indeed, in molecular orbital calculations, the low negative charge for the carbon at the 2-position (−0.12 e) supports a strong delocalization of the negative charge. The highly stabilized A⁻ structure makes the anion probably very soft and, as a consequence, the propagation of the epoxy polymerization is not inhibited. In CN_2, the carbonyl group is a less favorable electron withdrawing group than the dicyanovinyl, leading to a reduced stabilization of the basic form i.e. the negative charge is −0.19e and the proton release is less favorable. In Fig. 4, the highest occupied molecular orbital of the anion (A⁻) is more delocalized in CN_1 than in CN_2 in agreement with its more stabilized character. In CN_3, used here as a reference, no protons are present in the 2-position and accordingly, this structure is not able to release a proton. These results explain why CN_2 and CN_3 are not able to initiate a ROP reaction.

Fig. 4 Highest Occupied Molecular Orbitals (HOMO) of the anions (A⁻) associated with CN_1 and CN_2. The carbon 2 is indicated by an arrow.

ii The light activated process

Upon irradiation of CN_1/Iod at 532 nm, the EPOX polymerization profile is better than that obtained with CN_1 in the absence of light (Fig. 5 curve 1 vs. curve 2) which suggests that the anion A⁻ is able to photosensitize the iodonium salt decomposition (see below). No polymerization is observed for Iod alone at 532 nm; Iod does not absorb light at this latter wavelength. In this experiment, the formulation is prepared and immediately exposed to the light. The film rapidly turns blue, indicating the presence of A⁻ (at 532 nm, A⁻ leads to a significant light absorption; see Fig. 3). Therefore, both a thermal and a photochemical initiation simultaneously occur. The coating is tack free for t > 60 min.

Fig. 5 (A) Polymerization profiles of EPOX using different initiating systems: (1) CN_1 (2% w/w) at RT without light; (2) CN_1/Iod (2%/2% w/w) upon a 532 nm laser diode irradiation. (B) IR spectra recorded in the course of the polymerization process for (2).

The free energy change ΔG calculated for the ¹A⁻/Iod interaction is highly favourable (−1.07 eV; from eqn (1) using E_red (Iod) = −0.2 V;¹⁷ E_ox (A⁻) = 0.7 V (from this work) and E_S = 1.97 eV as derived from the UV-visible absorption and fluorescence spectra of A⁻ in acetonitrile – maximum emission wavelength at 628 nm). The A⁻/Iod interaction occurs according to reactions (5)–(7) as suported by the Ph˙/PBN radical adduct observed in ESR-ST upon irradiation of a A⁻/Iod solution (the hyperfine coupling constants are a_N = 14.2 G and a_H = 2.2 G in agreement with reference data^14,18). Phenyl radicals being excellent structures for hydrogen abstraction reactions, (8) is plausible and leads to initiating cations for the ROP process through (9). The presence of these additional cations explains the better polymerization profile obtained upon light irradiation (Fig. 5A curve 1 vs. curve 2).

A⁻ → ¹A⁻ (hν) (5)

¹A⁻ + Ph₂I⁺ → A˙ + Ph₂I˙ (6)

Ph₂I˙ → Ph˙ + Ph–I (7)

Ph˙ + EPOX → Ph–H + EPOX˙_(–H) (8)

EPOX˙_(–H) + Ph₂I⁺ → EPOX⁺_(–H) + Ph₂I˙ (9)

iii A new iodonium salt (NIS) based on an original counter-anion

The counter-anion (A⁻) being very soft (i.e. it does not inhibit the propagation of ring opening polymerization; see above) and being able to photosensitize the iodonium salt decomposition, a new iodonium salt (NIS) where PF₆⁻ is changed for A⁻ has been prepared (Scheme 4). The synthesis of NIS is based on a very classical cation exchange (as already proposed for other iodonium salt synthesis¹⁹) and presented in ESI.†

Scheme 4 The new proposed iodonium salt (NIS).

Remarkably, NIS exhibits excellent light absorption properties i.e. this deep blue compound has a strong visible absorption (centered at 580 nm – Fig. 6) due to the presence of A⁻. Compared to Iod, NIS presents a much better absorption in the UV or visible wavelength range (Fig. 6 curve 1 vs. curve 2). In Fig. 6, an important photolysis of NIS is noted upon a visible light exposure; the presence of several isobestic points (262; 322; 454 and 660 nm) indicates that no other side reactions occur. NIS works through a very fast intra ion-pair electron transfer (10) leading to the generation of a phenyl radical: indeed, the Ph˙/PBN radical adduct (a_N = 14.2 G and a_H = 2.2 G) is easily observed (see Fig. 7).

NIS → A˙ + Ph˙ + Ph–I (10)

Fig. 6 UV-visible absorption spectra of (1) Iod and (2) NIS in acetonitrile. Insert: photolysis of NIS in acetonitrile upon a laser diode 635 nm exposure from t = 0 to 20 min.

Fig. 7 ESR spectra obtained upon irradiation of NIS in tert-butyl-benzene (under N₂). Phenyl-N-t-butylnitrone (PBN) is used as a spin trap: (a) experimental and (b) simulated ESR spectra (A˙ being a highly delocalized radical, the addition onto PBN is probably not favourable).

As shown in Fig. 8, NIS behaves as an excellent photoinitiator for the epoxyacrylate polymerization upon a UV-visible light exposure (final conversion ∼60%; Fig. 8A curve 1 vs. 2; when Iod is used as a photoinitiator in such conditions, no polymerization obviously occurs). The IR spectra recorded during such a photopolymerization (Fig. 8B) shows the consumption of the acrylate functions at 1640 cm⁻¹. The film gradually fades in agreement with the bleaching of NIS upon light exposure (Fig. 6). A tack free coating is obtained for t > 200 s. The initiating radicals are the Ph˙ generated in (10); A˙ being a highly delocalized radical does not participate in the initiation process.

Fig. 8 (A) Radical photopolymerization profiles of E 605 in laminate: (1) without NIS and (2) in the presence of NIS (1% w/w) upon UV-visible light (λ > 330 nm; Xe–Hg lamp); (3) in the presence of 2,2-dimethoxy-2-phenylacetophenone (1% w/w); (B) IR spectra recorded during t = 0 at 200 s for (2).

The initiating ability of NIS upon UV-visible light (Xe–Hg lamp) remains lower than that of a well known Type I photoinitiator (2,2-dimethoxy-2-phenylacetophenone – DMPA) (Fig. 8A curve 3) but the final conversion is very similar (60%). However, in contrast to DMPA which is not active under visible lights, NIS can still be used with 450–650 nm lights (Fig. 6). NIS can also operate upon a 635 nm laser diode irradiation. Compared to the methylene blue/methyldiethanolamine (MB/MDEA) two-component photoinitiating system that is a well known reference for polymerization under red lights,¹ NIS exhibit a slightly lower reactivity (30% of E605 conversion with NIS vs. 45% with MB/MDEA after 600 s).

For NIS in EPOX, no polymerization occurs upon light irradiation i.e. reaction (10) does not generate initiating cations for ROP.

Conclusions

In this paper, 1,3-bis(dicyanomethylidene)indane appears as a new efficient one-component and metal-free initiator for the ring opening polymerization of epoxides at RT. This structure can also be used as a chromophore for the design of a new iodonium salt that is (i) shelf stable, (ii) easily soluble in acrylates, (iii) characterized by unprecedented visible light absorption properties, (iv) more conveniently handled compared to the multi-component combinations and (v) able to initiate a radical polymerization under light up to 635 nm. This concept of a deblockable acid owing to the protic character of the medium and the synthesis of an intra ion-pair compound involving an iodonium moiety and a chromophoric counter anion open the way for further researches. A possible thermal/photochemical controlled dual cure process based on such systems might also be explored in the future for the simultaneous curing of shadow/illuminated areas. The design of other initiating organic systems for ROP at RT as well as new onium salts possessing counter-anions with excellent blue-to-red light absorptions will be developed in forthcoming papers.

Acknowledgements

JL thanks the Institut Universitaire de France for the financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra42819b

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