On the Use of Bis(cyclopentadienyl)titanium(IV) Dichloride in Visible

Article pubs.acs.org/Macromolecules

On the Use of Bis(cyclopentadienyl)titanium(IV) Dichloride in VisibleLight-Induced Ring-Opening Photopolymerization Mohamad-Ali Tehfe,† Jacques Lalevée,†,* Fabrice Morlet-Savary,†,* Bernadette Graff,† and Jean-Pierre Fouassier‡ †

Institut de Science des Matériaux de Mulhouse, LRC CNRS 7228, ENSCMu-UHA 15, rue Jean Starcky, 68057 Mulhouse Cedex, France ‡ University of Haute Alsace, 68057 Mulhouse Cedex, France ABSTRACT: A titanocene derivative (bis(cyclopentadienyl)titanium(IV) dichloride Cp2TiCl2), for visible light induced polymerization of cationic resins working through a free radical promoted process is presented. This new photoinitiating system is based on the Cp2TiCl2 free radical initiator, a silane (tris(trimethylsilyl)silane: TTMSS), and an onium salt (diphenyliodonium hexafluorophosphate: Ph2I+) and appears suitable under long wavelength irradiation (λ > 400 nm). The polymerization reaction is highly efficient in aerated conditions (almost 100% conversion) and dramatically better than in laminate. The system works under exposure to a Xe lamp, a diode laser or a household LED bulb. The mechanisms are investigated by ESR experiments. The specific role of the generated free radicals is highlighted.

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R• + Ph2I+ → R+ + Ph• + PhI

INTRODUCTION Bis(cyclopentadienyl)titanium(IV) dichloride (Cp2TiC12) (Scheme 1) was prepared in 1954 by Wilkinson and Birmingham.1 This titanium-based compound is one of the simplest, cheapest, and most commercially available transition metals; this product is also nontoxic and environmentally friendly. This latter property strongly contrasts with the high toxicity of many transition metals2a−e and has allowed the use of Cp2TiC12 in different medical applications,2f,g removal of toxic metals2h and prostheses.2i The present paper aims at introducing Cp2TiCl2 in the photopolymerization area where it could be used for applications requiring safe conditions. Its performance will be compared to that of the known titanocene derivative bis(cyclopentadienyl)-bis[2,6-difluoro-3-(1-pyrryl)phenyl]titanium (Irgacure 784). Ring-opening photopolymerization is actually well-known in radiation curing.3 However, a known drawback is related to the lack of absorption of cationic photoinitiators in the emission spectral range of classical UV lamps (λ > 300 nm). Photosensitization of these salts has long been recognized4 and some recent works have shown an important progress for the development of photoinitiating systems that are able to efficiently start the reaction under visible lights (refs 5 and 6; see also a recent review in ref 3h). An elegant sensitization process is the free radical promoted cationic photopolymerization (FRPCP):4a−f the basic idea is to produce a radical (from a photoinitiating system) which in turn should be oxidized by the iodonium salt 1. In this process, the resulting cation (R+) is considered as the polymerization initiating structure.4a−h,5a,b,d,e,7−10 A suitable selection of the radical source obviously allows to tune the absorption in the near UV or/and visible wavelength range (e.g., using benzoin ethers, phosphine oxides, acridine diones, thiopyrylium salts, metallocenes, ...).11 © 2011 American Chemical Society

(1)

Another drawback in FRPCP remains the oxygen sensitivity5d,e,6a,b,12,13 of the system as the addition of radicals to oxygen generating hardly oxidable peroxyls is usually highly efficient, i.e., the rate constant for this reaction is usually close to the diffusion limit.14 FRPCP is usually rather inhibited in aerated conditions.5a,e,6a,12 Recently, it has been reported that the incorporation of silyl radicals in new photoinitiating systems can be highly worthwhile to overcome this oxygen inhibition.6 This prompted us to investigate the efficiency of the Cp2TiCl2/ silane (TTMSS)/iodonium salt (Ph2I+) photoinitiating system in the FRPCP of an epoxide monomer (EPOX) upon long wavelength excitation (under visible LED bulbs, xenon lamp and laser diodes). The overall mechanism will be discussed using ESR and ESR spin trapping experiments (ESR-ST).

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EXPERIMENTAL SECTION

The compounds investigated here are presented in Scheme 1 and used with the best purity available. Bis(cyclopentadienyl)titanium(IV) dichloride (Cp2TiCl2), tris(trimethylsilyl)silane (TTMSS) and diphenyl iodonium hexafluorophosphate (Ph2I+) were obtained from Aldrich. The monomer (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX or UVACURE 1500) was obtained from Cytec. i. Free Radical Promoted Cationic Polymerization Processes. The two- and three-component photoinitiating systems are based on Cp2TiCl2/Ph2I+ (1%/2% w/w) and Cp2TiCl2/TTMSS/ Ph2I+ (1%/3%/2% w/w). The experimental conditions are given in the figure captions. As in ref 6a, the laminated or aerated films (25 μm thick) deposited on a BaF2 pellet were irradiated with the polychromatic light (incident Received: October 14, 2011 Revised: November 17, 2011 Published: December 9, 2011 356

dx.doi.org/10.1021/ma202307x | Macromolecules 2012, 45, 356−361

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Scheme 1. Investigated Compounds

light intensity: I0 ≈ 60 mW cm−2; 400 nm < λ < 800 nm) of a xenon lamp (Hamamatsu, L8253, 150 W). To ensure a visible light irradiation, a cut off filter has been used to select λ > 400 nm. The evolution of the epoxy group content is continuously followed by real time FTIR spectroscopy (Nexus 870, Nicolet) as reported in.6a,15 The absorbance of the epoxy group was monitored at about 790 cm−1. The Si−H conversion for TTMSS is followed at about 2050 cm−1. Some experiments using a diode laser (473 nm; MGL-III-473-Bfioptilas; I0 ∼ 100 mW/cm2) and a blue LED bulb (I0 = 15 mW/cm2 at a distance of 4 cm) irradiation are also presented. ii. ESR and ESR Spin Trapping (ESR-ST) Experiments. The ESR experiments were carried out here using a X-Band spectrometer (MS 200 from Magnettech-Berlin, Berlin, Germany) at room temperature. The radicals were generated through photolysis (xenon lamp) under argon or under air in an inert solvent (tert-butylbenzene). In ESR-ST, the generated radicals were trapped by phenyl-Ntbutylnitrone (PBN); this procedure is also described in detail in ref 16: R• + PhCHN+(tBu)O− → PhCHR − N(tBu)O•

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RESULTS AND DISCUSSION 1. Photopolymerization Experiments. Because of its absorption spectrum and molar extinction coefficients (Figure

Figure 2. Polymerization profiles of (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX) upon a Xe lamp irradiation (λ > 390 nm) in the presence of bis(cyclopentadienyl)titanium(IV) dichloride (Cp2TiCl2)/tris(trimethylsilyl)silane (TTMSS)/diphenyl iodonium hexafluorophosphate (Ph2I+) (1%/3%/ 2% w/w) photoinitiating system. Inset: Conversion of the Si−H functions, in laminate (1) and under air (2).

390 nm) in the presence of two photoinitiating systems under air: (1) bis(cyclopentadienyl)titanium(IV) dichloride (Cp2TiCl2)/diphenyl iodonium hexafluorophosphate (Ph2I+) (1%/2% w/w); (2) bis(cyclopentadienyl)titanium(IV) dichloride (Cp2TiCl2)/tris(trimethylsilyl)silane (TTMSS)/ diphenyl iodonium hexafluorophosphate (Ph2I+) (1%/3%/ 2% w/w). Inset: conversion of the Si−H content in case 2.

observed that this radical is clearly centered on the titanium (spin density =1.09; spin density for the other atoms 55% is reached within 180 min of irradiation, and a completely tack free coating is formed after 20 h of irradiation. The formed polyether network is easily characterized by its absorption band at 1080 cm−1. For this new proposed system, a complete conversion can be achieved (>95%), this was not the case for a previous system based on a bis(cyclopentadienyl)-bis[2,6-difluoro-3-(1-pyrryl)phenyl]titanium with about 60% of final conversion.6c 2. Chemical Mechanisms. The mechanism for the photolysis of Cp2TiCl2 has been investigated in few works. Photoproduct analysis have been reported in.17 The cyclopentadienyl derivatives of a variety of transition metals have been shown to undergo a homolysis of the Cp-metal bond under visible lights:17 this reaction has been exploited as a source of substituted cyclopentadienyl radicals (Cp•) and/or metal-centered radicals in ESR studies.17e−h As expected from,17b different free radicals are observed here by ESR spectroscopy during the photolysis of Cp2TiCl2 under argon (Figures 5A and 5C): two titanium radicals (singlet spectra at g ∼ 1.97 and g ∼ 1.98) and Cp• (Figure 5A). For ESR-spin trapping experiments, the PBN spin adduct of Cp• is characterized by aH = 3.1 G; aN = 14.6 G (Figure 5C). The main titanium radical is CpTi•Cl2 in agreement with Scheme 2. The second Ti• species was not clearly ascribed.17b CpTi•Cl2 is a rather long-lived species (>10 s under argon). From the calculated singly occuped molecular orbital (SOMO), it can be 358

Cp• + (TMS)3 Si − H → Cp − H + (TMS)3 Si•

(2)

(TMS)3 Si• + Ph2I+ → (TMS)3 Si+ + Ph• + PhI

(3)


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Figure 5. (A) ESR spectra obtained during irradiation (under argon) of bis(cyclopentadienyl)titanium(IV) dichloride (Cp2TiCl2) in tertbutylbenzene under visible light (irradiation with a Xenon Lamp; λ > 400 nm). (B) ESR spectra obtained during irradiation (under air) of bis(cyclopentadienyl)titanium(IV) dichloride (Cp2TiCl2) in tert-butylbenzene under visible light (irradiation with a Xe Lamp; λ > 400 nm). (C) ESR Spin Trapping spectrum obtained after irradiation of bis(cyclopentadienyl)titanium(IV) dichloride (Cp2TiCl2) in tert-butylbenzene under visible light (under argon) (Xe Lamp exposure; PBN 0.05 M; λ > 400 nm). See text.

compared to laminated conditions. The fast peroxidation reaction avoids the recombination processes (which are much slower for peroxyl radicals)19 and increases the global free radical yield: this is in agreement with the better behavior observed when using the three-component system in photopolymerization in aerated conditions.

Scheme 2

Cp•, CpTi•Cl2, Ph• + O2 → CpOO•, CpTiCl2OO•, PhOO•

(6)

CpOO•, CpTiCl2OO•, PhOO• + (TMS)3 Si−H → CpOOH, PhOOH, CpTiCl2OOH

Figure 6. SOMO for CpTi•Cl2 for two different views (UB3LYP/ LANL2DZ geometry optimization).

(TMS)3 Si+ + M → polymer

(4)

Ph• + (TMS)3 Si − H → Ph − H + (TMS)3 Si•

(5)

+ (TMS)3 Si•

(7)

The specific behavior of the silyl radicals under air has been investigated in different works:6a,12 these structures can efficiently generate siliylium cations without oxygen inhibition. A complete reaction scheme was given in ref 11a. The formation of free radicals in Scheme 2 is also in full agreement with the photoinitiator ability of Cp2TiCl2 in free radical polymerization FRP of trimethylolpropane triacrylate TMPTA (Figure 7). Cp• and CpTi•Cl2 can be considered as interesting potential initiating radicals in FRP.

In aerated media, the different radicals generated (Cp•, CpTi•Cl2, and Ph•) are easily converted into peroxyl radicals (see also above for the CpTi•Cl2/O2 interaction) (6) and then into silyl radicals by hydrogen abstraction with TTMSS (7): this is a general behavior of the peroxyl/silane couple.18 These reactions explain the higher Si−H conversion in aerated media 359


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Figure 7. Conversion vs time curves for the photopolymerization of trimethylolpropane triacrylate (TMPTA) in laminate) using Cp2TiCl2 (1% w/w). Sample thickness = 20 μm; Xe lamp (λ > 390 nm) irradiation.

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CONCLUSION Compared to previous systems proposed for FRPCP, the proposed Cp2TiCl2/TTMSS/diphenyl iodonium salt system exhibits an excellent performance in aerated media and a remarkable final conversion close to 100%. This compound is also nontoxic and can be used for medical applications. Other monomers (limonene dioxide, epoxidized soybean oil) can also be polymerized by the presented approach. It also opens up a way to carry out cationic photopolymerization under air and upon visible wavelengths, e.g., for laser imaging area or formation of holographic optical elements (in this last case, the lower shrinkage of the cationically cured polymers could be an advantage compared to free radical polymerization reactions).

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AUTHOR INFORMATION

Corresponding Author *E-mail: (J.L.) [email protected]; (F.M.-S.) [email protected].

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ACKNOWLEDGMENTS J.L. thanks the Institut Universitaire de France; all the authors thank the French Agency for Research ANR (ANR-BLAN0802; ANR SILICIUM).

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On the Use of Bis(cyclopentadienyl)titanium(IV) Dichloride in Visible

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