Time-Resolved Laser Spectroscopy of Synergistic Processes in

Chapter 5

Time-Resolved Laser Spectroscopy of Synergistic Processes in Photoinitiators of Polymerization

Downloaded by UNIV OF ARIZONA on May 6, 2017 | http://pubs.acs.org Publication Date: December 28, 1990 | doi: 10.1021/bk-1990-0417.ch005

Jean-Pierre Fouassier and Daniel-Joseph Lougnot Laboratoire de Photochimie Générale, Unité Associée au Centre National de Recherche Scientifique numéro 431, Ecole Nationale Supérieure de Chimie, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France

The synergic effects which are generally invoked to account for the specific features of a system of two ketones used as polymerization photoinitiators are reconsidered. The increase of reactivity observed when mixing these two initiators is reinterpreted in terms of a simultaneous energy and electron transfer in the pair. The relative efficiencies of these processes depends on the energy gap between the triplet states involved, which is known to be influenced by the polarity of the medium. A general discussion on the efficiency of various couples photoinitiator/photosensitizers is presented.

The wide use of photoinduced radical or cationic polymerization of unsaturated monomers in UV curing technologies [1-3] have stimulated many detailed investigations of the processes involved [4-10]. Time-resolved laser-spectroscopy has been extensively employed [7 and references therein][8][11-20] to study in real time, the dynamics of the excited states of molecules used as photoinitiators. Reaction models accounting for the photoinitiation steps of the polymerization have been proposed which allow relationships between the excited state reactivity and the practical efficiency as photopolymerization initiators to be discussed thoroughly. Most of the photoinitiator families have been investigated during the past decade. In a general way, the main photoprocesses taking place in the excited states after absorption of light are, for the most part, well understood. However, the same does not hold true for the different reactions leading to excitation transfer in combinations of photoinitiators where the enhancement of the reaction efficiency is commonly referred to as "synergism" in the field of UV curing. The major problem encountered in the study of these reactions which take place in mixtures of several molecules exhibiting closely related absorption spectra is the selectivity of the excitation. This problem has been overcome very recently by using the emission of a Nd-YAG laser pumped dye laser [21] as the excitation light. In the present paper, a general discussion on synergistic effects will be presented, using a selection of systems recently studied in our laboratory. 0097-6156/90/0417-0059$06.00/0 o 1990 American Chemical Society Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

RADIATION CURING OF POLYMERIC MATERIALS

60

HOW SYNERGISM CAN BE DESCRIBED ? Under UV-light exposure, a radical photoinitiator is promoted to its first excited singlet state, S i and then, through a fast intersystem crossing, is converted to its lowest triplet state, Τ·]. This latter triplet state can yield radicals through cleavage, electron and proton transfer or Η-abstraction. Extension of the spectral sensitivity of a photoinitiator, I, can be achieved by adding a photosensitizer, S. Such an energy transfer process must be exothermic with the energy level of the excited donor exceeding that of the acceptor by a few Kcal M" (with a difference of 3 Kcal M~ , the energy transfer is almost diffusion controlled) : 1

1


Photosensitizer (T-| ) + Photoinitiator (Srj) Photosensitizer (SQ)

-»

Photoinitiator (T-j ) +

However, the two terms photoinitiator and photosensitizer are often used in a more general sense to define any process involving either energy transfer (to generate from I the same radicals as those obtained through direct excitation) or electron transfer followed by proton transfer (to form new initiating radicals) {Scheme 1). This mechanism is exemplified [22] by the mixture benzophenone 1 and Michlers Ketone 2 in which synergism is accounted for by the formation of a triplet exciplex which dissociates into a benzophenone ketyl radical and an aminobenzophenone radical as : 2[S ] 0

•

•

2[SJ

—^—•

exciplex

Chemical reactions with the radicals formed during the primary processes are also found to be responsible for synergistic effects, e.g. in a combination of 1 and 1 -benzoyl cyclohexanol 3. in aerated medium. A possible explanation [23] is based on the decomposition by 1 of the hydroperoxides arising from the oxidation of the radicals generated through cleavage of S- The weak reactivity of these peroxy radicals compared to alkoxy and hydroxy radicals (Scheme 2) and the concommittant reduction of air inhibition due to the consumption of dissolved oxygen by excited benzophenone can be also invoked to account for the experimental observations.

1

Very recently, it was shown that mixing substituted thioxanthones 1 or S with the morpholino ketone fi extends the photosensitivity towards the near visible part of the spectrum and accelerates the curing of pigmented coatings [24].

Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

FOUASSIER & LOUGNOT

Time-Resolved Laser Spectroscopy

À OD

—I

S„


Excitation Transfer :

- Energy transfer ς

hv

s

»

- Electron and proton transfer hv 5

Λ Λ Λ Λ >

S* 1

!

•

[S**!**] or [ S M * ]

Scheme 1

hv

400 nm - where only the substituted thioxanthone Ζ (R1 = R = R3 = H ; R4 = COO(CH2CH20)3 H) absorbs demonstrates the increase of the polymerization efficiency [25]. Under exposure, the mixture of Ζ + £ leads to a considerable polymerization enthalpy whereas, in the presence of Ζ or fi alone, the exothermic signal remains very small. The same is true when using a laser light at λ = 440 nm [26] for the excitation of a mixture 5 + £. No polymerization occurs in the presence of £ or £. The relative reactivity of £ at 363 nm and £ + £ at 440 nm shows a 35 :1 ratio, thus defining a low quantum efficiency of excitation transfer. 2

Evidence for the α-cleavage of £ either in the absence or in the presence of thioxanthone derivatives has been shown in NMR-CIDNP spectra [24][25] and through GCMS [26], which supports at least, the view of an energy transfer process between £ and Ζ (or i) : excitation with a filtered light at λ > 400 nm leads to the formation of methyl thiobenzaldehyde which proceeds, in non hydrogen donating solvents, from a cage process in the radical pair generated after cleavage. In photopolymerization experiments of methylmethacrylate (MMA) in degassed toluene solution, the increase of the initiation quantum yield j j (φ| « R 2) does not exceed 10-15% [26]. p

TIME-RESOLVED LASER-SPECTROSCOPY OF TRANSFER BETWEEN THIOXANTHONES AND £

EXCITATION

Investigating directly the processes involved between 4 or £ and £ during the exchange of excitation energy requires a pumping pulsed source of light which can excite 4 or £ without exciting £ (Figure 1) : this is readily achieved by using a light beam delivered by a dye laser pumped with a Nd-YAG laser [21]. A - EXCITED STATE PROCESSES IN 5 The triplet state of 5 is long-lived in deaerated solution [26] ; the absorption maximum of the T-T transition is blue shifted upon going from a non-polar to a polar solvent (670 nm

Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

5. FOUASSIER&LOUGNOT

Time-Resolved Laser Spectroscopy

63

in toluene and 625 nm in methanol). The fluorescence intensity increases and is red shifted with increasing solvent polarity : this can be readily understood in terms of a change of the spectroscopic character of the lowest lying excited state (an inversion of the relative position of the ηπ and πκ states) or of a modification of the dipole moment (in the excited singlet and triplet state) upon photoexcitation. Fluorescence and triplet absorption are quenched when methyldiethanolamine (MDEA), ten- butylmorpholine (TBM) and thioanisole (TA) are added to a solution of 5 and ketyl species are formed. The data are listed in Table 1. Scheme 3 summarizes the main photoprocesses occuring in the excited states of &.


Β - PRIMARY PROCESSES IN 6 In the case of 6, a fast cleavage occurs from a very short lived triplet state {

Time-Resolved Laser Spectroscopy of Synergistic Processes in

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