Photochemistry and photopolymerisation activity of novel aromatic carbonyl photoinitiators.
The photochemistry and photopolymerisation activity of eight novel aromatic carbonyl
photoinitiators have been examined. Five of these were t-butyl perester derivatives, three
of 9-fluorenone and these were compared with fluorenone itself and the other two of
benzophenone and this was used for comparison also. The remaining three compounds
incorporated acetophenone type chromophores bound onto a polysiloxane backbone and
these were compared to diethoxyacetophenone. The photochemistry was examined using UV/Vis absorption spectroscopy, luminescence, and flash photolysis, both conventional
microsecond and laser nanosecond. The photopolymerisation activity was studied using
different model monomers and concentration levels and analysis of the cure rate was
measured using a gravimetric method. Investigation of the resulting polymers were
undertaken using gel permeation chromatography, UV absorption spectroscopy and,
where appropriate, fluorescence for end group analysis. For the t-butylperester
derivatives differential photocalorimetry, real time FfIR and pendulum hardness testing
Spectroscopic and fluorescence data showed that the perester substitutions on the
fluorenone chromophore had an electron withdrawing effect and increased the charge
transfer content of the lowest excited singlet state. Absorption and phosphorescence
spectra for the benzophenone derivatives compared to benzophenone itself were similar
although the lower quantum yields and longer life times were indicative of the presence of
an increase in charge transfer content of the lowest excited triplet state with increasing
substitution. For the polysiloxane compounds the extinction coefficients and
phosphorescent quantum yields tended to increase with increasing molecular weight.
Photoreduction quantum yields were higher for the substituted fluorenones than
fluorenone itself, that for the di substituted being twice that of the mono suggesting
homolysis of the peroxyester groups to yield reactive benzoyloxy and t-butoxy radicals.
For the benzophenone derivatives, the benzophenone had much higher values than the
peresters indicating that benzophenone undergoes hydrogen abstraction more efficiently
whereas the derivatives again undergo photoc1eavage about the peroxyester groups.
Conventional microsecond flash photolysis showed that all the t-butylperester initiators
were capable of forming a ketyl radical via hydrogen abstraction. For the benzophenone
compounds this formation follows the order benzophenone>mono->tetraperester and is
consistent with the phosphorescence measurements and other data. For the polysiloxanes
conventional flash data indicated the formation of benzoyl radicals via photofragmentation
and again the intensity of the transient absorption was proportional to the molecular
weight of the photoinitiator. On nanosecond laser flash photolysis of the fluorenone
derivatives in nitrogen saturated non-hydrogen donating solvents, transient absorptions
corresponding to the triplet states were seen and residual absorption probably due to
benzoyloxy radicals. Triplet-triplet absorption occurs for the benzophenone initiators
under the same conditions but this is much stronger for benzophenone than the
derivatives. In oxygenated solutions the triplet absorptions were effectively quenched but
an interesting phenonemon occurs for all the t-butylperester derivatives in that a long lived
transient growth was observed which is tentatively assigned to that of aromatic peroxy
radical formation. On laser flash photolysis of the polysiloxane compounds very long
lived transients were observed in both oxygenated and nitrogen saturated solutions.
When nitrogen saturated, stable benzoyl radicals appear to have formed whereas when
oxygen saturated some type of complexation seemed to occur with the oxygen.
For all compounds the photopolymerisation activity was shown to be highly dependent
on the structure of the initiator, the extinction coefficient in the relevant region of
excitation and the degree of self termination reactions which can occur. All the tbutylperester
derivatives and polysiloxane compounds were effective at photoinitiating the
polymerisation of acrylate monomers. The derivatives of fluorenone initiated
polymerisation without the use of a tertiary amine co-initiator unlike fluorenone itself.
The derivatives of benzophenone also initiated polymerisation effectively without the
presence of an amine co-synergist with the overall order being
monoperester>tetraperester>benzophenone. Consistent with other results the efficiency
of the polysiloxane compounds to initiate polymerisation was proportional to the
molecular weight of the initiator.