photocuring is the key technique for the 
 preparation of films and 
 coatings as it offers a broad and economic application spectrum for 
 industry . during the curing process
it absorbs energy from 
 a photon either in a direct or an indirect process , transferring it 
 into chemical energy . after the excitation process a reactive radical 

can be formed , which is able to induce the polymerization of a wide 
 range of monomers . among the bimolecular type ii pis for radical photopolymerization , 

excitable chromophores like benzophenone in combination with tertiary 
 amines as co - initiators are commonly applied .
the ketone  amine interactions proved to be highly efficient 
 concerning radical formation due to electron transfer .
the efficiency 
 of the photochemical process depends on the rate constant of electron 
 and proton transfer as well as the reactivity of the -amino 
 alkyl radical toward reactive double bonds and quenching by side reactions . a wide range of co -
thiols and si  h groups are also described as efficient co - initiators in the literature , 
 but only limited storage stability is given for such formulations .
unfortunately , the efficiency of this system is usually reduced 
 by back electron transfer ( bet ) , the solvent cage effect , and limited 
 diffusion capability in highly viscous formulations or water - based 
 systems . molecular oxygen from the atmosphere 
 easily inhibits the polymerization process .
consequently , pi systems , 
 which are unperturbed by oxygen are preferred because it is unfavorable 
 to work under a nitrogen atmosphere .
using n - phenylglycine 
 as co - initiator for benzophenone , the decarboxylation step delivers 
 enough co2 to displace o2 from the curing material .
recently , a very efficient class of pis has been created by covalently 
 linking benzophenone and n - phenylglycine , thus keeping 
 the co - initiator in close proximity .
also , 
 oxime esters might produce co2 after cleavage 
 of the n  o bond and decarboxylation of the acyloxy radical . 

accordingly , this type of functional group in combination with photosensitizers 
 was considered as an alternative concept for type ii initiator systems . 

several investigations on the photolysis of o - acyl 
 oximes in the presence of photosensitizers can be found in the literature .
yoshida et al . described the nature 
 of the triplet states and the subsequent photochemistry of aromatic o - acyl oximes .
they postulated a requirement of close triplet 
 state energies of the oximes ( et = 289305 
 kj mol ) and their ketone - based sensitizers , displaying 
 a  * character .
moreover , the photolysis of aldoxime 
 esters in presence of a sensitizer like 4-methoxyacetophenone was 
 investigated by mccarroll and walton , performing epr measurements 
 and radical trapping experiments , thus presenting a new class of radical 
 precursors for spectroscopic studies ( scheme 1 ) .
furthermore , such sensitized o - acyloximes were 
 tested in a patent , but unfortunately 
 no exact data on their performance was given .
structure related -keto - o - acyloximes have found specialized 
 industrial applications , such as color filter resists , however , thermal 
 stability of such compounds is always crucial .
very recently , we have investigated some benzaldoxime esters 
 in combination with 4-methylbenzophenone as sensitizer and have found 
 surprisingly high photoreactivity in photo - dsc experiments .
all reagents were purchased from 
 sigma - aldrich and were used without 
 further purification .
hdda and 2-hydroxy-2-methyl-1-phenyl-1-propanone 
 ( darocur 1173 ; d1173 ) were received as a gift from ivoclar 
 vivadent and basf , respectively .
nanosecond transient absorption spectroscopy 
 was carried out using 
 the third harmonic ( 355 nm ) of a q - switched nd : yag laser ( spectra - physics 
 lab-150 ) with a pulse duration of 8 ns .
transient absorbances were 
 measured in a right - angle setup using a cell holder with incorporated 
 rectangular apertures defining a reaction volume of dimensions 0.17 
 cm ( height ) , 0.32 cm ( width ) , and 0.13 cm ( depth ) within the cell . 

pulse energies between 0.1 and 4 mj / pulse were used , the typical value 
 for the measurement of transient spectra being 2 mj / pulse .
pulse energies 
 were measured using a ballistic calorimeter ( raycon - wec 730 ) .
time - resolved continuous - wave electron paramagnetic resonance 
 ( cw 
 tr - epr ) experiments were performed using a frequency - tripled nd : yag 
 laser ( continuum surelite ii , 20 hz repetition rate ; 355 nm ; ca . 10 
 mj / pulse ; ca .
10 ns ) , a bruker esp 300e x - band spectrometer ( unmodulated 
 static magnetic field ) , and a lecroy 9400 dual 125 mhz digital oscilloscope . 

the tr - epr spectrum is obtained by scanning the desired magnetic field 
 range recording the accumulated ( usually 50100 accumulations ) 
 epr time responses to the incident laser pulses at a given static 
 magnetic field .
the system is controlled using a program developed , 
 kindly provided and maintained by dr .
argon - saturated solutions were pumped through a quartz tube ( inner 
 diameter 2 mm , flow ca .
the epr spectra were simulated using the easyspin package for matlab ( version 3.1.7 ) .
the hyperfine 
 coupling constants ( hfcs ) of the free radicals were 
 calculated using the gaussian03 package .
all calculations ( geometry optimizations and single point calculations ) 
 were conducted at the b3lyp level of theory with 
 the basis set tzvp .
photo - cidnp 
 experiments were performed on a 200 mhz bruker avance 
 dpx spectrometer .
irradiation was carried out using a frequency - tripled 
 spectra - physics nd : yag indi laser ( 355 nm , ca
10 
 ns ) . the following pulse sequence was used : presaturation  laser 
 flash30 rf detection pulse ( 2.2 s)free 
 induction decay .
the concentrations of the initiators were typically 
 0.01 m in d3-acetonitrile or d6-benzene , deaerated by bubbling argon through the solution .
dsc photocuring experiments were carried out in hdda with 0.12 
 m of each component ( 2 wt % of 1 ) under 
 a nitrogen atmosphere , using an exfo omnicure 2001 uv lamp with a 
 200500 nm filter and a netzsch dsc 204 f1 phoenix with autosampler .
nanosecond transient absorption 
 spectroscopy of solutions of benzophenone and the co - initiating o - acyloximes 1 and 2 as well as 
 solutions of the covalently bound o - acyloxime 4 was used to gain information on the early photochemical 
 steps . in case of the co - initiating oximes ,
the experiments were performed 
 in deaerated mecn in equimolar ( 5  10 and 
 1  10 m ) and different concentrations of 
 sensitizer bp ( 1  10 m ) and 
 oxime ( 1  10 m ) , respectively .
the irradiation 
 wavelength ( 355 nm ) exclusively excites benzophenone chromophores 
 into the n ,  * state . upon 
 irradiation of solutions bp/1 and bp/2 , 
 as expected , the first transient appearing at the nanosecond time 
 scale was the characteristic benzophenone triplet  triplet absorption 
 around 520 nm . at a longer time scale ,
however , the microsecond - range lifetimes of these transients 
 suggested that they originated from radical - type precursors .
the degradation 
 kinetics of the benzophenone triplet ( bp ) in the conducted experiments under variation of sensitizer 
 and co - initiating oxime concentrations gave no indication of ground - state 
 complex formation . regarding transients originating from bp ,
no traces of benzophenone ketyl radicals 
 or radical anions could be detected . because of the lack of product 
 species originating from h abstraction reactions or electron transfer 
 processes , the sensitization presumably proceeds via an efficient 
 energy transfer .
in contrast to the co - initiating 
 systems bp/1 and bp/2 , no local triplet 
 state of the benzophenone moiety was 
 detectable in the photolysis of covalently bound benzophenone  o - acyloxime 4 . here the sensitization process 
 is successfully shifted from a diffusion - controlled , bimolecular process 
 to an intramolecular energy transfer due to the spatial arrangement 
 of the chromophore system and the oxime moiety . as a result of this 
 fundamental change in the sensitization mechanism ,
the triplet lifetime 
 is shortened to an extent (  < 5 ns ) that is no longer directly 
 assessable by the time resolution of our laser flash photolysis setup . 

nevertheless , other transient absorption bands on a longer time scale 
 remained completely identical . in regard to possible primary 
 cleavage products in the photolysis 
 of the co - initiating o - acyloxime 1 , 
 a transient subsequent to triplet states on the nanosecond time scale 
 obviously originates from the triplet bp .
the visible spectrum of this transient ( 6 ) exhibited 
 a slow but continuous progress of absorption in minor intensity from 
 below 550 nm up to and above 800 nm , resembling the absorption of 
 the benzoyloxyl radical obtained by photolysis of dibenzoyl peroxide . in case of the oxime 2 , which should 
 produce a less stable alkoyloxyl radical , an analogue transient to 6
the degradation of the triplet bp at 530 nm and the buildup of the 
 absorption of 6 at 830 nm in the photolysis of o - acyloxime 1 are illustrated in figure 2 .
the kinetic congruence between both traces is 
 a strong indication that the benzophenone triplet bp is the precursor of the benzoyloxyl radical 6 .
temporal absorbance change of bp at 530 nm and 6 at 830 nm upon 355 nm laser flash 
 photolysis of an equimolar solution of benzophenone and the o - acyloxime 1 in mecn ( 1  10 m ) .
unfortunately , 6 displays a low extinction 
 coefficient 
 in the accessible wavelength range , and related values given in the 
 literature vary significantly .
nevertheless , the 
 yield of the benzoyloxyl radical 6 was determined as 
 the product 830
 (6 ) in an equimolar solution of 1 and benzophenone 
 in mecn ( 1  10 m ) .
the results are shown 
 in figure 3 in the form of the absorbance of bp at 530 nm and 6 at 
 830 nm , versus the energy of the laser pulse .
absorption of bp at 530 nm 
 obtained at 20 ns and 6 at 830 nm at 50 ns after 355 
 nm laser flash photolysis of an equimolar solution of benzophenone 
 and the o - acyloxime 1 in mecn ( 1  
 10 m ) as a function of laser pulse energy .
the dependence determined for both the yield of 
 triplet benzophenone bp and 
 benzoyloxyl radical 6 is linear .
the value of the product 
 830  (6 ) 
 in mecn was found to 
 be 200 m cm .
additionally , 
 an important conclusion that can be drawn from this experiment is 
 that the formation process is purely monophotonic . in the photo - dsc 
 experiments
the co - initiating 
 oximes 1 and 2 displayed a higher activity 
 as pis compared to the covalently bound o - acyloxime 4 . therefore , two different conclusions are possible : pi activity 
 of 4 is reduced due to ( 1 ) a reduction of quantum yield 
 for the cleavage or ( 2 ) a less reactive initiating species produced . 
 in nanosecond
transient absorption spectroscopy , the covalently bound o - acyloxime 4 yields a transient , which is 
 completely congruent to the benzoyloxyl radical 6 but 
 shows accelerated buildup kinetics .
apart from the differences in 
 sensitization , 4 follows an analogue photodegradation 
 chain as 1 . however , in comparison to the co - initiating 
 oxime 1 , the product 830
thus , the 
 spatial arrangement of the benzophenone and oxime moieties in the 
 structure of 4 causes a reduction in radical quantum 
 yield in comparison to the co - initiating system . it must be 
 assumed that the benzoyloxyl radical 6 is 
 produced by a homolytic scission of the n  o bond upon photolysis 
 of 4 and 1 .
consequently , this type of photodegradation 
 should result in the formation of an iminyl radical .
nevertheless , 
 no transient absorption in the measured wavelength range ( 300840 
 nm ) could be assigned to a primary formed iminyl radical . since iminyl 
 radicals are rarely reported in the literature as transients in laser 
 flash photolysis ,
it can be reasonably argued that this species shows 
 no or an insufficient absorbance for a detection in the applied analytical 
 setting .
apart from primary photolysis products , secondary 
 radicals appear 
 as subsequent transients at the nanosecond time scale in the photolysis 
 of all o - acyloxime pis .
these transients possess 
 absorptions between 350 and 500 nm and resemble adducts of benzoyloxyl 
 radicals to aromatic rings as described in the literature .
the degradation of the benzoyloxyl radical 6 at 830 
 nm and the buildup of one of these transients ( t1 ) at 390 nm in the photolysis of o - acyloxime 1 are illustrated in figure 4 .
the kinetic resemblance between the traces is 
 another strong indication that the benzoyloxyl radical 6 is the precursor of the secondary adduct t1 ( see scheme 2 ) .
temporal absorbance change of bp and t1 at 390 nm and 6 at 830 nm upon 355 nm laser flash 
 photolysis of an equimolar solution of benzophenone and the o - acyloxime 1 in mecn ( 1  10 m ) .
the decay of the transient 6 , as shown 
 in figure 4 , proceeded with a first - order rate 
 constant k1 = 4  10 s. this rate has to be regarded as the product of a 
 superposition 
 of two well - known concurrent reactions : the first - order decarboxylation 
 process after escape from the solvent cage and the pseudo - first - order 
 addition to aromatic structures of the photolysis solution .
the trace 
 of t1 in figure 4 is partially distorted due to a spectral overlap 
 by at least a second , similar benzoyloxyl radical adduct t2 which exhibits different 
 degradation kinetics .
in contrast to t1 , an adduct transient which resembles t2 was also detected in the 
 photolysis of o - acyloxime 4 . in the 
 case of the covalently bound o - acyloxime ,
the congruence 
 between the degradation of the benzoyloxyl radical and the buildup 
 kinetics of t2 could 
 be more easily studied since no overlap with a triplet spectrum occurs . 
 in the photolysis of the co - initiating o - acyloxime 2 , which produces an alkoyloxyl radical , the relative absorbance 
 of transients comparable to t1 or t2 is significantly lower .
this is the logical consequence of an acceleration 
 of the concurrent reaction , the decarboxylation of the less stable 
 alkoyloxyl radical . to obtain insight into the early 
 stages of polymerizations , continuous - wave tr - epr and h
via epr one is able to establish the radicals formed within 
 the first 50 ns after irradiation , whereas using 
 cidnp one can gain information about the products formed via the primary 
 radical pair . as the general model , the system bp/1 was investigated by both tr - epr and h cidnp . 

additionally , we present h cidnp results for bp/2 , bp/3 , and 4 . upon irradiation of a mixture of bp ( 10 mm ) and 1 ( 10 mm ) in acetonitrile in an epr spectrometer , 
 employing a 355 nm pulsed light source , the time - resolved epr spectrum 
 shown in figure 5 ( lower panel ) is recorded . 

all signals appear in absorption with similar signal intensities in 
 the low- and high - field portions of the spectrum , indicating that 
 the radicals are formed via the triplet mechanism .
the nitrogen - centered 
 radical 5 dominates the spectrum and is characterized 
 by two triplets centered at 338.2 mt .
the benzoyloxyl radical 6 gives rise to a broad peak at 336.8 mt , with a high g value characteristic for oxygen - centered radicals .
additional 
 signals stemming from phenyl radical 7 in the center 
 of the spectrum are shown in figure 6 . from 
 the associated time
traces it can be inferred that 6 produces 7 via decarboxylation within about 500 ns .
the extracted hyperfine 
 coupling constants ( hfcs ) for 7 are in agreement with 
 published data and calculations ( see 
 figure 8) .
no signals stemming from ( reduced ) bp or related radicals could be identified , again indicating 
 that bp only acts as a sensitizer .
tr - epr spectrum of a 
 solution of 1 ( 10 mm ) and bp ( 10 mm ) in 
 acetonitrile in the absence ( lower panel ) and 
 presence ( upper panel ) of 0.1 m butyl acrylate .
the upmost panel represents 
 the spectrum at the time delay ( 150 ns ) indicated by the dashed red 
 line in the 3d spectrum , showing the nitrogen - centered radical ( and 
 its simulation ) as well as the benzoyloxyl radical .
zoom - in on the central part of the tr - epr spectrum shown 
 in figure 5 in the absence of butyl acrylate . 
 for better visibility 
 of the low intensity radical
7 signals the color map 
 was cut at values far below the maximum of the radical 6 signal maximum . the blue and green spectra and time traces are cuts 
 along the field and time axis at values indicated by the colored dashed 
 lines . to study the time profiles of the addition of radicals 
 to monomers , 
 butyl acrylate ( ba )
was added to the bp/1 system . upon this addition ( epr spectrum in figure 5 , upper panel )
one can immediately see that the time evolution 
 of radicals 5 and 6 is hardly affected . 
 but
a change in the signal pattern of the central part of the spectrum 
 can be clearly discerned ( figure 7 ) . in the 
 presence of 0.1 m
ba , 7 vanishes and gives 
 rise to a new signal , which can be assigned to the addition product 
 of phenyl radical 7 to ba , 7-ba .
no direct hyperfine data for the adduct 7-ba could 
 be found in the literature , but epr data for the addition of phenyl 
 radicals to methyl acrylate and benzoyl 
 radicals to ba(38 ) as well as 
 theoretical calculations are in agreement with this assignment .
thus , we were able to establish the primary radicals formed directly 
 upon photolysis of 1 by tr - epr . by the addition of ba
it could be shown that there is no reactivity of the nitrogen - centered 
 iminyl radical 5 toward the monomer in the observed time 
 scale .
additionally decarboxylation of 6 toward 7 is fast enough that no addition of 6 to ba could be discerned from the spectra .
the addition of phenyl 
 radicals 7 to ba is fast enough to repress 
 the epr signal of 7 ( figure 7 ) .
zoom - in on the central part of the tr - epr spectrum shown in figure 5 in the presence of butyl acrylate ( 0.1 m ) .
the 
 2d spectrum and time traces are obtained by cuts along the axis at 
 the indicated colored dashed lines .
initially experiments irradiating only the benzaldoxime 
 esters 1 , 2 , and 3 were performed . 
 except for some small polarized signals of parent compounds ,
no reaction 
 products could be discerned , reflecting the necessity of a photosensitizer . as the reference experiment , the mixture benzophenone bp / benzaldoxime benzoate 1 was photolyzed inside the nmr 
 spectrometer .
the nmr spectrum before irradiation ( see figure 9a ) shows a distinct signal for the aldoxime hydrogen 
 at  = 8.70 ppm .
the rest of the spectrum is made up of two 
 separated groups in the aromatic region the one with the higher shift 
 (  = 8.098.15 ppm ) stemming from the protons of the 
 benzoate moiety .
the signals of bp as well as the other 
 aromatic hydrogens of 1 make up the rest of the spectrum .
it is caused by an interaction of the electron and nuclear spin in 
 the radical pair .
therefore , the determining factors to the effect 
 are the sign and magnitude of the radicals magnetic properties 
 ( g value , hyperfine , and j coupling ) 
 as well as the initial spin state and the reaction pathway (  cage  
 or 
the resulting cidnp signals can 
 easily be rationalized employing kaptein s rules . immediately after irradiation using a 
 single nd : yag laser pulse , 
 a cidnp spectrum was recorded ( see figure 9b ) .
it can be assigned as follows : the emissive peak at  = 
 8.70 ppm corresponds to the reformation of parent compound 1 .
a second emissive peak can be found at  = 8.03 ppm , which 
 is assigned to the z - isomer 1i of the 
 parent compound ( e - isomer ) , also generated by in 
  cage  recombination .
the shift difference of  
 = 0.67 ppm is characteristic for the two isomers . of the signals in absorption ,
it is generated by the 
 decarboxylation of benzoyloxyl radical 6 to phenyl radical 7 and followed by hydrogen transfer from iminyl radical 5 , the most likely hydrogen donor ( see scheme 3 ) .
benzonitril 8 , the byproduct of this reaction , 
 is not distinguishable owing to its aromatic hydrogens stemming from 
 iminyl radical 5 where they only had small hfcs , thus 
 leading to low polarizations .
the two absorptive signals at  
 = 8.55 and 8.42 ppm can be assigned to the e- and z - isomers of n - benzylideneaniline 11 and 12 , respectively .
those are formed from 
 the recombination of phenyl radical 7 and iminyl radical 5 . the e - isomer displays higher signal intensity 

in addition , the e - isomer is thermodynamically favored leading to thermal 
 isomerization of the z - isomer .
there are also another few possibilities of recombination , 
 which are indicated in scheme 3 .
the recombination 
 of two phenyl radicals 7 leads to biphenyl 14 which can be accommodated by the occurrence of small signals at 
 around  = 7.4 ppm .
the low intensity is expected because the 
 phenyl ring does not carry a lot of polarization in addition to 14 being an  escape  product .
recombination of two radicals 6 is not expected because 
 of the fast decarboxylation reaction .
radical 5 can also abstract a hydrogen ( most likely 
 from another 5 ) and form benzaldimine 10 .
the cidnp spectra of 1 , 2 , and 3 are all expected to give identical follow - up products via 
 radical 5 .
this is corroborated by identical peaks at 
  = 7.89 and 8.30 ppm in all three spectra .
( b ) h cidnp spectrum obtained immediately 
 after 
 the laser pulse ( 355 nm ) .
benzaldoxime acetate 2 shows a 
 similar behavior as 1 , except for the differing carbon - centered 
 radical fragment . 
 to avoid a signal overlap appearing in d3-acetonitrile ,
the reaction 
 is compatible in both solvents . in the nmr spectrum ( figure 10a ) of the mixture
bp/2 there are only 
 two identifiable singlet peaks , namely the signal of the aldoxime 
 hydrogen at  = 7.81 ppm and the singlet of the methyl end group 
 at  = 1.73 ppm .
the remaining signals stem from the aromatic 
 hydrogens of both compounds , which are not individually attributable . 

the solvent signal is visible at  = 7.16 ppm . upon irradiation 
 of the solution ( nd : yag laser - single pulse ) 
 there are more signals in the cindp spectrum ( figure 10b ) than in the case of 1
. there are signals in 
 emission at  = 7.81 and 7.21 ppm , which stem from recombination 
 of the primary radicals 5 and 15 to the 
 parent compound 2 and its isomer 2i , respectively 
 (  = 0.60 ppm ) .
the acetoyl radical 15 resulting 
 from triplet cleavage of 2 undergoes decarboxylation 
 generating methyl radical 16 .
the first one is hydrogen 
 transfer from radical 5 , which leads to methane 19 and can be found in absorption at  = 0.15 ppm . as 
 for 1
, benzonitrile 8 is a byproduct but 
 is not clearly distinguishable in the spectrum .
another absorptive 
 signal in the aliphatic region is found at  = 0.79 ppm and 
 can be assigned to the  escape  reaction of two methyl 
 radicals forming ethane 20 . moreover , the recombination 
 of radicals 5 and 16 leads to the e- and z - isomer of n - benzylidenemethanamine , 17 and 18 , respectively .
the corresponding signals 
 are two quadruplets at  = 8.48 and 7.89 ppm as well as two 
 doublets at  = 3.35 and 3.27 ppm . because these signals stem 
 from coupled polarized protons , they both show the multiplet effect 
 a / e ( first absorption , then emission ) .
this behavior , also described 
 in kaptein s rules , is easily 
 visible for the two methyl doublets .
for the two iminyl quadruplets 
 there is also overlying enhanced absorption ; thus , only the latter 
 peaks are visible in emission .
the two isomers could be assigned using 
 the j coupling constant , which is known 
 for the e - isomer ( j = 
 1.6 hz ) .
therefore , the signals 
 at  = 7.89 and 3.27 ppm belong to 17 ( j = 1.6 hz ) and the signals at  = 
 8.48 and 3.35 ppm to 18 ( j = 2.2 hz ) . two other signals at  = 8.31 and 7.82 
 ppm are assigned to substances 9 and 10 in 
 analogy to 1 ( see above ) .
the slightly different shift 
 values are caused by the different solvent ( d6-benzene vs d3-acetonitrile ) .
( b ) h cidnp spectrum obtained immediately after 
 the laser pulse ( 355 nm ) .
but , contrary to the 
 other substances investigated in this work , 3 features 
 an end group including a reactive vinyl moiety , causing additional 
 follow - up products and polarization effects .
the h nmr spectrum of bp/3 shows the aldoxime 
 hydrogen signal at  = 8.56 ppm , and the protons of the aromatic 
 ring are concentrated in the region  = 7.447.81 ppm . 

further features of the nmr spectrum are two multiplets at  
 = 5.75 and 6.19 ppm assigned to the e- and z - hydrogen of the vinyl bond , respectively . additionally 
 a multiplet stemming from the methyl group can be seen at  
 = 2.01 ppm .
the h cidnp spectrum shows typical features 
 of recombination 
 reactions of the initial radical pair of 5 and 21 , leading to polarized signals of parent 3 at 
  = 8.56 ppm and of isomer 3i at  = 7.93 
 ppm (  = 0.63 ppm ) .
also substances 9 and 10 appear in the spectrum as two polarized doublets at  
 = 7.89 and 8.30 ppm .
the iminyl hydrogens of products 23 and 24 stemming from recombination of radicals 5 and propen-2-yl radical 22 after decarboxylation 
 can be found at  = 8.32 and 8.11 ppm , respectively .
there are 
 some additional signals visible in this region , which are probably 
 generated by additional recombination reactions of 5 . 

all the other fragmentation products lead to compounds containing 
 one or more double bonds generating a number of polarized multiplets 
 for vinyl and aliphatic hydrogens . for those it is impossible to discern 
 the single components , owing to several overlapping resonances ( see supporting information ) .
the unimolecular 
 initiator 4 shows similar behavior 
 and spectra as the bimolecular type bp/1 .
the h nmr spectrum ( see figure 11a ) shows one 
 isolated resonance signal at  = 8.80 ppm stemming from the 
 aldoxime hydrogen .
the remaining protons of the molecule are attached 
 to aromatic rings and occur as one block of signals (  = 7.528.15 
 ppm ) . upon irradiation
, the initiator 4 undergoes 
 cleavage 
 of the n  o bond analogous to 1 ( see laser flash photolysis section ) .
the most prominent 
 signal in the h cidnp spectrum ( figure 11b ) is benzene 13 at  = 7.37 ppm in absorption 
 generated via decarboxylation of benzoyloxyl radical 6 .
the recombination isomer of 4 ( emissive signal at 
  = 8.80 ppm ) , 4i , can be found in emission at 
  = 8.15 ppm .
the shift difference of  = 0.65 
 ppm is in accordance with the isomerization of the other three molecules 13 .
the two remaining absorptive signals 
 are assigned to the recombination reactions of iminyl radical 27 and benzyl radical 7 after decarboxylation . 
 in the same way as for the bimolecular system bp/1 ,
the 
 two e- and z - isomers ( 31 and 32 , respectively ) can be found at  = 8.67 
 and 8.54 ppm , 31 showing the higher intensity . 

furthermore , signals stemming from biphenyl 14 can be 
 seen at low intensity at the base of the benzene signal .
there are 
 also products 2830 , analogous to 810 formed from the iminyl radical 5 stemming from 1 .
the 4-cyanobenzophenone 28 did not show polarization in the cidnp spectrum , but the 
 peaks at  = 8.05 and 8.42 ppm are tentatively assigned to 29 and 30 .
( b ) h cidnp spectrum obtained immediately after 
 the laser pulse ( 355 
 nm ) . to study the reactivity of the generated radicals 
 toward monomers , 
 cidnp experiments were performed in the presence of two acrylates , 
 butyl acrylate ( ba ) and 3,3-dimethyl-2-methylenebutanoate 
 ( t - bam ) , analogous to the 
 tr - epr measurements .
the bulky tert - butyl substituent 
 of t - bam hampers the polymerization 
 process and thus does not lead to a propagating polymer chain .
this 
 is desired , because it reduces the number of follow - up reactions and 
 therefore the influence of the growing chain on the studied reactions 
 can be followed .
it can be seen in figure 12 that the addition 
 of the monomer ba leads to an easily identifiable effect 
 in the cidnp spectrum .
the benzene signal intensity is reduced by 
 about 60% . a similar reduction of intensity
can be seen for the two 
 products 11 and 12 , which are recombination 
 products formed after decarboxylation .
in contrast , the two emission peaks 
 of the parent compound and its isomer are not affected by the addition 
 of quencher ( the monomer ) .
this reveals that the formation of  cage  
 products is hardly influenced by the polymerization process .
the clearly 
 diminished intensity of the peaks corresponding to  escape  
 products 1113 shows that phenyl 
 radical 7 adds to the acrylates .
consequently all products 
 formed at longer time scales from 7 are outcompeted by 
 the monomer , which is present at much higher concentration compared 
 to photoinitiator radicals .
there was no effect seen on the two minor 
 peaks at  = 7.89 and 8.30 ppm ( compare above ) , which are attributed 
 to compounds 9 and 10 .
therefore , the cidnp 
 spectra reflect that the iminyl radical is not reactive toward the 
 acrylate .
this behavior of the two radicals 5 and 6 is in agreement with the tr - epr results above .
zoom - in of 
 the h cindp spectrum of the initiator system bp/1 and the same with the addition of butyl 
 acrylate ( spectrum horizontally shifted ) .
the same experiment performed with t - bam gave comparable relative intensities 
 of the different 
 signals .
furthermore , the reaction was performed with styrene as 
 monomer , but the cidnp spectra only gave very small signals compared 
 to noise .
this is attributed to the ability of styrene to act as a 
 quencher of the bp triplet while the oxime benzoate itself is not able to efficiently generate 
 radicals under irradiation .
preliminary experiments have indicated 
 that 4-methyl benzophenone a is an efficient sensitizer 
 for 1 . to establish the range of suitable sensitizers , a 
photo - dsc provides insights into the overall performance 
 of polymerizing 
 systems . here the parameter tmax , the 
 time needed to reach the maximum heat of polymerization , comprises 
 effects which can be traced back to the efficiency of the photoinitiator . 

a comparison of tmax values for selected 
 sensitizer / initiator combinations for the polymerization of hdda ( 1,6-hexanediol diacrylate ) is shown in figure 14 .
sensitizer c does not reveal any 
 effect , owing to its rather low triplet energy of 173 kj mol .
the type ii initiating system , a and triethanolamine 
 ( tea ) , leads to tmax = 10 
 s , and even a itself shows activity , but tmax amounts to 26 s. this can be traced back to the hydrogen - abstraction 
 ability of the corresponding excited triplet state .
however , when 
 sensitizers a and b ( itx , 2-isopropylthioxanthone ) 
 are utilized in combination with 1 , tmax reaches the level of reference systems a / tea and darocur 1173 ( d1173 ) .
similar results were achieved 
 for benzil and 2-ethylanthraquinone as sensitizer ( data not shown ) .
photo - dsc 
 data for oxime ester 1 with equimolar amounts 
 of the sensitizers a and b as well as reference 
 pi systems a / tea , a , and d1173 .
in this study we have shown that benzaldoximes 
 like o - benzoyl benzaldoxime ester 1 act 
 as highly reactive 
 type ii photoinitiators when combined with an appropriate sensitizer . 

reactivities similar to classical monomolecular compounds are possible , 
 with the advantage that the spectral range of sensitivity can be easily 
 tuned .
it could be established that the radical initiation proceeds 
 via triplet energy transfer from the sensitizer , followed by cleavage 
 of the oxime n  o bond . in first instance
the latter radical decarboxylates to give a carbon - centered 
 radical , which induces polymerization . on the other hand ,
the iminyl 
 radical does not contribute to the initiation at the observed time 
 scale .
trans photoisomerization at the c = n bond 
 of the oxime moiety is observed in the cidnp experiments . in 
 summary ,
the benzaldoxime - based initiators reveal efficient 
 systems , which circumvent the complication of  back electron 
 transfer  in type ii systems .
 typical bimolecular photoinitiators ( pis ) for radical 
 polymerization 
 of acrylates show only poor photoreactivity because of the undesired 
 effect of back electron transfer . to overcome this limitation , pis 
 consisting of a benzaldoxime ester and 
 various sensitizers based on 
 aromatic ketones were introduced . 
 the core of the photoinduced reactivity 
 was established by laser flash photolysis , photo - cidnp , and epr experiments 
 at short time scales . according to these results , the primarily formed 
 iminyl radicals are not particularly active . 
 the polymerization is 
 predominantly initiated by c - centered radicals . 
 photo - dsc experiments 
 show reactivities comparable to that of classical monomolecular type 
 i pis like darocur 1173 . 
Introduction
Experimental Section
Results
Summary