Data of the synthesis and U.V. properties of the vinylsilyl terminated esters used. Ester .... The polymers are usually waxy at room temperature when modified by.
Pure & A p p / . Chem., Vol. 62, No. 8, pp. 1603-1614,1990. Printed in Great Britain. @ 1990 IUPAC
Synthesis and properties of photoreactive polysiloxanes containing pendant functional groups Xavier COQUERET, Adel HAJAIEJ, Alain LABLACHE-COMBIER, Claude LOUCHEUX, Regis MERCIER Lydie POULIQUEN AND Lili RANDRIANARISOA-RAMANANTSOA. Laboratoire de Chimie Organique et Macromoleculaire, U.R.A. C.N.R.S. 351 Universite des Sciences et Techniques de Lille Flandres-Artois 59655 Villeneuve d’Ascq Cedex France.
-
Abstract - Functional polysiloxanes containing photo-reactive pendant groups were prepared by multi-step modification of silicone copolymers containing methylhydrosiloxanes units. A first method based on the platinum Catalyzed hydrosilylation of vinylsilyl terminated photoreactive esters allowed to prepare liquid silicone polymers containing photo-dimerizable esters. Another method based of the esterification of pendant epoxy groups previously grafted on the silicone main chain was found a s a general and very powerful alternative route to prepare photo-reactive polysiloxanes. The method has been applied to synthesize polysiloxanes containing various functional side-groups such a s dimerizable esters, aromatic carbonyl compounds or dyes which possess properties of photochemical interest. The reactivity, the ability to be photosensitized a s well practical properties of different photo-crosslinkable polysiloxanes either by cinnamic, furacrylic or a cyano p styrylacrylic ester were examined. A kinetic investigation of the reactivity polysiloxane-bound dimerizable chromophores allows to point effect of the silicone matrix in comparison with more hydrocarbon pho topolymer 8.
as some modified groups of the out the classical
IN TR OD UCTI O N Polysiloxanes containing pendant functional groups receive increasing interest because they possess some outstanding properties making extremely attractive their use in various applications a s biomedical material (ref.1)’ surface-coating resins (ref.2) or liquid crystalline polymers (ref.3). Surprisingly it is almost exclusively in the patent literature or in technical papers (ref.4) that a r e described such silicone-based systems having additional photochemical properties allowing process in the molten state and subsequent immobilization by UV curing. We now report on the basic problem of the synthesis of well-defined photosensitive polysiloxanes and on the study of some of their properties.
SYN TH ESI S OF PHOTO-CROSSLINKABLE PCILYSILOXANES BY HYDROSILYLATION: A FIRST APPROACH One of the most usual methods affording such modified silicones involves the one-step fuctionalization of presynthesized (co)polymers or prepolymers containing hydrosiloxane units, by hydrosilylation of a terminal carbon-carbon double bond which binds the low molecular weight compound to the macromolecular backbone (ref.5). Some hydrosiloxane copolymers are commercially available starting material, but for systematic studies, these polymers a r e preferably taylored a t will by acid catalyzed redistribution of siloxanic units a s depicted in Scheme 1 (ref.6)’ where the homopolymer MD’q-M affords the reactive D’ units and where the cyclic oligomer D4 affords the dimethylsiloxane comonomeric units. The molecular weight is controlled by introducing the appropriate amount of M2 which acts a s a chain regulator. When the equilibrium is reached, the solid catalyst is removed to avoid further redistribution reaction and the produced volatile oligomers a r e distilled off to give taylor-made copolymers I. 1603
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Scheme 1
P
F&
4
CH3 30 .-2!.
lo
/.
/'
.
C v)
01
0
I
10
30
I
I
L
50
Fig. 1. Influence of t h e a v e r a g e molecular weight of t h e s t a r t i n g p o l y m e r on t h e sensitivity of FAVS functionalized polysiloxanes : M P 1 (o),M P 2 ( W ) , M P 5 ( A ) . See table 3.
I
1
30
50
100
I
I
70
Fig. 2. Dependence of the practical sensitivity of some CSAVS functionalized polymers with t h e s q u a r e of t h e molar r a t e in sensitive u n i t s : M P 8 (o), M P 9 (I), M P l O ( A ) . See table 3.
From t h e comparison of some selected r e s u l t s of table 3, i t i s possible t o point o u t t h e influence of t h r e e basic constitutional parameters on t h e practical photosensitivity of t h e modified polysiloxanes : f o r a fixed content in given sensitive units, t h e a v e r a g e molecular weight of t h e p a r e n t copolymer and f o r a fixed value of t h e chain length, t h e d e g r e e of functionalization with a given photo-reactive group. According to t h e theoretical treatment of Tsuda (ref.13) developped f n r hydrocarbon polymers, it was found t h a t , for small variations of t h e two f i r s t parameters, t h e sensitivity has a linear dependence o n t h e molecular weight (Fig.1) a n d o n t h e s q u a r e of t h e molar c o n t e n t i n sensitive u n i t s (Fig.2). The last basic parameter i s naturally t h e n a t u r e of t h e photodimerizable e s t e r . The comparison of t h e sensitograms of f i g u r e 3 obtained b y photosensitivity measurements u n d e r monochromatic light shows t h e intrinsically more sensitive n a t u r e of cyanostyrylacrylic g r o u p s giving a maximum of sensitivity of 50 cm2.J-1 a t t h e wavelength of t h e absorption maximum, when t h e value of S is only 20 cm2.J-1 f o r t h e furylacrylic derivative and 8 cm2.J-' for t h e cinnamic one.
S J-1 cmz)
0.C
S ( J -' c ml)
0.[ 1
J2 55
I
/
~
\\ \
'.
I
0.9
I
/ /
\
/ /
I
\ \
/
\
./
-L
I
I
\
4ba
300
X (nm) Fig. 3. Sensitograms (---) of polysiloxanes containing 2 mol % of sensitive units: M P 3 ( a ) , M P 2 ( b ) , M P 4 (c) ( s e e table 3 ) . The corresponding UV s p e c t r a are shown b y solid lines.
The improvement of t h e photosensitivity of t h i s t y p e of negative working photopolymers by t h e optimization of t h e above mentioned parameters i s nevertheless limited
Photoreactive polysiloxanes
1607
b y t h e consecutive viscosity i n c r e a s e i n t h e molten s t a t e as shown o n t h e diagram of f i g u r e
4. The relation of practical importance between t h e polychromatic sensitivity (500 Watts
-
high p r e s s u r e H g s o u r c e ) and t h e dynamic viscosity of various polymers containing cyanostyrylacrylic e s t e r s shows t h a t t h e more sensitive of t h e s e r i e s h a s a viscosity exceding 10 Pa,s at 60 'C, a high value which does not allow solvent-free coating applications.
0
Fig. 4. Relation between t h e practical sensitivity u n d e r polychromatic light a n d t h e dynamic viscosity at 60'C of some CSAVS functionalized polymers : MP14 ( 0 ) :
1 ' .
In addition to t h i s limitation concerning t h e rheologic p r o p e r t i e s of t h e modified polymers, t h e functionalization reaction described in scheme 3 was found to give r i s e to some unwanted side-reactions involving t h e necessary excess of Si-H g r o u p s with r e s p e c t to t h e sensitive e s t e r . This problem was especially a c u t e when t h e reactions were carried o u t with systems designed to give high values of S. The side-reactions c a n lead to chain branching which i n c r e a s e s t h e molecular weight distribution of t h e polymer all along t h e modification as evidenced by size exclusion chromatography (ref.14) and finally induces t h e gelation of all t h e reaction mixture by chemical cross-linking. A spectroscopic evidence of one t y p o of possible side-reaction obtained by NMR analysis ,of some polymers modified by cinnamic e s t e r s C V S clearly indicated (ref.10) t h a t t h e hydrogenation of t h e double bond of t h e chromophore c a n t a k e place to some significant extent. The s t u d y of a model reaction indeed gave confirmation of t h e formation of dihydrocinnamic esters i n t h e p r e s e n c e of Si-H, as depicted in scheme which involves a t f i r s t a 1,4-hydrosilylation p r o c e s s followed b y hydrolysis or alcoholysis of t h e silyl enolether.
Scheme 4
y 3
y 3
fH3
H C -S i - 0 - S i-0- S i - C H 3 1 I I 3 H CIi3 CH3
H3C-Si-O-Si-O-Si-CH 1 ' 1 CH3 0 CH3
3
+ 0
R-OH
step ii
FH3
CH3
CH I 3
H3C-Si-O-&i-O-Si-CH3 ' I LH3 t 0 I1
aCHz-CH2-C-O-CH
3
-
-
tautomerization
of
CH3
R
H 0-H \ / / c=c CHZ
Mechanism of t h e hydrogenation p r e s e n c e of hydrosilane functions.
0
the
'O-CHJ
cinnamic
chromophore in
the
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These various problems d u e to ill-controlled processes, together with t h e cost of preparation of t h e vinylsilyl terminated e s t e r s , prompted u s t o s e a r c h f o r a g e n e r a l method of s y n t h e s i s of multifunctional liquid polysiloxanes in which t h e n e c e s s a r y hydrosilylation s t e p would be performed with a n olefin of low functionality in o r d e r to avoid s u c h sidereactions.
S Y N T H E S I S OF PHOTOREACTIVE POLYSILOXANES BY ESTERlFlCATlON OF P E N D A N T GLYClDlC G R O U P S The catalyzed addition of a functional carboxylic acid on a p e n d a n t epoxy g r o u p to give a p-hydroxy e s t e r a p p e a r s a s a versatile key-step (ref.15) to be introduced in t h e reaction sequence of scheme 5. According to t h i s method, taylor-made copolymers I are modified in a first s t e p b y hydrosilylation of ally1 glycidyl e t h e r to give with quantitative conversion and high selectivity polymers of t y p e 11. I n a second s t e p , t h e addition of a carboxylic acid is performed to afford t h e expected hydroxyester p e n d a n t g r o u p s observed in polymers I11 as a mixture of t h e two position isomers.
Scheme 5
I
CH,
CHI
,
- CH-
" \
CH,
Pt cat
m
CH,-0
- CH,-
R-COOH
CH=CH,
I
CH, I
I
CHI
?HZ
CH,
AH,
I 0 I
0
I
II
CH,-C-O-
CH I CHz I
0 IV
0
I
=c I R
0 I iI CH-0-C-R I 1
0 I
c=o I
I
I
CH,
FHz
0 I
0
DMP
0
y
AcZO
CH,
I
CH, I
H O - CH
I
CH, I
CH - 0 - C -
I
I
CHI
FH,
I
0 I
OH
0 II
R 111
0=c I
R
Functionalizntion of polysiloxanes by esterification of p e n d a n t glycidic groups. We have studied t h e main esterification f e a t u r e of t h e p e n d a n t glycidic g r o u p s b y model carboxylic acids either in t h e p r e s e n c e of a t e r t i a r y amine as a catalyst in low polarity media (ref.16) or with tetrabutylammonium bromide a s a catalyst f o r reactions c a r r i e d in polar aprotic solvent (ref.17). N o important problem of chemical selectivity could be observed d u r i n g t h e c o u r s e of t h e epoxy-carboxy reaction, b u t t h e recovered polymers of t y p e I11 were obtained in most cases a s untractable gummy p r o d u c t s h a r d l y soluble a f t e r s t a n d a r d purification workup. This phenomenon which was shown t o only r e s u l t from physical associations between t h e chains by intermolecular hydrogen bonding, was finally avoided by t h e ultimate acetylation of polymers I11 to give liquid or waxy polysiloxanes IV whatever t h e length of t h e chain or t h e content in functional p e n d a n t g r o u p s . This novel method was used to p r e p a r e a v a r i e t y of multifunctional liquid polysiloxanes among which photosensitive polymers of t y p e I11 or IV with cinnamic (CIN) or cyanostyrylacrylic ( C S A ) p e n d a n t e s t e r g r o u p s (ref.18) were studied in some details. The values of the practical sensiti-*.ity (S), determined by insolubilization t e s t s , a r e collected i n Table 4. The comparison of t h e r e s u l t s obtained with t h e two s e r i e s of polymers I11 a n d IV indicates t h a t t h e acetylation reaction h a s only little influence o n t h e practical sensitivity of polymers containing e s t e r g r o u p s deriving from VIa. A somewhat more significant loss i s
Photoreactive polysiloxanes
1609
observed when t h e acetylated polymer IVg with a s h o r t silicone chain o r IVc containing cinnamic e s t e r g r o u p s of lower polarity h a s a low viscosity as a consequence of weaker intermolecular interactions. The insolubilization of t h e network, therefore, r e q u i r e s a more important number of covalent cross-links leading to a lowered value of t h e practical sensitivity, a s measured for t h e two acetylated polymers. The intrinsically more sensitive n a t u r e of t h e e s t e r deriving from CSA a p p e a r s from t h e comparison of t h e values of S obtained with polymers IIIb (or IVb) with r e g a r d to IIIc ( o r IVc), t h e l a t t e r being a b o u t ten times l e s s sensitive. The d e g r e e of polymerization DPn of t h e functionalized polymers has a prime effect on their practical sensitivity, while t h e functionalization polymers r a t e seems to be of s e c o n d a r y importance in t h e r a n g e of DPn covered b y t h e studied samples. Table 4. Results of practical sensitivity and ments on photosensitive polymers of t y p e I11 and IV. Polymer of Sa Polymer of S DPnb type I11 cm2J-1 type IV m2J-1 a
a
450 250 23 200 155 100 50
b C
d e f g
435 240 18 140 90 40
b C
d
e f 8
40 1 24 1 24 1 251 209 129 33
dynamic Sensitive ester CSA
CSA CIN CSA
CSA CSA
CSA
viscosity
measure. Con t en t C i n mol% 4.1 5.8 6.5 10d 7.5 9.5 14.2
a ) P r a c t i c a l s e n s i t i v i t y determined by p h o t o r e s i s t tests under monochromatic l i g h t = 345 mn, except f o r polymers I I I c and IVc) ( X = 280 nm). b ) Deduced from GPC measurements performed with t h e parent polymers 2. c ) Deduced from UV absorbance measurements. d ) Not measured. Theoretical maximum value.
C
$
C
.$
I
.:loo
C .-
80
80
0
L
m
C
C
(fl 60 C
" 60
P
LO
LO
20
20
0
10
20
30 f/rnin
Fig. 5, Disappearance (deduced from UV absorption measurements) of the photosensitive g r o u p s i n polysiloxanes 111 o r IV upon irradiation with 280 nm UV light (I = 220.10-6 W.cm-2). IIIb ( A ), IVb ( ), with 345 nm light (I=230 W.cm2) 1% ( 0 ), IVg ( 0 ).
0 t/rnin
Fig. 6. Disappearance (deduced from UV absorption measurements) of the photosensitive groups in acetylated polymer IVb ( m ) upon irradiation with 345 nm UV light (I 230.10-6 W . c m - 2 ) a n d (0) immediately a f t e r s h o r t heating at 6070'C.
The plots of Fig. 5 and 6 show t h e variations of conversion a g a i n s t t h e irradiation time f o r plates coated with polymers I11 o r IV (initial optical d e n s i t y in t h e r a n g e 1.7 - 2). The experimental conditions of t h e s e f i r s t semi-quantitaive measurements do not allow a detailed kinetic analysis of t h e process b u t t h e following comments have to be pointed out. The comparison of t h e conversion v s time c u r v e s obtained with polymers IIIg and IVg, a n d on t h e o t h e r hand IIIb a n d IVb (Fig. 5) indicates t h a t t h e acetylation reaction h a s only little influence o n t h e conversion r a t e of polymers modified with CSA e s t e r s when, in a p p a r e n t contradiction with the results obtained from practical sensitivity measurements (insolubilization t e s t s ) , t h e r a t e was shown to be increased for t h e cinnamic derivative. This phenomenon h a s to be connected with t h e increase in mobility favourable t o t h e bimolecular photo-chemical reaction ( r e f .19).
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The change in t h e conversion c u r v e of polymer IVb irradiated a f t e r annealing b u t without chemical degradation of t h e sample (bottom c u r v e of Fig. 6 ) s u g g e s t on t h e c o n t r a r y , t h a t t h e polysiloxanes containing cyanostyrylacrylacrylic e s t e r s possess a c e r t a i n d e g r e e of organization inducing a higher reactivity b u t a r e sensitive to t h e thermal history of t h e sample. The influence of t h e physical s t a t e of t h e polymeric material on t h e reactivity i s exemplified in t h e next section.
REACTIVITY OF POLYSILOXANE W I T H PENDANT PHOTODIMERIZABLE ESTER GROUPS Polymers with pendant cinnamic esters The photoreactivity of polyvinylcinnamate (PVCi) o r related hydrocarbon systems h a s been studied by Reiser (ref.20). The addition of t h e p e n d a n t g r o u p s i s t h e almost single reaction o c c u r r i n g in t h e solid matrix when films of t h e polymer are exposed to UV light i n t h e g l a s s y state. A typical limit value f o r t h e overall conversion lies in t h e r a n g e of 50 mol%. Even when PVCi i s irradiated in dilute solution, t h e dimerization - oligomerization i s still t h e main process .despite t h e low concentration, t h e deactivation of excited chromophores by reversible cis t r a n s isomerization of t h e conjugated olefin competing only t o a slightly lower extent.
H
?c=o
CHl I
0
'C'
I
I1
(CyH
HO
- CH
I CHl I
PVC i
0 I 0 =c I
Polymer of type 111 CIN
Ph
I n dilute solutions, with initial concentration in cinnamic e s t e r i n the r a n g e of 50 vmol.l-l, polysiloxanes of t y p e I11 or IV showed reversible E-2 isomerization as observed on t h e UV s p e c t r a showing a n isobestic point. The absence of addition reaction between t h e cinnamic g r o u p s c o n t r a s t i n g with t h e behaviour of PVCi i s d u e to t h e length of t h e s p a c e r connecting t h e sensitive moiety to t h e highly flexible main chain. When examining t h e photoreactivity of neat films of t h e sensitive polymers, i t is possible to follow b y UV spectroscopy at t h e two wavelength corresponding t o t h e maximum of absorption of t h e p u r e E chromophore (279 nm) a n d t o t h e isobestic point of t h e isomerization respectively (249 nm), t h e variations of t h e mol ratio of t h e different species, E a n d 2 forms of t h e cinnarnate a n d of t h e dimcrs. For v e r y thin films giving a n initial optical density below 0.1, t h e phatochemical excitation is homogeneous ( n o significant g r a d i e n t of light intensity depending on t h e d e p t h of t h e film) and t h e variations of species a r e totally r e p r e s e n t a t i v e of t h e overall process. The c u r v e s of Fig. 7 , obtained from a s e r i e s of t h r e e polymers of t y p e 111, show t h a t t h e main reaction taking place d u r i n g t h e v e r y f i r s t moment!, of t h e irradiation i s t h e isomerization of t h e p u r e E cinnamate to g i v e la@ddly a mixture of ' t h e , two isomers i n dynamic equilibrium. The dimerization starts b u t much more slowly, accounting f o r a collision controlled p r o c e s s totally different from t h e topochemical control by t h e c r y s t a l lattice of t h e dimerization of cinnamic acid (ref.21) or from t h e reaction in t h e glassy matrix of PVCi. Indeed, t h e initial absolute rate of t h e dimerization roughly correlates with t h e weight content i n photosensitive group. The comparison between t h e limit values of t h e dimerization oligomerization process would not be relevant in t h e p r e s e n t case, since t h e t h r e e polymers h a v e v e r y different DPn. The formation of t h e network which r e d u c e s t h e mobility of t h e reactive g r o u p s d u r i n g t h e c o u r s e of t h e dimerization is s t r o n g l y dependent on t h i s macromolecular parameter a n d h a s naturally a determinant influence o n t h e plateau value.
-
1611
Photoreactive polysiloxanes
tI
l'QOr
Molar fraction
1.00
!. 00
0.75
0. 75
1-
0. 75
\
0. 50
0. 50
0.25
3.25
0.0Q
0. 00
*
Irradiation time Fig. 7. Conversion v s time plots of cinnamic. e s t e r s i n neat films of polysiloxanes of t y p e I11 with 25 (a), 44 ( b ) and 100 mol % ( c ) of p e n d a n t cinnamic g r o u p s u n d e r t h e light of a 900 Watts Xenon source. Polymers with pendant CSA esters UV light irradiation of dilute solutions ( C < 5 M) of f r e e CSA e s t e r s or of copolymers containing CSA p e n d a n t g r o u p s leads t o a c h a n g e in t h e i r UV s p e c t r a with persistence of a n isobestic point at 301 nm a t t r i b u t e d to t h e isomerization of t h e chromophore. When irradiated u n d e r similar conditions, neat films of sensitive polysiloxanes MP (See Table 3 ) or of t y p e I11 o r IV - whatever t h e n a t u r e of t h e s p a c e r linking t h e CSA e s t e r to t h e chain - g a v e UV s p e c t r a with a n isobestic point at a s h o r t e r wavelength (288 nm) as shown o n Fig.8a.
CSA e s t e r s t h u s show two v e r y clean-cut t y p e s of behaviour : in solution one o b s e r v e s only a monomolecular isornerieation a t t r i b u t a b l e process when, i n neat polymeric samples, t a k e s place a single different process to t h e dimerieation to give well-defined cyclobutane products. The retro-dirnorization known to be photo-activated by 255 nm light irradiation of crystalline model e s t e r s (ref.22) i s indeed observed, a s shown on Fig. 8b. The reversibility i s high if t h e conversion into t h e dimer h a s been limited to a value below 50%. I n t h e case p r e s e n t e d i n Fig. 8 b t h e recovered optical density a t 345 nm, more t h a n 80% of t h e initial value accounts f o r a good selectivity s u g g e s t i v e of a matrix controlled reaction (ref.23). F u r t h e r work i s c u r r e n t l y in p r o g r e s s to correlate kinetic parameters of t h e dimerization process with t h e micromorphology of t h e s e systems. I t i s likely t h a t t h e polar n a t u r e of t h e photosensitive e s t e r induces phase separation of domains having a partially ordered structures. 1
a 0
0.5
0 250
h
(4
350
300
h
400
(nm)
Fig. 8. ( a ) Changes in t h e U.V. s p e c t r a of MP8 upon 348 nm irradiation : (b) reversibility of t h e [2+2] photo-cycloaddition reaction by 255 nm irradiation of t h e crosslinked polymers.
X. COQUERET e t a / .
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Photosensitization of the cross-linking process For many applications as negative photoresists, hydrocarbon polymers crosslinkable b y a [2+2] cyclo-addition process a r e used with photosensitizers in o r d e r to give a b e t t e r overlap between industrial light s o u r c e s and t h e s p e c t r a l r e s p o n s e of t h e whole formulation (ref,24). W e have used typical triplet s e n s i t i z e r s a s t h e different phenones mentioned i n t h e legend of Fig. 9 a n d some o t h e r compounds t o enhance t h e reactivity of cinnamic polysiloxanes. A clear effect of s p e c t r a l sensitization i s o b s e r v e d with t h e s e systems despite t h e heterogeneous n a t u r e of t h e mixture. The efficiency i s naturally extremely poor with polymers of low r a t e of functionalization ( 2 20 mol%) i n reason of t h e low concentration in reactive g r o u p s which dramatically affects t h e probability of encounter of a pair of e s t e r s with t h e excited sensitizer. The influence of t h e n a t u r e of t h e photosensitizer clearly a p p e a r s from t h e comparison of t h e c u r v e s obtained with a polysiloxane of t y p e 111 containing 44 mol% of cinnamic groups. The r e s u l t s confirm t h e good efficiency of MK a n d BN (ref.24) which give with a filtered incident light a r a t e of disappearence of t h e cinnamic e s t e r s i n t h e same r a n g e t h a n f o r t h e direct excitation of t h e chromophore using t h e whole emitted spectrum of t h e light source.
-
60
.-
TX
./--*-fT
Blank -
10
a
20
t /mn
Fig. 9. Variations of t h e conversion rate of a polysiloxane of t y p e I11 with 44% molar of photosensitive cinnamic e s t e r units, in t h e p r e s e n c e of various photosensitizers ( 5 wt %). @ i s t h e c u r v e obtained when t h e 900 W a t t Xenon lamp i s used without light filter. Tho o t h e r c u r v e s were obtained with incident light of wavelength A > 320 nm). BP = Benzophenone, BN N-methyl 2-benzoylmethylidenenaphtothiazoline, MK Michler’s ketone, P = p y r e n e , TX = thioxanthone, 1P = 2,4,6-triphenyl-2,4,6 pyrylium perchlorate, 1T I: 2,4,6-triphenyl-thiapyrylium perchlorate,
The sensitization of polysiloxane with p e n d a n t CSA g r o u p s by different pyrylium s a l t s has also been tested. The photocross-linking t h e n proceeds with loss of t h e isobestic point i n t h e UV spectra recorded d u r i n g t h e c o u r s e of t h e irradiation. Pyrylium s a l t s which a r e know t o give rise to electron t r a n s f e r , presumably induce additional radical processes leading t o t h e addition of t h e double bonds (ref.25). Practical photoresist tests show anyway t h a t t h e sensitized compositions a r e a b o u t 3 times more sensitive t h a n t h e p u r e polymer u n d e r t h e polychromatic light of a Xenon lamp. To examine if t h e i n c r e a s e in practical sensitivity by s p e c t r a l s e n s i t i z e r s could be enhanced by inducing a b e t t e r compatibility between t h e functional polysiloxano and t h e a d d e d compound we have p r e p a r e d , using t h e different s y n t h e t i c methods described in t h i s paper, t h e sensitizing systems shown i n scheme 6. a) BP o r 1P added i n bulk p) vinyl derivative of BP o r carboxylic derivative of 1P fixed on a n oligomeric siloxane 7 ) vinyl derivative of BP o r carboxylic derivative of 1P fixed o n t h e polysiloxane chain as comonomeric unit.(3P)
1613
Photoreactive polysiloxanes
Scheme 6
Different modes of sensitization.
The c u r v e s of Fig. 10 allow to compare t h e r a t e of disappearance of t h e cinnamic e s t e r s of a polymer of t y p e I11 show t h a t t h e initial slope in systems 7 ) is s t e e p e r t h a n with modes of sensitization a) a n d p) which give i n t u r n v e r y similar r e s u l t s whatever t h e t y p e of sensitizer, aromatic carbonyl BP o r pyrylium salt 1P. I n c r e a s e s in practical sensitivity with f a c t o r s in t h e r a n g e from 4 to 11 could be measured with s u c h systems.
6P
5 a)
3P
I 5 10 t
I 10 t/mn
b)
Fig. 10. Variations of t h e conversion r a t e of polysiloxane I11 with p e n d a n t cinnamic g r o u p s (44 mol%) sensitized b y benzophenone ( a ) or pyrylium salt derivatives ( b ) in various manners, t h e percentage of sensitizer being c o n s t a n t ( 5 wt %) u n d e r t h e polychromatic light of a 900 Watts Xenon lamp.
CONCLUSION
We have described a general method for functionalisation of polysiloxanes which allowed us to prepare a variety of liquid photocrosslinkable resins. Some of the properties of these materials have been examined with a view to their practical application. A more basic study based on kinetic measurements points out some special features of photodimerisation of polysiloxane-bound chromophores. Other applications of photochemical interest have been developed. Multifunctional polysiloxanss with pendant acrylic groups have been used as source of radically polymerisable monomer in UV-curable resins. Grafting on silicone chains mesogenic side-groups together with dyes and photocrosslinkable esters affords novel material for non linear optics (ref.26). Finally, macromolecular but liquid photoinitiators of the polymerisation of acrylic monomers have been prepared by grafting aromatic carbonyl compounds onto silicone main chains. The study of the photochemical processes occurring with these efficient systems in the presence of amino compounds are currently in progress.
REFERENCES
1) H. INOUE: AND S. KOHAMA. J . Appl. Polpz. Sci. 29, 877 (1984) 2) Y. Chujo, K. MURAI, Y. YAMASNITA and Y. OKUMURA,
bf3h-ODO1.
Chem. 186, 1203
3) H. FINKEIMANN and G. REHAGE. Mokromol. Chem. Repid Commun. 1, 31 (1980)
(1985)
X. COQUERET e t a l
1614
4) U. MELLER, H.J. TIMPE, H. ROSLER, P l a s t e Kautsch 34, 1134, (1987) 5 ) E. LUKEVICS, Z.V. BELYAKOVA, M.G. POMERANTSKA and M.G. VORONKOV. Organometal. Chem. Rev. 5, I , (1977). P.N. RYLANDER. Organic Syntheses w i t h Noble Metal C a t a l y s t , p . 274, Academic P r e s s , New York (1973)
6) S. PENCZEK P. KUBISA, K. MATYJASZEWSKI, Adv. Polpn. Sc. 68/68, Catioriic Ring Opening Polqrmerization, Part II : S y n t h e t i c App.lications pp. 216-225, Springer Verlag, Berlin, (1935) - L/ WILCZEK, J. CHOJNOWSKI, Macromolecules 14, 9, (1981,); Makromol. Chem. 184, 77, (1983) : J. CHOJNOWSKI, S. RUBINSZTAJN, L. WILCZEK, Macromolecules 20, 2345 (1987) - P. SIGWALT, G. SAUVET, Polym. J, 12, 651 (1980) ; J.J. LEBRUN, G. SAUVET, P. SIGWALT, Makromol. CJiem., Rapid Commun., 3, 7
