Coreactive Photoinitiators for Surface Polymerization - ACS Publications

Chapter 9

Coreactive Photoinitiators for Surface Polymerization

Downloaded by MONASH UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: December 28, 1990 | doi: 10.1021/bk-1990-0417.ch009

M. Koehler andJ.Ohngemach E. Merck, P.O. Box 4119, D-6100 Darmstadt 1, Federal Republic of Germany

Coreactive photoinitiators are prepared by the intro duction of a reactive functionality to the (2-hydroxy2-propyl)phenone initiator unit. Triethoxysilyl, epoxy and azido groups cover a broad range of coreactivity toward different substrates and surfaces. Triethoxy silyl substituted initiators give a covalent coupling reaction on silica gel as a model. By this method the photoinitiator activity is transfered to the solid support and a surface polymerization on such modified silicas becomes possible, undiluted monomers (DMAEMA, NVP) and their ethanolic solutions are investigated in photopolymerizations under unstirred conditions. The amount of polymer which segregates on the silica is regulated by the monomer concentration and the initia tor content of the silica is also of importance in the case of monomer solutions. The extent of the initia tion and polymerization on silica (measured as weight percent polymer of the solid material) is explained by a radical transfer which depends on the intermolecular distance of the reactants.

Most of the photoinitiators for UV curing of acrylate based formula tions belong to the chemical class of aromatic ketones which are able to generate reactive r a d i c a l s by an UV radiation induced process. 2-Hydroxy-2-methylpropiophenone l a i s known for about ten years as an e f f e c t i v e photoinitiator (1). A benzoyl and a (2-hydroxy-2propyl) r a d i c a l are formed by a photochemical cL -cleavage ( 2 - 4 ) .

0097-6156/90/0417-0106$06.00/0 ο 1990 American Chemical Society

In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

9. KOEHLER & OHNGEMACH

Photoinitiators for Surface Polymerization

The (2-hydroxyethoxy) substituted phenone lb and i t s acrylate Ic are examples of more advanced (2-hydroxy-2-propyl)phenone initiators (5, 6). 0 CH lb, R = 0-CH-CH OH (ethanol) ^ •> ν

Ig

N

_ N -CH V OOOH 3 2 ~-CH 2 ~-0 - ^^=7" _ C H o

o C H

o

U

The acid should be formed by an ck -cleavage with subsequent oxida tion and hydrogen abstraction of the resulting radical. Ihe azido group shows a remarkable stability under these conditions but the situation might be different in a UV-curable formulation.


Coupling of initiators to s i l i c a Coreactive photoinitiators seem to be promising tools to investigate UV induced processes of ketone structures with a permanent bond to solid substrates. Under the conditions of the thermal substrate coup ling (coreaction) the photoinitiator activity should be conserved and a subsequent photochemical cL-cleavage in the presence of a suit able monomer should initiate a polymerization process. Several alkoxysilane reagents have been coupled recently to s i lica and related materials (1Q-.12,) but the introduction of a photoinitiator group to a surface has not been reported so far. Silica gel in a quality which i s commonly used for column chromatography (par ticle distribution 40-63 ym) was selected as a starting material and the triethoxysilane initiators Id,e were applied at two different concentrations to influence fche initiator loading of the s i l i c a . A certain amount of the silanes Id,e will react on heating and the ex cess i s removed by repeated solvent washing. The reaction might be due to condensation with silanol groups and polysiloxane formation. OH + (C H 0)SUCH ^ 0(CH -CH -O^R 2

J \ ^ S

I

°

2

J

^

5

2

3

2

2

R :^V)-C-Ç-OH

I heating

O-Si«CH ),-0(CH -CH -0)- R 0

o

3

o

$

It i s assumed that the ethoxy groups are lost by partial hydrolysis, condensation with surface silanols and by self-condensâtion. Initial amounts of 0.5 and 0-1 weight equivalent silane Id,e per s i l i c a were used in the preparation of the four silicas Ila-d. Ihe structure of coupled agents was investigated by diffuse reflectance Fourier Transform Infrared spectroscopy and i t has been outlined that a weak sharp band around 3740 cm i s due to the free Si-OH surface group (10). Such an absorption was not detectable in the i . r . spectjrum of s i l i c a l i a but i t i s obvious in the case of l i e (at 3738 cm ) due to the presence of unreacted silanol groups (Figure 1). Elemental analysis (e.a.) and thermal gravimetric analysis (t.g.a.) were used as alternative methods to calculate the percentage photoinitiator species per s i l i c a (Table II). An inaccuracy in the results of the elemental analysis i s possible by the influence of adsorbed ethanol (from the washing process) on the carbon values. In the thermal gravimetric analysis absorbed species should be removed below 200 °C.


r

KOEHLER & OHNGEMACH

PhotoimtiatorsforSurj ace Polymerization

Table I. Pendulum Hardness o f UV Cured Layers (measured according to DIN 53 157) Depending on I n i t i a t o r Compound and I r r a d i a t i o n Time Initiator


Irradiation Time (s)

la

Id

Ie

Remark

Compound If

ig

-

(a)

1.0

205

111

127

178

2.5

214

155

213

216

42

(a)

5.0

215

214

216

220

88

(a)

213

193

(a)

10.0

208

209

210

(a) Pendulum Hardness, i n seconds.

Table I I . P h o t o i n i t i a t o r Loading o f the Modified S i l i c a s I I a-d (The weight equivalent s i l a n e I d,e a f f e c t s the f i n a l initiator content.)

Weight Percent I n i t i a t o r on S i l i c a by e.a. by t . g. a. 12.9

Ha

η

Weight Equivalents (a) Id,e per S i l i c a

Ha

0

0.5 Id

11.9

lib

1

0.5 Ie

11.9

lib

12.2

He

6.3

7.4

He

0

0.1 Id

8.5

8.8

Hd

1

0.1 Ie

Hd

(a) g I n i t i a t o r

/ g Silica


111

RADIATION CURING OF POLYMERIC MATERIALS


112

+ la) extract but the carbon value of the extracted s i l i c a was not decrea sed (probably by the adsorption of ethanol at refluxing). Ihe i . r . spectrum of this s i l i c a was almost identical with a reference of un treated material Si 60. A different behaviour was observed with the epoxide derived ma terial ( Si0 + If ). Ihe carbon value i s decreased after extraction but the characteristic initiator absorptions are s t i l l present in the i . r . spectrum. A new compound which was identified as the water adduct of the epoxide If i s found in the ethanolic extract by t.l.c. The trihydroxy compound was also formed by refluxing the epoxide If with water under acidic conditions in the absence of s i l i c a . 2

2

2

2

It i s assumed that the nonextractable part is due to such highly po lar initiator species derived from the epoxide by ring opening. Ihe interaction to the s i l i c a should be based mainly on physical forces (hydrogen bonding).



19

1.22

ι 1.35

I

\

46.

X

1

mg)

rc

148'. 8*C"

11 .91 % 2 mg)

—ft

Ν

\

\y V\ s,


\

\

\\ Πβ sldi β: χ

-AO

Si l i :a 11< ι (

1C 0

2C 0

3()0

4C10 5C>o Temperature

6()0 (*C)

7C 0

8C)0 T6A

9C 0

10

VI.IF

Dupont 990

22 0

3 43L toll

3 .80 X S00*C Yifr42 mg\

iilfca! Jit

100

200

300

400 500 Temperature

600 CC)

700

BOO TGA

900

1000

VI.IF

Dupont 990

Figure 2. Thermal gravimetric analysis (t.g.a.) of the modified silicas Ha and lie. The percent photoinitiator per silica is directly measured by the weight loss of a sample during controlled heating in an air stream.



Photoinitiatorsfor Surface Polymerization

115

Surface polymerization on photoinitiator mcdified s i l i c a The silicas Ila-d were used for photopolymerization of monofunctional monomers which were applied as undiluted liquids or as ethanolic solutions. Several monomers were tested f i r s t without a solvent on their ability to polymerize. CH=CH-COO-CH-CH-OH

(2-Hydroxyethyl)acrylate HEA

CH =-^-èoO-CH -CH -OH

(2-Hydroxyethyl)methacrylate HEMA

2

2

2

2

2

2

CH =CH-C H Downloaded by MONASH UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: December 28, 1990 | doi: 10.1021/bk-1990-0417.ch009

2

6

Styrene

5

ÇH. CH =0-COO-CH -CH -N(CH ) 2

2

H C—CH 2 2 0

2

3

(2-Dimethylaminoethyl)methacrylate DMAEMA

2

0

χ

H

/

CO (N-Vinyl)pyrrolidone NVP CH=CH

2

HEA underwent a vigorous polymerization reaction on irradiation i n the presence of s i l i c a II which led to a sticking and lumping on par ticles. Such a process can hardly be controlled and the resulting material was not very useful for a quantitative characterization (e.a., weight increase). Nevertheless the reaction i s due to the photoinitiator because in a control experiment with untreated s i l i c a no polymerization took place. The corresponding methacrylate HEMA did not react and the s i l i c a II was recovered unchanged (but s t i l l active toward other monomers). Styrene also did not polymerize under the applied conditions. Satisfactory results were obtained with the nitrogen containing monomers DMAEMA and NVP. The poly-DMAEMA (Ilia) and poly-NVP (Illb) silicas with more than 30 weight percent polymer conserve the s i l i c a morphology and were easy to separate from the monomer excess. Ο CH 0

U \

3

-C-C-OH \ ^ 0-Si4CH 4 0(CH -CH -0) --^3--C I SiO - 7 I 23 2 2 η W CH ? / \ l l a - d (η = 0,1) AEMA ^ Χ ^ o

Q

9

9

M

?

2

3

y

+

Λ ^ - ( poly-DMAEMA) Si0

9

2

/ /ilia

^ (poly-NVP) Si0

9

Hlb

As an additional result i t was confirmed that photoinitiator silicas II which are derived from silanes Id,e with different spacers show an almost identical behaviour. To simplify the further discussion



116

t h e f o r m e r f o u r s i l i c a s I l a - d c a n be d e s i g n a t e d as o n l y two d i f f e r e n t m a t e r i a l s : IIa,b = I l h (higher i n i t i a t o r content) I I c , d = I I I (lower i n i t i a t o r c o n t e n t )


U n d i l u t e d DMAEMA and NVP From t h e w e i g h t p e r c e n t poly-DMAEMA on s i l i c a formed under condensed phase c o n d i t i o n s ( u n d i l u t e d DMAEMA) a minor i n f l u e n c e o f t h e i n i t i a t o r s i l i c a t y p e and t h e i r r a d i a t i o n time became a p p a r e n t ( T a b l e I V ) . The polymer c o n t e n t o f t h e m a t e r i a l s I l i a was f o u n d i n c l o s e r a n g e between 30 and 37 wt % and i t seemed t o be i m p o s s i b l e t o i n c r e a s e t h e polymer l o a d i n g b y extended i r r a d i a t i o n . As a p r o o f on the v a l i d i t y o f the experimental c o n d i t i o n s s e v e r a l p o l y m e r i z a t i o n s were r u n w i t h u n d i l u t e d DMAEMA on t h e s i l i c a s l l h , l and an average v a l u e o f 34.1 wt % w i t h a s t a n d a r d d e v i a t i o n o f 1.71 was c a l c u l a t e d as a maximum c a p a c i t y o f t h e s i l i c a f o r poly-DMAEMA. On SEM images ( o b t a i n e d w i t h a H i t a c h i Model S-570 S c a n n i n g E l e c t r o n M i c r o s c o p e ) no s i g n i f a c a n t d i f f e r e n c e was o b s e r v e d between t h e m o r p h o l o g i e s o f a t y p i c a l m a t e r i a l I l i a and normal s i l i c a g e l a t d i f f e r e n t m a g n i f i c a t i o n s i n a range from 150 t o 1 urn. I t i s c o n c l u d e d t h a t t h e polymer c o a t i n g on t h e s i l i c a i s homogeneous and a considerable part o f the polymeric m a t e r i a l i s l o c a t e d at t h ^ inner s u r f a c e ( s p e c i f i c s u r f a c e o f S i 60 i s g i v e n (14) a t 550 m g ). In t h i s c a s e t h e s u r f a c e p o l y m e r i z a t i o n on p h o t o i n i t i a t o r modif i e d p a r t i c l e s o c c u r e d w i t h o u t a g g l o m e r a t i o n and t h e p o l y m e r i z a t i o n seems t o be r e s t r i c t e d m a i n l y on t h e p a r t i c l e s u r f a c e ( i m m o b i l i z e d polymer) b u t t h e f o r m a t i o n o f a s o l u b l e polymer ( d i s s o l v e d i n t h e monomer e x c e s s ) cannot be e x c l u d e d . T h i s b e h a v i o u r depends a l s o on t h e c h e m i c a l s t r u c t u r e o f t h e monomer (compare a g g l o m e r a t i o n w i t h HEA) and i t i s an o b j e c t i v e o f f u r t h e r work t o i n v e s t i g a t e v a r i o u s monomers. The a c t i v i t i e s o f t h e s i l i c a s ( S i 0 + l a ) and ( S i 0 + I f ) t o ward DMAEMA were examined f o r c o m p a r i s o n . S u r p r i s i n g l y t n e y p r o v i d e s i m i l a r amounts o f polymer as o b t a i n e d w i t h t h e s i l a n e m o d i f i e d mat e r i a l s I I h , l and t h e polymer was n o t e x t r a c t a b l e from t h e s i l i c a . After the intensive extraction (refluxing with ethanol overnight) t h e s i l i c a ( S i 0 + l a ) l o s t most o f i t s i n i t i a t o r a c t i v i t y when i t was u s e d a g a i n i n a p h o t o p o l y m e r i z a t i o n e x p e r i m e n t . The amount o f poly-NVP on t h e m a t e r i a l s I l l b i s f o u n d i n a range comparable t o t h e I l i a s e r i e s above. The m a c r o s c o p i c shape o f t h e mat e r i a l s I l i a and b i s a l s o v e r y s i m i l a r . The i . r . s p e c t r a o f t h e components I l i a and I l l b a r e r e p r e s e n t e d i n F i g u r e 3. I h e spectrum o f I l l b s t i l l shows a l e s s i n t e n s i v e a b s o r p t i o n o f t h e i n i t i a t o r c a r b o n y l g r o u p a t 1643 cm" . I n t h e spectrum o f I l i a t h e a b s o r p t i o n o f t h e w a v e l e n g t h 1721 cm" i s a t t r i b u t e d tOj p o l y DMAEMA and i n t h e spectrum o f I l l b t h e a b s o r p t i o n a t 1706 cm" i s a t t r i b u t e d t o poly-NVP. In g e n e r a l from t h e s e e x p e r i m e n t s w i t h u n d i l u t e d DMAEMA and NVP i t became o b v i o u s t h a t t h e p o l y m e r i z a t i o n p r o c e s s which tends t o a c e r t a i n l i m i t o f polymer formed on t h e s i l i c a does n o t depend much on t h e amount o f p h o t o i n i t i a t o r . The i n i t i a t i o n ( r a d i c a l t r a n s f e r from t h e i n i t i a t o r r a d i c a l p a i r t o t h e n e i g h b o u r monomer) and s u b s e quent p o l y m e r i z a t i o n ( r a d i c a l c e n t e r t r a n s f e r along the growing p o l y mer c h a i n ) a r e promoted b y t h e c l o s e c o n t a c t o f t h e m o l e c u l e s . 9

2

2


KOEHLER & OHNGEMACH

Photoinitiotors for Surface Polymerization

T a b l e I I I . I n t e r a c t i o n o f t h e P h e n o l e s l a , I f and DMPA w i t h


SiO_

/ \X j

l a /

(Si0

f

/

\

2

2

+ la) (Si0

+ If)

Silica

Materials

% Carbon (Initiator)

% Carbon ( a ) (Initiator)

(Si0

+la)

2.5 (3.4)

3.1 (-)

7.8 (11.4)

1.4 (7.0)

-

-

2

(Si0

2

+I f )

(Si0

2

+ DMPA)

(a) a f t e r r e p e a t e d e x t r a c t i o n ( r e f l u x i n g w i t h e t h a n o l o v e r n i g h t )

T a b l e I V . Weight P e r c e n t Poly-DMAEMA on S i l i c a I n i t i a t o r S i l i c a Type and I r r a d i a t i o n Time Photoinitiator Modified Silica/Monomer

Irradiation Time ( m i n )

Depending on

Weight P e r c e n t Polymer o n S i l i c a

I l h / DMAEMA

3

32.0

I l h / DMAEMA

1

30.9

I l h / DMAEMA

10

35.9

I I I / DMAEMA

3

32.6

(Si0

+ l a ) / DMAEMA

3

35.4

2

(Si0

+ I f ) / DMAEMA

3

37.1

2

I l h / NVP

3

33.3

I I I / NVP

3

36.9

(Si0

3

40.1

2

+ I f ) / NVP


118



4000

Figure 3. 1

3S00

3000

2500 2000 VRVENUMBERS Crt-1

1500

1000

Infrared spectra of the materials Ilia and Illb. Tire absorbance at 1

1721 cm" (Ilia) is attributed to poly-DWEW and at 1706 cm" (Illb) to poly-NVP.




119

Initiation by radical transfer: For the process i t should not be very important which radical (R# or *R ) will primarily react with a monomer molecule. A covalently surface-bound polymer only results through initiation by the radical R..


1

Z

R » »R initiator radical^pair formed by photochemical oC-cleavage ^ of the molecule R — R _ y monomer, ^\~~^~*^ oligomer radical For such a process i t i s not necessary to assume a considerable motion or diffusion of the reacting species which i s less probable under the applied conditions. Recently the photoinitiated polymerization of multifunctional monomers in condensed phase (usually called UV curing) was characterized as an extremely fast reaction with a kinetic chain length in the order of 100 000 acrylate functions per initiating radical ( 15.). The results of the photoinitiated surface polymerization indicate short chain lengths which might be due to a high degree of radical combination on the porous substrate (cage conditions). Monomer dilution In the presence of a solvent (ethanol) a different situation was expected because the average distance between the reacting molecules (initiator-monomer as well as monomer-monomer) i s enlarged. Ethanol was considered as a diluent only, other influences on the reaction are excluded. The resutls, listed in Table V, were obtained from the i n i t i a tor s i l i c a Ilh with the higher loading and DMAEMA concentrations in the range between 50 and 10 volume percent monomer. The final polymer content of the resulting materials I l i a ' i s remarkably decreased i n comparison to I l i a . A similar dilution effect i s noted with NVP solutions. For a comparison see the results with 100 % (undiluted) monomers: Ilh,1/DMAEMA and Ilh/NVP. In the i . r . spectrum of a material I l i a ' (12.9 wt % poly-DMAEMA on silica)_^he typical photoinitiator carbonyl absorption i s present at 1642 cm (Figure 4). Such a material should be s t i l l active toward undiluted DMAEMA and NVP i f the photoinitiator group i s accessible to the applied monomer. Ihe corresponding photopolymerization experiments provided the materials I l i a and Illa'/b". With DMAEMA the polymer content was increased to 32.6 wt % poly-DMAEMA (Ilia) and NVP led to a composite Illa'/b' with 27.7 wt % poly-NVP.



120

T a b l e V. Weight P e r c e n t Poly-DMAEMA ( I I I a ' , a " ) ( I I I b') on S i l i c a O b t a i n e d w i t h Monomer S o l u t i o n s

and Poly-NVP

(C H OH) 2


Ilh

+

5

DMAEMA

-*

wt % poly-DMAEMA (on s i l i c a )

v o l . % DMAEMA (in ethanol)

Ilh

Ilia'

50

28.0

30

21 .9

20

17.4

10

12.9

+

NVP

(C H OH) 2

5

—•

Illb" wt % p o l y - N V P (on s i l i c a )

v o l % NVP (in ethanol) 50

30.2

30

26.9

20

19.8

10

15.1 (C H OH) 2

III

+

DMAEMA

v o l . % DMAEMA (in ethanol) 50

5

Ilia'' wt % poly-DMAEMA (on s i l i c a ) 6.7

30

3.9

20

2.8

10

2.2


KOEHLER & OHNGEMACH



9.


121

122


Second p o l y m e r i z a t i o n o n m a t e r i a l I l i a ' w i t h u n d i l u t e d DMAEMA and NVP


Ilia'

The i n f l u e n c e o f r e d u c e d i n i t i a t o r l o a d i n g became o b v i o u s i n e x p e r i ments w i t h t h e i n i t i a t o r s i l i c a I I I and DMAEMA s o l u t i o n s . The p o l y DMAEMA c o n t e n t s o f t h e r e s u l t i n g m a t e r i a l s I l i a ' a r e much l o w e r comp a r e d t o I l i a . The d e c r e a s e d number o f i n i t i a t i o n c e n t e r s on t h e s i l i c a s u r f a c e t o g e t h e r w i t h t h e monomer d i l u t i o n r e s u l t i n a r e d u ced p r o b a b i l i t y f o r a s u c c e s s f u l r a d i c a l t r a n s f e r ( i n i t i a t i o n ) . I n g e n e r a l t h e e x p e r i m e n t s w i t h monomer s o l u t i o n s p r o v i d e deeper i n s i g h t t o t h e mechanism o f t h e p h o t o i n i t i a t i o n a t t h e s i l i c a / m o n o m e r i n t e r f a c e . A d i f f u s i o n a l mixing o f t h e r e a c t i n g molecules can be exc l u d e d i f t h e i n i t i a t o r i s c o v a l e n t l y anchored t o a s o l i d m a t e r i a l . Ihe s i t u a t i o n i s d i f f e r e n t f r o m a homogeneous r e a c t i o n w h i c h i s u s u a l l y p e r f o r m e d b y s t i r r i n g t h e s t a r t i n g m a t e r i a l s . I n t h e absence o f s t i r r i n g and under t h e d i m e n s i o n a l c o n s t r a i n t o f a s u r f a c e r e a c t i o n t h e i n i t i a l c o n d i t i o n s and t h e n e a r e s t - n e i g h b o u r distance o f t h e r e a c t a n t s p e c i e s become o f i m p o r t a n c e (Jj6 ). 1

1

The n e x t r e a c t i o n scheme d e s c r i b e s t h e d i f f e r e n t b e h a v i o u r o f two s u r f a c e c o u p l e d i n i t i a t o r r a d i c a l p a i r s and randomly d i s t r i b u t e d monomer m o l e c u l e s a t t h e i n t e r f a c e .

d i l u t e d monomer (S = s o l v e n t ) polymerization

polymer f o r m a t i o n and recombination

condensed phase polymerization

I f an i n i t i a l r a d i c a l p a i r g e t s no o p p o r t u n i t y f o r a r a d i c a l t r a n s f e r i t w i l l recombine a f t e r a c e r t a i n l i f e t i m e . By monomer d i l u t i o n t h e i n t e r m o l e c u l a r d i s t a n c e between t h e i m m o b i l i z e d i n i t i a t o r and



Photoinitiators for Surface Polymerization

t h e monomers i s enhanced. The d e g r e e o f p o l y m e r i z a t i o n i s l i m i t e d and a c o n s i d e r a b l e p a r t o f recombined i n i t i a t o r i s s t i l l a v a i l a b l e . A s e c o n d i n i t i a t i o n o n s u c h a m a t e r i a l ( I l i a ) was p o s s i b l e b y c h a n g i n g t o condensed phase c o n d i t i o n s . The r a d i c a l t r a n s f e r ( i n i t i a t i o n ) and t h e subsequent p o l y m e r i z a t i o n a r e f o r c e d now b y t h e r e d u c e d i n termolecular distance o f the species. 1


Conclusions From t h e f i r s t e x p e r i m e n t s w i t h s u r f a c e i m m o b i l i z e d i n i t i a t o r s ( d e r i ved from c o r e a c t i v e p h o t o i n i t i a t o r s ) some g e n e r a l a s p e c t s became apparent. A r a d i c a l c h a i n process which i s s t a r t e d on t h e s o l i d s u r f a c e r e s u l t s i n a s e g r e g a t i o n o f o r g a n i c m a t e r i a l ( u s u a l l y a polymer) w i t h a s t r o n g i n t e r a c t i o n t o t h e s u b s t r a t e . The s u r f a c e i n i t i a t i o n may b e a v e r y complex p r o c e s s from a m e c h a n i s t i c p o i n t o f v i e w b u t t h e p r a c t i c a l a p p l i c a t i o n a l r e a d y becomes o b v i o u s . S u r f a c e m o d i f i c a t i o n s o f p a r t i c l e s w h i c h a r e commonly u s e d a s m a t e r i a l s f o r chromatography, a s pigments o r f i l l e r s a r e o f c o n s i d e r a b l e t e c h n i c a l i n t e r e s t . By t h e s e l e c t i o n o f a p p r o p r i a t e monomers and t h e i r s u r f a c e p o l y m e r i z a t i o n d i f f e r e n t c h e m i c a l functionalities o r p h y s i c a l p r o p e r t i e s a r e i n t r o d u c e d . S i l i c a g e l w i t h an e x t e n d e d s u r f a c e a r e a ( S i 60, Merck) i s a u s e f u l s u b s t r a t e f o r t h e s t u d y o f p h o t o i n i t i a t e d s u r f a c e p o l y m e r i z a t i o n s b u t l e s s porous particles, microcrystalline materials or f i b e r s are the objects o f investigat i o n s w i t h a more p r a c t i c a l b a c k g r o u n d . In p r i n c i p l e m a c r o s c o p i c s u r f a c e s o f d i f f e r e n t c h e m i c a l nature s h o u l d b e a c c e s s i b l e t o c o r e a c t i v e i n i t i a t o r c o u p l i n g and s u b s e q u e n t photopolymerization. The c a p a c i t y o f a m a c r o s c o p i c surface like g l a s s , m e t a l , c e r a m i c s o r p l a s t i c s f o r i n i t i a t o r c o u p l i n g might b e v e r y low compared t o p o r o u s s i l i c a g e l . N e v e r t h e l e s s few i n i t i a t i o n c e n t e r s c o u l d l e a d t o a h i g h degree o f p o l y m e r i z a t i o n e s p e c i a l l y i f t h e monomer i s a p p l i e d a s condensed p h a s e . Acknowledgment The A n a l y t i c a l C e n t r a l L a b o r a t o r y o f E . Merck i s g r a t e f u l l y acknowledged f o r t h e performance o f thermal g r a v i m e t r i c a n a l y s i s , d i f f u s e r e f l e c t a n c e i n f r a r e d s p e c t r a and e l e m e n t a l m i c r o a n a l y s i s o f t h e s i l i ca based m a t e r i a l s . Literature

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1. J. Gehlhaus and M. Kieser, German Pat. 2722 264 (1978) 2. J. Eichler, C.P. Herz, J. Naito and W. Schnabel, J. Photochem. 12, 225 (1980) 3. A. Salmassi, J. Eichler, C.P. Herz and W. Schnabel, Polym. Photochem. 2, 209 (1982) 4. J.P. Fouassier, P. Jaques, D.J. Lougnot and T. Pilot, "Lasers, Photoinitiators and Monomers: a Fashionable Formulation", Photochemistry and Photophysics in Polymers, Elsevier Appl. Sci. Publ. London, 1984, p. 57 5. M.Koehler, J. Ohngemach and G. Wehner, German Pat. Appl. 3512179 (1985)


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7. M. Koehler, J. Ohngemach, E. Poetsch, R. Eidenschink, G. Greber, D. Dorsch, J. Gehlhaus, K. Dorfner and H.-L. Hirsch, Europ. Pat. Appl. 0 281 941 (1988) 8. L.N. Lewis and N. Lewis, J. Amer. Chem. Soc. 108, 7228 (1986) 9. M. Koehler and J. Ohngemach, Polymers Paint Colour J. 178, 203 (1988) 10. S.R. Culler, H. Ishida and J.L. Koenig, J. Colloid. Interface Sci. 106, 334 (1985) 11. J.D. Miller and H. Ishida, Langmuir 2, 127 (1986) 12. Y. Okahata, K. Ariga, H. Nakahara and K. Fukuda, J. Chem. Soc., Chem. Commun. 1986, 1069 13. C.S. Goldstein, K.D. Weiss and R.S. Drago, J. Amer. Chem. Soc. 109, 758 (1987) 14. E. Wellner, D. Rojanski, M. Ottolenghi, D. Huppert and D. Avnir, J. Amer. Chem. Soc. 109, 575 (1987) 15. C. Decker and K. Moussa, J. Appl. Polymer Sci. 34, 1603 (1987) 16. R. Kopelman, Science 241, 1620 (1988) RECEIVED October 27, 1989


Coreactive Photoinitiators for Surface Polymerization - ACS Publications

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