Synthesis and Photoinitiated Cationic Polymerization of Monomers

Chapter 18

Synthesis and Photoinitiated Cationic Polymerization of Monomers Containing the Silsesquioxane Core 1

James V. Crivello 1

2

and Ranjit Malik

Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY 12180 Adhesives Research, Glen Rock, PA 17327

Downloaded by UNIV OF PITTSBURGH on June 12, 2016 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch018

2

A synthetic scheme was developed to prepare c a t i o n i c a l l y polymerizable octafunctional monomers with cubic silsesquioxane (T ) cores. E p o x y and 1-propenoxy functional groups were attached 8

H

to the core by the hydrosilation of T8 with an appropriate precursor. The steric constraints and the requirements for the hydrosilation reaction are discussed. The monomers were fully characterized and then polymerized by exposure to ultraviolet irradiation in the presence of onium salt photoinitiators. The polymerization conditions for the monomers were optimized and compared with each other using real-timeinfrared spectroscopy. Thermal analysis was also performed on the resulting crosslinked polymers.

In recent years, there has been a great deal of interest i n functionalizing silsequioxanes to prepare interesting monomers and polymers. ' Bassindale and G e n t l e hydrosilated a l l y l - and vinyl-functional molecules onto the hydrogen functional silsesquioxane (Ts ) cage to obtain eight arm "octopus" molecules. Sellinger and L a i n e employed a similar strategy to prepare crosslinkable v i n y l functional silsesquioxane monomers. Previously, we have reported on the synthesis of siloxane containing epoxy resins for coatings and composite applications . These resins were prepared by the hydrosilation of v i n y l epoxides onto siloxane substrates bearing S i - H groups and these resins displayed exceptional reactivity on exposure to ultraviolet or electron beam radiation i n the presence of onium salt inititors . The present work focuses on the synthesis of novel multifunctional epoxy and v i n y l ether monomers bearing the Ts silsesquioxane core and a study of their behaviour under photoinitiated cationic polymerization conditions. 1

2

3

H

4

5

6

284

© 2000 American Chemical Society

Clarson et al.; Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

285

Results and Discussion. H

Synthesis and Characterization of Monomers. T s was the key starting material for the preparation o f a series o f cationically photopolymerizable multifunctional monomers prepared during the course o f this work. Previously, Bassindale and G e n t l e had prepared this compound by the ferric chloride catalyzed hydrolysis o f trichlorosilane i n solution. A 12% yield was obtained. W e have found that a higher yield (23%) can be obtained when this reaction is carried out under essentially high dilution conditions. This is achieved by passing a stream o f nitrogen saturated with trichlorsilane vapor into a methanolic solution solution of ferric chloride. 3


H ,Si

O—Si

^ S f - - 0 — S i - Tr H

oH.:

o

!

T.O«-°1-^H T

H 8

Octasubstituted silsesquioxane I was prepared as a model compound by the hydrosilation o f 1-octene with T s (eq. 1). H

I

eq. 1 H

The hydrosilation o f 4-vinylcyclohexene oxide with T s using W i l k i n s o n ' s catalyst was carried out under a variety o f conditions. Even in the presence o f excess epoxide, only four o f the eight S i - H bonds could be successfully hydrosilated onto the Tg core (eq. 2). H

II

eq. 2

The product, II, is presumed to be symmetrically substituted, although this has not been fully established. M o r e aggressive hydrosilation conditions resulted i n the opening o f the epoxide groups with consequent gelation. W e ascribe the inability to fully functionallize the remaining S i - H groups as due to steric hindrance. Indeed, computer modeling o f the the target octasubstituted molecule revealed it to be highly hindered due to the presence of the bulky epoxycyclohexyl groups. In contrast, the hydrosilation o f T s with the linear, open chain epoxide, 1,2epoxy-5-hexene took place to give the fully octafunctional epoxide I I I . H


286

H


Since steric hindrance appeared to be a major factor in the hydrosilation of Ts , we prepared the novel vinyl epoxy compound V shown below in which the vinyl and a bulky cycloaliphatic epoxy groups are separated by a spacer group. r^N

NaH

Hv

H

reflux IV

eq. 3

H

Hydrosilation of the above compound IV with T s using the platinum containing Karstedt's catalyst followed by epoxidation with 3-chloroperoxy ben zoic acid yielded octafunctional epoxy monomer V with a Tg core.

It was also observed that a similar strategy was effective for the synthesis of monomer VI shown below.

VI H

Condensation o f T s w i t h 4 - v i n y l c y c l o h e x e n e gave an intermediate silsesquioxane compound bearing eight ethylcyclohexene groups. Thus, reducing the steric bulk o f 4-vinylcyclohexene oxide by removal of the epoxy group was effective i n achieving a fully functionallized core. Subsequent epoxidation o f the


287

cyclohexenyl groups using O x o n e ® readily gave the desired monomer V I bearing eight epoxyeyelohexyl groups. In addition to epoxy functional silsesquioxane monomers, we wished to also examine analogous monomers bearing even more reactive functional groups. In recent years, we have found that 1-propenyl ether monomers display exceptional reactivitiy in photointiated cationic polymerization. * Moreover, these monomers are easily prepared by straightforward synthetic reactions. Accordingly, the precursor, 1propenoxy-2-vinyloxyethane, V I I I , was prepared using the methods shown i n equations 3 and 4. The key step i n this reaction sequence is the catalytic isomerization of the allyl ether V I I using (Ph3P)3RuCl2-


7

HO-CH CH -0-CH=CH + Br-CH -CH=CH L2

2

z2

z

2

L

8

a

Q

H

^ » benzene

2

CH =CH-0-CH CH -0-CH -CH=CH 2

2

2

2

VII x7ti

(Ph P) RuCl 3

3

2

eq. 4

2

154°C

*

CH =CH-0-CH CH -0-CH=CH-CH 2

2

2

VIII

3

eq. 5

H

The hydrosilation o f T g with l-propenoxy-2-vinyloxyethane V I I I takes place regeoselectively at the v i n y l group. A l l eight S i - H groups o f Tg react to give an octafunctional monomer I X bearing cationically polymerizable 1-propenyl ether groups. This monomer is a mixture of isomers due to cis and trans isomerism at the terminal 1-propenyl ether groups. 9

u

IX C h a r a c t e r i z a t i o n o f M o n o m e r s . A l l the monomers and the model compound I bearing the Tg silsesquioxane core were characterized by * H and C NMR spectroscopy, by gel permeation chromatography as well as by elemental analysis. Typical N M R spectra are shown in Figure 1 for model compound I and Figure 2 for monomer I I I . Definitive assignments for each o f the resonances in the H N M R spectra can be made based on model compound I. The C N M R spectrtum of I I I (Figure 2 B ) consists of two silicon resonances at 66.6 ppm and 66.0 ppm which correspond respectively, to silicon atoms which have undergone hydrosilylation at the 1

3

l

1 3

a - and (3-carbon atoms of the double bond. A l l the monomers, with the exception of monomer V I were liquids. The fact that these monomers are liquids despite their rather high molecular weights can be ascribed to the presence in these monomers of multiple chiral centers and geometrical isomers. Elemental analyses for a l l the monomers are within experimental error limits o f the calculated values. The only exception to this is, again, monomer V I which is within 1-2% o f the theoretical values for both carbon and silicon. Further insight into the structure of the monomers is provided by G P C analysis. The results of that study are shown in Figure 3. It may be noted that the G P C traces for all the monomers indicate that i n addition to the Clarson et al.; Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2000.


288



289

Figure 2. a ^ H , b)

1 3

2 9

C , c) S i N M R spectra for III.


2.410


3

Time (min) Figure 3. GPC traces of functionalized Ts monomers.


291

expected molar mass which comprises the major component present, there is also a minor component at approximately twice the expected molar mass. This is indicative of the presence of dimers. In the case o f monomer V I , the amount of this dimer is appreciable and there are further indication of the presenceof higher molecular weight species as well. W e speculate that these species arise by the platinum or rhodium reaction of the S i - H groups with water during the hydrosilation reaction to give silanols followed by the further condensation of the silanols to give siloxane bridged Ts cores. W i t h the exception of monomer V I , we have estimated that the other monomers contain less than 3% of such bridged species. Photoinitiated C a t i o n i c P o l y m e r i z a t i o n . The cationic photopolymerization of the monomers synthesized above was studied using real-time infrared spectroscopy (RTIR). This technique involves monitoring the decrease of an IR absorption characteristic of the functional group undergoing polymerization. In these studies, 2 m o l % (4-decyloxyphenyl)phenyliodonium SbFfi" was used as the photoinitiator. Figure 4 gives individual plots of the percent conversion of the various Tg monomers as a function o f time at the optimum photoinitiator concentration for each o f the monomers. The rate of photopolymerization of 1-propenyl ether functional monomer IX is the fastest followed by III, V and VI. The limiting conversions achieved by epoxy and 1-propenyl ether functional Tg monomers range from 78-90%, It is surprising to achieve conversions as high as these for octafunctional monomers. Typically, as higher and higher functional monomers are photopolymerized, the limiting conversions decrease markedly. This is generally attributed to the decrease in mobility of the reactive functional groups as the highly crosslinked network develops. One possible explanation for the present results is that a considerable amount of intramolecular reaction may be taking place. The resemblence of the silsequioxane monomers with polymerizable epoxy and 1propenyl ether functional groups described i n this communication to spherical dendrimeric materials is striking. In both cases, the concentration o f the reactive functional groups at the surface o f each monomer molecule is very high facilitating intramolecular reaction. In addition, since the functional groups are located at the same distance from the core, they are "preoriented" further favoring intramolecular reaction. A t the same time, the extreme steric crowding present in these molecules together with the large steric bulk of each molecule prevents interpenetration of the arms containing the functional groups. This results in diminished intermolecular reaction. It is also surprising to observe that monomer III containing open-chain epoxy groups are more reactive than those monomers (V and V I ) w h i c h have epoxycyclohexyl groups. A g a i n , the larger steric bulk of the arms containing the epoxycyclohexyl groups may inhibit the approach of one reactive functional group to another during the ring-opening polymerization reaction. It should be noted that a cationic epoxide ring-opening polymerization is a SN2 reaction which requires highly specific backside approach of the incoming epoxide group on one o f the carbons bearing the positively charged epoxide oxygen.


1 0

T h e r m a l A n a l y s i s . The weight loss and T g data for the polymers obtained by optimally photopolymerizing all four Tg monomers are given in Table 1. If it is postulated that polymerization o f the octafunctional monomers takes place by predominantly an intramolecular process, then the resulting polymers may not exhibit glass transitions because the rigidity of the resulting highly crosslinked structure may restrict chain mobility. This rationale may explain why a T was not observed for g

photopolymerized samples of V and VI on heating from 4 0 ° C to their decomposition (>350°C). However, T s of 2 3 6 ° C and 2 2 8 ° C were observed for polymers obtained from III and IX respectively. This again supports the hypothesis that the polymers obtained from III and IX have relatively low crosslink densities even at conversions g


292

100


M

M

M

M

N

M

IX

•

80 H

• in A A

A

60

VI

40

20 H

*

o ° —"l

20

40

•

1

60

«

1

80

•

1

' —

100

120

IRRADIATION TIME (sec) Figure 4. RTIR study of monomer conversion as a function of irradiation time.



293

of 90 and 94 % because the majority of the reaction takes place by an intramolecular process. Thermal gravimetric studies of the polymers derived from photopolymerization of four Ts monomers were carried out in nitrogen at a heating rate of 20°C/minute. The results are given in Table 1 and show that the polymer obtained from V I has the highest thermal stability. High weight retentions i n these polymers can be attributed in part to the inherent stability of the polymers, but mainly to the low carbon content of the polymers. 5% Weight losses observed at 366°C and 3 3 9 ° C for polymers from V I and III respectively, can attributed to their high crosslink density as well as to a low hydrocarbon content. If, as has been suggested earlier, III reacts primarily through intramolecular mode and for this reason has a low crosslink density, the only factor contributing to a lower weight loss for this monomer is the low hydrocarbon content of the polymer. The higher weight loss seen for V appears due to fact that it has the highest hydrocarbon content among the four Tg monomers. Similarly, the comparatively poor thermal stability of IX can be explained by the inherently poor thermal stability expected with polymers derived from 1-propenyl ethers. Table 1 Results of the thermal analysis of polymers from Ts monomers. Monomer

TGA (N ,20°C/min) 5% wt. loss 10% wt. loss 2

DSC (20°C/min) Tg

III

339°C

382°C

236°C

IX

217°C

283°C

V

278°C

335°C

228°C not observed

VI

366°C

392°C

not observed

Conclusions. A series of octafunctional epoxy and 1-propenyl ether monomers bearing the Tg silsesquioxane core have been prepared and characterized. Despite the high functionality and reactivity o f these monomers, the efficiency with w h i c h the monomers undergo crosslinking is considerably less than expected. Instead, due to steric inhibition effects and the proximity of the functional groups to one another, photoinduced cationic polymerization proceeds mainly by an intramolecular process. Experimental. P r e p a r a t i o n of Ts . Ts was prepared by a modification of the method described by Bassindale and G e n t l e . A 3 L, three necked round bottom flask fitted with a mechanical stirrer, reflux condenser and a gas dispersing tube was charged with ferric chloride (75 g), methanol (98.6 g), 37% hydrochloric acid (74.4 g), toluene (300 g) and hexane (1000 g). W h i l e this mixture was stirred at high speed, a stream of nitrogen which was first passed through a reservoir containing trichlorosilane (83 g) was bubbled into the reaction mixture at such a rate that it required approximately 5 hours for all of the trichlorosilane to evaporate. The reaction mixture was filtered and dried first over potassium carbonate (46 g), then calcium chloride (31 g). The solvents were removed on a rotary evaporator until 50 m L of solution remained. O n cooling, colorless crystals of Tg were collected, dried and subjected to purification by sublimation at 180-200 °C/15 m m H g . A 23% yield (7.5 g) of T was obtained. H

H

3

n

H

8


294


Synthesis of l-Vinyloxy-2(2-propenoxy)ethane VIII. A 100 mL, 3 neck round bottomed flask fitted with a mechanical stirrer, addition funnel and a dry ice/acetone cooled condenser was charged with 4.4 g (50 mmol) of 2-hydroxyethyl vinyl ether, 5 mL of benzene and 3 g (75 mmol) of powdered sodium hydroxide. Allyl bromide (6.05 g, 50 mmol) was added dropwise to the flask and the flask was warmed slightly to initiate the reaction. The exothermic reaction that set in was controlled by cooling in a water bath. After the exotherm had subsided, the reaction was continued at reflux for 18 hours. The solid residue was filtered and the filtrate fractionally distilled to isolate the product, l-vinyloxy-2(2-propenoxy)ethane (b.p. 79°C/40 mm Hg; yield 6 g, 97%). Synthesis of 4(2-Vinyloxyethoxy)cycIohexene IV. A 1000 rnL three neck round bottomed flask fitted with a nitrogen inlet, mechanical stirrer, addition funnel and a dry ice/acetone cooled condenser was charged with 100 mL of dry toluene, 4 g (0.17 mol) of sodium hydride and 0.5 g of 18-crown-6 ether.. Nitrogen was passed through the reaction vessel and 1,2,3,6-tetrahydrobenzylalcohol (18.7 g, 0.17 mol) was slowly added via the addition funnel until no further hydrogen evolution could be detected. The exotherm of the reaction was controlled by adjusting the addition rate. When the addition had been completed, the contents of the flask were heated to 80°C and then 17.7 g (0.17 mol) of 2-chloroethylvinyl ether was added. Heating was continued at 80°C for about 18 h. The reaction mixture was cooled, filtered, washed with water in a separatory funnel and dried over anhydrous magnesium sulfate. The toluene was removed using a rotary evaporator and the product was purified by first column chromatography (silica gel, 10:90 ethyl acetaterhexene) followed by fractional distillation (b.p. 77-78°C/0.25 mm Hg). The product, (yield: 15 g, 50%) was 99.3% pure as determined by gas chromatography. The synthesis given below for the sythesis of monomer V is typical for the preparation of all the Tg monomers in this communication. Preparation of Monomer V . 4(2-Vinyloxyethoxy)cyclohexene (5 g, 0.027 mol) and 1.09 g (0.0026 mol) of T s were placed in a vial together with a magnetic stirrer and sealed with a screw cap. Approximately 50 mL of Karstedt's platinum catalyst was added and the mixture heated to 80°C. The initially insoluble T 8 went in solution as the reaction proceeded. The reaction was monitored by following the disappearance of the 2254 cm" infrared band assigned to the Si-H bond. After approximately 30 minutes, the reaction was complete. Excess 4(2-vinyloxyethoxy)cyclohexene was removed under vacuum leaving V as a colorless oil. H

H

1

1

IR (NaCl); 3021, 1652 cm" (cycloaliphatic double bond).

*H-NMR (CDC1 .200MHz); 8 (ppm) 5.65 (s, H ) ; 3.55 (br., H . ); 3.35 (d, H ); 2.21.6 (m, H ) ; 1.35-1.15 (m, H ); 1.15-1.05 (t, H i ) . 3

910

7 A 1 1

2

4

5

6

C - N M R (CDCI3, 50MHz); 8(ppm) 126.5 (C ); 125.5 (C, ); 76 (C ); 70 (C ); 69.5 (C ); 66 (C ); 33 (C ); 28 (C„); 25 (Q); 24 (C ); 14 (CO. 13

9

3

29

2

6

0

5

7

Si-NMR (CDCI3, 40 MHz); 8(ppm) -68.5.


4

295

Elemental A n a l . Calc. for CggHjsAgSig: C , 56.14%; H , 8.14%. Found: C , 56.29%; H , 8.20%.. Cationic Photopolymerization. Real-Time infrared spectroscopy was used to monitor the kinetics and conversions of the polymerizations of the functionalized Tg monomers prepared in this investigation. The techniques employed were originally described by Decker and Mousa. A thin film of the monomer containing 2 mol % (4decyloxyphenyl)phenyliodonium SbF6" as photoinitiator was drawn onto a sodium chloride salt plate placed in the sample holder. Photopolymerizations were carried out at room temperature and at 100°C using a light intensity of 16 m W / c m . During polymerization the epoxide band at 850 c m and the 1-propenyl ether band at 1660 cm" were monitored. 7

2

1


1

Thermal Analysis. Samples for thermal gravimetric analyses and differential scanning calorimetry were prepared in the following way. Optimized amounts of a photoinitiator as determined by a previous R T I R study were dissolved in the four Tg monomers (0.5 mole % I O C l O / f u n c t i o n a l group for I X , 1.0 mole % (4thiophenoxyphenyl)diphenylsulfonium SbF6" (SS)/functionality for V I , 2.0 mole % IOClO/functional group for both I I I and V . These compositions were cast as thin (-25mm) films onto glass plates and polymerized by exposure to an unfiltered 200 W medium pressure H g arc lamp for 4 minutes. The distance between the U V source and the sample was adjusted to give a radiation intensity of 16 m W / c m at the position o f the sample as measured by a radiometer sensitive to 363 nm. The polymerized films were removed from the glass plates and ground to a fine powder which was used in the thermal analysis studies. Glass transition temperatures and thermal stabilities were obtained using a Perkin-Elmer D S C - 7 Thermal Analysis System equipped with D S C - 7 and T G A - 7 modules.

References.

1. Feher, F. J.; B u d z i c h o w s k i , T. A., J. Organometallic Chemistry, 1989, 379, 3340. 2. D i t t m a r , U, H e n d a n , B. J., F l o r k e , U., M a r s m a n n , J. Organometallic Chemistry, 1995, 489, 185-194. 3. Bassindale, A. R.; G e n t l e , T. E., J. Mater. Chem., 1993, 3(12), 1319-1325. 4. Sellinger, A.; L a i n e , R. M., Polymer Preprints, 1994, 665-666. 5. C r i v e l l o , J. V.; L e e , J. L., J. Polym. Sci.: Part A: Polym. Chem. Ed., 1990, 28, 479-503. 6. C r i v e l l o , J. V.; F a n , M., Bi, D., J. Applied Polym. Sci., 1992, 44, 9-16. 7. C r i v e l l o , J.V.; Jo, K. D. J. Polym. Sci., Polym. Chem. Ed., 1993, 31(6), 1473. 8. C r i v e l l o , J.V.; Jo, K. D. J. Polym. Sci., Polym. Chem. Ed., 1993, 31(6), 1483. 9. C r i v e l l o , J.V.; Y a n g , B.; Kim, W.-G., J. Polym. Sci., Polym. Chem. Ed., 1995, 33(14), 2145. 10. D e c k e r , C; M o u s s a , K., J. Polym. Sci.: Part A: Polym. Chem. Ed., 1990, 28, 3429.


Synthesis and Photoinitiated Cationic Polymerization of Monomers

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