The Effects of Initiator and Diluent on the Photopolymerization of 2

Chapter 4

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The Effects of Initiator and Diluent on the Photopolymerization of 2-Hydroxyethyl Methacrylate and on Properties of Hydrogels Obtained Yu-Chin Lai and Edmond T. Quinn Contact Lens Division, Bausch & Lomb, Inc., Rochester, NY 14692-0450

A variety of photo-initiators and water-soluble diluents were used to study the photopolymerization of 2-hydroxyethyl methacrylate (HEMA). The rate of polymerization can be correlated to the structures as well as the concentration of i n i t i a t o r s and diluents. I t was found that, i n terms of rate of polymerization, 2,2-dimethoxy-2-phenyl acetophenone and glycerine were the best i n i t i a t o r and the best diluent among all i n i t i a t o r s and diluents evaluated. The rate of polymerization was found to have a great impact on mechanical properties, while it showed only minor effects on water contents of hydrogels derived from HEMA. 2-Hydroxyethyl methacrylate,

(HEMA, 1), i s the most

widely used UV-polymerizable h y d r o p h i l i c monomer i n making hydrogel lenses (2). However, the k i n e t i c s of i t s p o l y m e r i z a t i o n was not w e l l s t u d i e d . The e f f e c t of UV i n i t i a t o r and i t s c o n c e n t r a t i o n ; and the e f f e c t of d i l u e n t and i t s concentration, on the r a t e of © 1997 American Chemical Society

In Photopolymerization; Scranton, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

35

36

PHOTOPOLYMERIZATION

p o l y m e r i z a t i o n and on p r o p e r t i e s o f hydrogels d e r i v e d , are not w e l l understood. In t h i s paper, these aspects were addressed. I n i t i a t o r s and d i l u e n t s which l e d t o f a s t e r photo-polymerization o f HEMA were a l s o assessed. Experimental


Monomers. HEMA was p u r i f i e d by aluminum oxide chromatography, followed by d i s t i l l a t i o n under reduced pressure. A l l UV i n i t i a t o r s and organic d i l u e n t s were used as r e c e i v e d . Monomer Mixes f o r P o l y m e r i z a t i o n S t u d i e s . A mixture c o n t a i n i n g HEMA and g l y c e r i n e a t a weight r a t i o o f 85/15, and ethylene g l y c o l dimethacrylate (EGDMA) a t 0.4 %, and an i n i t i a t o r a t 0.2 % o f the weight of HEMA was t h e standard monomer mix used i n photop o l y m e r i z a t i o n s t u d i e s . Other v a r i a t i o n s were made by v a r y i n g the i n i t i a t o r o r the d i l u e n t c o n c e n t r a t i o n while keeping other components a t t h e same r e l a t i v e c o n c e n t r a t i o n s , unless otherwise s p e c i f i e d . P o l y m e r i z a t i o n K i n e t i c s - by Photo D i f f e r e n t i a l Scanning C a l o r i m e t r y (Photo-DSC). In a DuPont d i f f e r e n t i a l scanning c a l o r i m e t r y u n i t , a monomer mix o f 40 u l (40 mg) was placed i n a sample pan a t ambient temperature. Under n i t r o g e n atmosphere, i t was polymerized under UV, u s i n g a S y l v a n i a F4T5 lamp (which g i v e s a broad emission between 310-400 nm with peak a t 350 nm). The e f f e c t i v e l i g h t i n t e n s i t y was measured as 410 microwatts under t h i s setup. The exothermic p r o f i l e was monitored. The time when maximum p o l y m e r i z a t i o n r a t e (peak time) and when the p o l y m e r i z a t i o n s t a l l e d ( s t a l l time) were recorded. The % conversion was obtained by the i n t e g r a t i o n of t h e exothermic p r o f i l e . A t y p i c a l exothermic p r o f i l e f o r HEMA p o l y m e r i z a t i o n i s shown i n F i g u r e 1 which a l s o e x e m p l i f i e d t h e d e f i n i t i o n s o f peak time and s t a l l time. P r e p a r a t i o n o f Hydrogel Films Containing HEMA. S e l e c t e d monomer mixes c o n t a i n i n g HEMA, EGDMA, g l y c e r i n e and d i f f e r e n t i n i t i a t o r s were prepared. The mixes were p l a c e d between two s i l a n e - t r e a t e d g l a s s p l a t e s and cured under UV (4000 microwatts) f o r 1 hour. The f i l m s were e x t r a c t e d with b o i l i n g water f o r 4 hours and then p l a c e d i n a phosphate b u f f e r e d s a l i n e (pH 7.3) before testing. C h a r a c t e r i z a t i o n o f Hydrogel F i l m s . Mechanical t e s t i n g was conducted i n b u f f e r e d s a l i n e on an I n s t r o n instrument, according t o the modified ASTM D-1708 ( t e n s i l e ) and D-1938 (tear) and were r e p o r t e d i n g/mm f o r modulus and g/mm f o r t e a r s t r e n g t h . The water contents and the amount o f e x t r a c t a b l e s were measured gravimetrically. 2



2ε ε

90 H

non

130-

—ι— 10

(Stall time)

15 Time (min)

—ι— 20

F i g u r e 1. A t y p i c a l exothermic p r o f i l e o f photopolymerization o f HEMA.

.56min

3.67min (Peak time)


J 25

30

δ

I I s, I

I

I

b

o

PHOTOPOLYMERIZATION

38 R e s u l t and D i s c u s s i o n

The E f f e c t o f I n i t i a t o r s on the P o l y m e r i z a t i o n o f HEMA. In terms of mechanism o f r a d i c a l generation, t h r e e types o f p h o t o - i n i t i a t o r are f r e q u e n t l y used i n t h e p o l y m e r i z a t i o n o f v i n y l monomers: P I ^ (o(-cleavage, u n i m o l e c u l a r ) , P I (Η-abstraction, b i m o l e c u l a r ) , and the r a d i c a l c a t i o n type. A Pin-type i n i t i a t o r i s an i n i t i a t o r which causes homolytic cleavage o f a chemical bond t o form r e a c t i v e r a d i c a l s . PI -type i n i t i a t o r s u s u a l l y are p a i r s of compounds c o n t a i n i n g a photos e n s i t i z e r and an amine with an -hydrogen. The s e n s i t i z e r ( u s u a l l y i s an aromatic ketone) forms an e x c i p l e x with the amine a f t e r absorbing UV l i g h t , which then c l e a v e s t o form an amine r a d i c a l and i n i t i a t e s p o l y m e r i z a t i o n . The r a d i c a l c a t i o n i n i t i a t o r s a r e aryloxonium s a l t s , such as t r i a r y l s u l f o n i u m hexafluoroantimonate, which generate a r a d i c a l c a t i o n ( u s u a l l y i s H ) and i n i t i a t e s p o l y m e r i z a t i o n . Table I l i s t s t h e chemical s t r u c t u r e , acronym, and a b s o r p t i o n maximum o f i n i t i a t o r s o f three types used i n t h i s study. Among them, benzoin methyl ether i s used most o f t e n f o r the p o l y m e r i z a t i o n o f HEMA t o g i v e contact l e n s . A l l i n i t i a t o r s were screened f i r s t by Photo-DSC u s i n g a standard HEMA monomer mix c o n s i s t s o f HEMA/Glycerine/EGDMA/photo-initiator a t 85/15/0.34/0.17 (by weight). The UV lamp used ( S y l v a n i a F4T5 lamp) was t h a t with a broad emission spectrum between 310-400 nm. The peak time i n the exothermic p r o f i l e , which i s t h e time when the r a t e of p o l y m e r i z a t i o n was f a s t e s t , was recorded. Because peak time i s i n d i c a t i v e o f t h e r a t e of p o l y m e r i z a t i o n , i n i t i a t o r s can be ranked by the peak time i n a photo-polymerization. Table 2 summarizes the peak times f o r the photo p o l y m e r i z a t i o n o f HEMA. Among i n i t i a t o r s with s t r u c t u r e s known, t h e ranking of more a c t i v e i n i t i a t o r s , i n terms of decreasing p o l y m e r i z a t i o n r a t e was: 2


2

+

2,2-dimethoxy-2-phenyl acetophenone > benzoin methyl ether > 1-hydroxycyclohexane acetophenone = 2-hydroxy2,2-dimethyl acetophenone > TXN/MDEA I t i s i n t e r e s t i n g t o note t h a t among i n i t i a t o r s s t u d i e d , a benzyl k e t a l type i n i t i a t o r (2,2-dimethoxy2-phenyl acetophenone) i s more a c t i v e than other i n i t i a t o r s o f acetophenone type. 2,2-Dimethoxy-2phenyl acetophenone i s more a c t i v e than BME, probably due t o the presence of the e x t r a methoxy group on the carbon, which g i v e s a more a c t i v e phenyldimethoxy methyl r a d i c a l (and i t f u r t h e r c l e a v e s t o g i v e an even more a c t i v e methyl r a d i c a l ) than the phenylmethoxy methyl r a d i c a l from BME (4). I t i s known t h a t the phenylmethoxy methyl r a d i c a l dimerizes e a s i l y and thus l o s e s some o f i t s r o l e as a r a d i c a l i n a


4. LAI & QUINN

Initiator & Diluent Effects on Polymerization of HEMA

Table I. U V initiators used in the photopolymerization of H E M A Initiator

2

(Acronym/Trade Name)

Structure

Abs Max (nm)

PI1 Type .—,


Benzoin M e t h y l E t h e r (BME)

O OCH .

.

3

2,2-dimethoxy-2phenyl acetophenone (Irgacure-651)

ο

323

335

OCH, —. /

OCH

3

2-hydroxy-2,2-dimethyl acetophenone (Darocur-1173)

320

Ο

CH,

1-hydroxycyclohexane acetophenone (Irgacure-184)

326

2,2-diethoxy acetophenone (DEAP)

4 - ( 2-hydroxyehtoxy) pheny1-2-hydroxy-2p r o p y l ketone (Darocur-2959)

323 1

O

^ 0

/=\ H

H

The Effects of Initiator and Diluent on the Photopolymerization of 2

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