Initiation of Polymerization - ACS Publications - American Chemical

26 Initiator-Accelerator Systems for Dental Resins G. M . BRAUER and H . ARGENTAR Downloaded by PURDUE UNIVERSITY on June 1, 2013 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch026

National Bureau of Standards, Washington, DC 20234

Initiators and initiator-accelerator systems for curing acrylic resins employed as dental restorative and prosthetic devices are reviewed. These resins are hardened by a free-radical polymerization that is activated by an initiator and heat or light, or by a redox initiator-accelerator system. Dentures are commonly cured by a heating cycle during which benzoyl peroxide (BP) initiator is decomposed to release sufficient radicals to yield a fully hardened device. Most room-temperature, chemically activated systems employ BP-tertiary aromatic amines. Many other potential redox systems are limited by the instability of the uncured components on prolonged storage or the doubtful biocompatibility of the ingredients. Visible or UV energy-cured systems do not require clinical mixing and allow unrestricted working time. Acrylic resins are the materials of choice for almost a l l dental applications wherever synthetic plastics are favored for the restoration of missing teeth or tooth structures. This is not surprising because polymers derived from methacrylate esters f u l f i l l most requisites of a restorative: adequate strength, resilience and abrasion resistance; dimensional stability during processing and subsequent use; translucency or transparency simulating the visual appearance of the oral tissue that i t replaces; satisfactory color stability after fabrication; resistance to oral fluids, food or other substances with which i t may come into contact; satisfactory tissue tolerance; low toxicity, and ease of fabrication into a dental appliance. The largest volume of plastics for dental applications is consumed in the construction and repair of dentures. Other uses include artificial teeth, restoratives—especially for anterior This chapter not subject to U.S. copyright. Published 1983, American Chemical Society In Initiation of Polymerization; Bailey, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Downloaded by PURDUE UNIVERSITY on June 1, 2013 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch026

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teeth, p i t - a n d - f i s s u r e s e a l a n t s , r e s i n cements, orthodontic s p l i n t s and bonding r e s i n s , m a x i l l o f a c i a l prostheses, o r a l implants and mouth p r o t e c t o r s . In dental a p p l i c a t i o n s , a f l u i d a c r y l i c r e s i n formulation i s hardened by means of a f r e e - r a d i c a l i n i t i a t e d polymerization that i s e f f e c t e d by one of three means: s u b j e c t i n g a thermally s e n s i t i v e i n i t i a t i n g i n g r e d i e n t to a higher than ambient temperature; s u b j e c t i n g a photoactive i n g r e d i e n t to v i s i b l e or u l t r a v i o l e t r a d i a t i o n ; or b r i n g i n g together i n the r e s i n s o l u t i o n a binary redox i n i t i a t i n g system composed of two chemic a l s , one a reductant (the " a c c e l e r a t o r " or "promotor") and the other an oxidant, i . e . , the i n i t i a t o r . Each of these systems has advantages as w e l l as disadvantages and consequently none i s s t r o n g l y p r e f e r r e d over the others. The i d e a l p o l y m e r i z a t i o n - i n i t i a t i n g system does the f o l l o w i n g : i t produces enough f r e e r a d i c a l s w i t h i n an acceptable time i n t e r v a l so that a mix with a d e s i r a b l e working and curing time f o r the s p e c i f i c end use i s obtained; y i e l d s a polymer with minimum r e s i d u a l monomer and a molecular weight d i s t r i b u t i o n that w i l l r e s u l t i n optimum p h y s i c a l p r o p e r t i e s of the dental r e s t o r a t i v e or device; and, f i n a l l y , produces no undesirable byproducts, such as t o x i c or colored m a t e r i a l s . The i n g r e d i e n t s of the i d e a l system meet the f o l l o w i n g requirements: biocomp a t i b l e and non-toxic; t a s t e l e s s and c o l o r l e s s ; storage s t a b l e f o r extended periods of time under environmental conditions that may be encountered i n t r a n s i t , i n warehouses, i n dental depots or i n the dental o f f i c e ; compatible chemically and p h y s i c a l l y with a l l the other i n g r e d i e n t s of the r e s i n which are encountered i n storage; and r e a d i l y a v a i l a b l e commercially at reasonable p r i c e s . I n i t i a t o r - a c c e l e r a t o r systems f o r a c r y l i c r e s i n s and comp o s i t e s i n dental use have been p r e v i o u s l y reviewed (1). This report updates the information on these systems i n d e n t a l polymers. Thermally I n i t i a t e d Resin Systems Employing Peroxides Heat-cured denture base m a t e r i a l s were introduced i n t o dental use i n 1937. These m a t e r i a l s are prepared from a powderl i q u i d s l u r r y . The l i q u i d i s methyl methacrylate to which are added a p l a s t i c i z e r , c r o s s l i n k i n g agent and i n h i b i t o r . The powder i s poly(methyl methacrylate) containing approximately one percent i n i t i a t o r , u s u a l l y benzoyl peroxide. By s u b j e c t i n g t h i s s l u r r y to elevated temperature (about 75°C to 100°C) f o r one or more hours depending on the temperature employed, s u f f i c i e n t f r e e r a d i c a l s are produced from the i n i t i a t o r to y i e l d a s a t i s f a c t o r y denture. Other i n i t i a t o r s have been proposed. These include the thermally l e s s s t a b l e d i a c y l peroxides, e.g., d i a c e t y l - , b i s ( 2 , 4 - d i c h l o r o b e n z o y l ) - or d i l a u r y l , peroxide. The f i r s t two of these are a v a i l a b l e commercially as a 50% paste

In Initiation of Polymerization; Bailey, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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d i s p e r s i o n i n a phthalate e s t e r , the d i s p e r s i o n being s a f e r i n handling than the neat m a t e r i a l . Binary Redox P o l y m e r i z a t i o n - I n i t i a t i n g Systems


The thermal decomposition r a t e of benzoyl peroxide at mouth or ambient temperature i s much too slow to cure a c r y l i c monomers. At such temperatures i n i t i a t o r a c c e l e r a t o r systems are commonly employed. Peroxide-Amine System. Although a great number of redox p o l y m e r i z a t i o n - i n i t i a t i n g systems have been suggested f o r d e n t a l a p p l i c a t i o n s , the d i f f i c u l t y i n meeting the requirements described above has e l i m i n a t e d a l l but a few. By f a r the most popular system i s that c o n s i s t i n g of a t e r t i a r y aromatic amine ( n i t r o g e n and r i n g - s u b s t i t u t e d a n i l i n e ) a c t i n g as the a c c e l e r a t o r and benzoyl peroxide (BP), as the i n i t i a t o r . T h i s system was o r i g i n a l l y suggested i n the e a r l y 1940 s by Schnabel (2, 3). f

Overview of Experimental Data. Most peroxide-amine systems impart c o l o r s to the cured polymers ranging from yellow f o r N,Nd i m e t h y l - p - t o l u i d i n e to b l a c k f o r N,N-dimethyl-p-phenylenediamine (4). Highly e l e c t r o n - d o n a t i n g groups i n the a c c e l e r a t o r molecule u s u a l l y cause the hardened m a t e r i a l to be e s t h e t i c a l l y u n s u i t a b l e . The e f f e c t of i n h i b i t o r , peroxide, i n i t i a t o r and amine a c c e l e r a t o r on the r a t e of p o l y m e r i z a t i o n of poly(methyl metha c r y l a t e ) s l u r r i e s has been s t u d i e d (5_, 6). Time r e q u i r e d to reach the exotherm i n d i c a t e s aromatic peroxides, e s p e c i a l l y j>chlorobenzoyl peroxide, to be the most e f f i c i e n t i n i t i a t o r s . Although p o l y m e r i z a t i o n i n the presence of t h i s compound and N,Ndimethyl-£-toluidine (DMPT) i s more r a p i d i n i t i a l l y , i t i s slower a f t e r the exotherm than i s a system employing BP-DMPT (7). P o l y m e r i z a t i o n using the l a t t e r i n i t i a t o r - a c c e l e r a t o r gives r e s i n s with lower r e s i d u a l monomer. Most chemically a c t i v a t e d denture r e s i n s and f i l l i n g mater i a l s employ the BP-DMPT or BP-N,N-bis(2-hydroxyethyl)-p-toluidine (DHEPT) system. Use of DHEPT i n c r e a s e s the s e t t i n g time somewhat. Methacrylate or dimethacrylate monomers using t h i s a c c e l e r a t o r have improved storage s t a b i l i t y (8) and thus w i l l not g e l prematurely even on exposure to elevated temperatures. Composite r e s t o r a t i v e s c o n t a i n i n g a c r y l i c monomers and i n o r g a n i c r e i n f o r c i n g agents are now being used as f i l l i n g materials. In t h i s a p p l i c a t i o n , l a r g e concentrations of c r o s s l i n k i n g dimethacrylates are incorporated. These l i q u i d s polymerize much more r a p i d l y than l e s s v i s c o u s monomethacrylates because of the Tromsdorff a u t o a c c e l e r a t i o n or g e l e f f e c t a t t r i b u t e d to lessened t r a n s l a t i o n a l m o b i l i t y of growing polymer r a d i c a l s with i n c r e a s i n g v i s c o s i t y of the medium (9^, 10). The aim of a number of recent s t u d i e s has been to develop more r e a c t i v e amines that y i e l d n e a r l y c o l o r l e s s polymers with good c o l o r s t a b i l i t y and improved b i o c o m p a t i b i l i t y .



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Amines with more r i n g s u b s t i t u e n t s , p a r t i c u l a r l y i n the 3 and 5 p o s i t i o n s , e.g. Ν,Ν-dimethyl-sym-xylidine, decrease the curing time and improve the c o l o r s t a b i l i t y of the polymer ( IX). A c c e l e r a t i n g a b i l i t y i s a f u n c t i o n of both r i n g and n i t r o g e n s u b s t i t u t i o n , although r i n g s u b s t i t u t i o n has the greater i n fluence ( 12); c o l o r s t a b i l i t y i s i n f l u e n c e d more by substituents on the r i n g . In general, increased s u b s t i t u t i o n and s t e r i c hindrance of aromatic amines produces l i g h t e r colored m a t e r i a l s . Composites containing aromatic amines having a 3,5-dimethylphenyl group d i s c o l o r l e s s than those with a 4-methylphenyl, and much l e s s than those with an unsubstituted phenyl group. N,N-dimethylp - t e r t - b u t y l a n i l i n e (12, 13) or Ν,N-bis(hydroxyalkyl)-3,5-di-tert b u t y l a n i l i n e s ( 1 4 ) y i e l d hardened r e s i n s that have a very l i g h t shade and e x c e l l e n t c o l o r s t a b i l i t y . T e r t i a r y aromatic amines with l a r g e s u b s t i t u e n t s on the nitrogen atom and molecular weight above 400 can be e f f e c t i v e a c c e l e r a t o r s (15) y i e l d i n g composites with r a p i d curing times. Because of t h e i r low v o l a t i l i t y and reduced d i f f u s i o n r a t e , such a c c e l e r a t o r s should not r e a d i l y penetrate body t i s s u e s . Thus, they would be a n t i c i p a t e d to cause less pulpal i r r i t a t i o n or t o x i c r e a c t i o n s than lower molecular weight amines. V o l a t i l i t y or d i f f u s i o n of the t e r t i a r y amine a c c e l e r a t o r may a l s o be reduced by using polymerizable amines such as those with N-methacryloxyethyl groups that are incorporated i n t o the cured r e s i n (16) or by s u b s t i t u t i n g f o r the low-molecular weight amine a polymeric t e r t i a r y aromatic amine (17). However, s u b s t i t u t i o n of the N-methyl group i n the amine by a b u l k i e r methacryl o y l o x y e t h y l group slows the polymerization process. Formation of i n s o l u b l e polymer i n d i c a t e s that the amines copolymerize, y i e l d ing a c r o s s l i n k e d polymer. Resins cured with an aminoethyl methacrylate a c c e l e r a t o r containing a j>-tolyl or 3,5-xylyl s u b s t i tuent on the n i t r o g e n atom have c o l o r s t a b i l i t y s i m i l a r to those of t h e i r low molecular weight counterparts. The storage s t a b i l i t y of the components of composite formu l a t i o n s i s mainly l i m i t e d by the poor s h e l f - l i f e of the BP i n g r e dient (8). Paste formulations, containing BP, but f r e e of a c c e l e r a t o r prematurely harden when stored at 60°C. Formulations using powder-liquid c o n s t i t u e n t s are more s t a b l e at these elevated temperatures. A f t e r extended storage at room temperature f o r two years, composite mixes showed delayed s e t t i n g and decreased mechanical p r o p e r t i e s i n the cured m a t e r i a l . The p u r i t y of the BP and the amine s e l e c t e d g r e a t l y i n f l u e n c e s parameters such as r e a c t i o n r a t e , c o l o r s t a b i l i t y and b i o c o m p a t i b i l i t y (18). With m u l t i f u n c t i o n a l t h i o l s such as p e n t a e r y t h r i t o l t e t r a ( 3 mercaptopropionate), added i n concentrations below 10 percent to t y p i c a l peroxide-amine cured r e s i n s , composites with s i g n i f i c a n t l y reduced s e t t i n g time and e x c e l l e n t c o l o r s t a b i l i t y have been obtained ( J£). Dodecyl mercaptan y i e l d s comparable r e s u l t s ( l l ) . The f r e e r a d i c a l a d d i t i o n of the t h i o l to the double bond of the monomer probably c o n t r o l s the molecular weight of the polymer.


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The polymeric methacrylate m a t e r i a l s , even 24 hours a f t e r hardening, contain a l a r g e number of unreacted groups (20). T h e i r c o n c e n t r a t i o n i n the cured r e s i n i s considerably higher f o r dimethacrylate monomers than f o r those with a s i n g l e methacrylate group i n the molecule. I n f r a r e d r e f l e c t a n c e measurements i n d i c a t e that the r e s i d u a l methacrylate group i n commercial dental com p o s i t e s with dimethacrylate i n g r e d i e n t s ranges from 30 to 48 percent. Peroxide-Amine Mechanism and Supporting R e s u l t s . Two chemical mechanisms have been proposed to e x p l a i n how s u b s t i t u ents, s o l v e n t s and e x t e r n a l f a c t o r s a f f e c t the p o l y m e r i z a t i o n i n i t i a t i n g rate. However, the only mechanism that appears to agree with a l l the experimental data so f a r reported i s that i n v o l v i n g an e l e c t o n t r a n s f e r as the key step (21, 22). Accord i n g to t h i s mechanism, the amine and peroxide molecules i n t e r a c t to form a c h a r g e - t r a n s f e r complex ( t h i s term i s used to i n d i c a t e a non-polar t r a n s i t i o n s t a t e ) c o n s i s t i n g of an e l e c t r o n d e f i c i e n t amine and a peroxide with an excess e l e c t r o n . The complex sub sequently breaks down to y i e l d an aminium c a t i o n , a peroxide f r e e r a d i c a l s u f f i c i e n t l y r e a c t i v e to combine with a monomer molecule to i n i t i a t e the p o l y m e r i z a t i o n and an i n e r t benzoate anion. This mechanism i s supported by the f o l l o w i n g f a c t s , which do not seem to be e x p l a i n a b l e by other mechanisms: The r e a c t i v i t y of the amine as a p o l y m e r i z a t i o n a c c e l e r a t o r depends upon the σ+ value of the meta- or para-substituent of the amine, where σ+ i s the e l e c t r o p h i l i c s u b s t i t u e n t parameter pre v i o u s l y described and tabulated (23). When the k i n e t i c r a t e constant (or the r e c i p r o c a l of the p o l y m e r i z a t i o n time with amine and peroxide i n i t i a l concentrations h e l d constant) i s p l o t t e d against the σ+ value on a semi-logarithmic p l o t (Figures 1 and 2 ) , an i n v e r t e d "V" shaped curve r e s u l t s . The k i n e t i c data f o r Figure 1 were taken from a published a r t i c l e d e s c r i b i n g the polymerization of methyl methacrylate i n the presence of BP and v a r i o u s t e r t i a r y aromatic amines as shown i n Figure 2 ( 5 ) . S i m i l a r p l o t s can be obtained from k i n e t i c data of unsaturated p o l y e s t e r r e s i n s (24, 25). T h i s behavior i s independent of the i d e n t i t y of the n i t r o g e n s u b s t i t u e n t s of the amine or the monomer. Previous p u b l i c a t i o n s from t h i s l a b o r a t o r y have pointed out that the σ+ value of the amine r i n g s u b s t i t u e n t y i e l d i n g maximum r e a c t i v i t y , i . e . , minimum p o l y m e r i z a t i o n (or cure) time, occurs at approximately -0.2 i n the cases examined (26-29). The o p t i mum value i s a n t i c i p a t e d to be f a i r l y i n s e n s i t i v e to the type of monomer and experimental c o n d i t i o n s . In Figure 2, p l o t s are shown f o r the p o l y m e r i z a t i o n and corresponding g e l times. The l a t t e r times do not r e v e a l a minimum at the σ+ value where the p o l y m e r i z a t i o n time i s minimum, but r a t h e r show a decided break i n the curve. Since g e l time i n d i c a t e s the c l i n i c a l working time and


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4h

-0.8

-0.6 -0.4 -0.2

0

0.2

σ+ Figure 1. Curing times of methyl methacrylate containing fixed initial concentra tions (in mol/L) of benzoyl peroxide and aryl-substituted N,N-dimethylaniline versus the σ + value of the aryl substituent of the amine. (Curing times from Ref. 5; σ-f values from Ref. 23.)

Figure 2. The gel (%) and polymeriza tion (O) times of unsaturated polyester containing a fixed initial concentration (in mol/L) of benzoyl peroxide and arylsubstituted N,N-bis(3-allyloxy-2-hydroxypropyl)aniline versus the σ + value of the aryl substituent of the amine. The symbols (Â, A) indicate the corresponding time values for the analogous ^-substituted 2-naphthylamine which in the usual sense is not a substituted aniline. (Time values from Refs. 24 and 25; σ + from Ref. 23, except for σ + value for 2-naphthyl ( — 0.61), recalculated from Bier (64).)

-0.8 -0.6

-0.4

-0.2



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polymerization time the p e r i o d i n which a hard polymer i s ob t a i n e d , the r a t i o of these two values should be near 1.0 so that "snap hardening" r e s u l t s . I d e a l l y , the r e s p e c t i v e times should be i d e n t i c a l but t h i s i s never r e a l i z e d . The g e l and polymeri z a t i o n times approximate each other most c l o s e l y when σ+ i s greater than about -0.2; i . e . , when the amine does not contain an o v e r l y electron-donating r i n g s u b s t i t u e n t . I f the σ+ value of the r i n g s u b s t i t u e n t i s very low, e.g., i n such compounds as t e r t i a r y aromatic amines derived from j>-anisidine or _o-phenylenediamine, g e l a t i o n occurs very r a p i d l y but f i n a l cure to the hard s t a t e takes a considerable time. Previous i n v e s t i g a t i o n s of the spectroscopic behavior of charge-transfer complexes derived from r i n g - s u b s t i t u t e d N,Nd i m e t h y l a n i l i n e have demonstrated the usefulness of the σ+ parameter i n c o r r e l a t i n g the data (30, 31). The s a t i s f a c t o r y f i t t i n g of polymerization r a t e s against the σ+ s u b s t i t u e n t i s i n d i c a t i v e of charge-transfer i n t e r a c t i o n of the aromatic amine. The e f f e c t of solvent upon the r a t e of r e a c t i o n of t e r t i a r y aromatic amines with BP i s not dependent upon the d i e l e c t r i c constant of the solvent or on the monomer formulation (32)· The data of previous i n v e s t i g a t o r s could be c o r r e l a t e d by a simple f u n c t i o n of the solvent r e f r a c t i v e index as the relevant indepen dent v a r i a b l e (32a). The c o r r e l a t i o n with the r e f r a c t i v e index a l s o i n d i c a t e s charge-transfer complex formation. This i s i n agreement with the spectroscopic evidence r e v e a l i n g the f a r greater s e n s i t i v i t y of the e l e c t r o n i c energy of charge-transfer complexes between uncharged molecules to the r e f r a c t i v e index of the solvent as compared to the d i e l e c t r i c constant (31). We tested the p r e d i c t i o n that t e r t i a r y aromatic amine a c c e l e r a t o r s c o n t a i n i n g r i n g s u b s t i t u e n t s with σ+ values c l o s e to -0.2 would be the most e f f e c t i v e . A compilation of the σ+ values of r i n g s u b s t i t u e n t (23) l i s t e d the j>-CH2C02C2H5 group as having a σ+ value of about -0.16, suggesting use of a t e r t i a r y aromatic amine w i t h t h i s s u b s t i t u e n t or the corresponding c a r b o x y l i c a c i d or i t s methyl e s t e r as an a c c e l e r a t o r . The o v e r a l l c h a r a c t e r i s t i c s of the composites (hardening time, strength and c o l o r s t a b i l i t y ) c o n t a i n i n g 4-N,N-dimethy1aminopheny l a c e t i c a c i d (DMAPAA) or i t s methyl e s t e r (MDMAPAA) as a c c e l e r a t o r i n g r e d i e n t s compared favorably to r e s t o r a t i v e r e s i n s cured with commonly used t e r t i a r y amines (26). Based on hardening times the approximate order of the a c c e l e r a t i n g a b i l i t y of the r e s p e c t i v e amines was: DMAPAA > Ν, Ν- dime thy 1- sym-xy 1 i d ine > DMPT > MDMAPAA >> DHEPT. This order of r e a c t i v i t y i s dependent on the components ( e s p e c i a l l y monomers) used i n the formulations. Minimal c u r i n g time and maximum t e n s i l e and compressive strength of the cured m a t e r i a l were obtained over a narrow concentration range of a c c e l e r a t o r i n the l i q u i d . This range, which i s depen dent on the type of d i l u e n t employed, was approximately the same f o r a l l amines. The b i o c o m p a t i b i l i t y of these amines as expected from s i m i -


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l a r i t y i n s t r u c t u r e s to compounds used m e d i c i n a l l y (33, 3 4 , ) i s good. No mutagenic or c y t o t o x i c e f f e c t s have been observed using the Ames t e s t f o r b a c t e r i a l mutagenicity and the agar overlay t e s t f o r c y t o t o x i c i t y with DMAPAA (35). A second s e r i e s of amines suggested by theory to be r e a c t i v e a c c e l e r a t o r s are the corresponding j)-aminophenethanols. The σ+ value f o r the 2-CH2CH2OH group has not been reported but should be somewhat l e s s than that f o r the 2-CH2CO2C2H5 group and be c l o s e to -0.2. The f o l l o w i n g homologues and d e r i v a t i v e s of DMAPAA(I) and Ν,Ν-dialkylaminophenethanol ( I I ) have been synthesized and tested as a c c e l e r a t o r s (37): (I)

and

f

where R=CH , C H and R =H,CH ,C H . Composites u s i n g a 3 to 1 powder/liquid r a t i o were prepared c o n t a i n i n g a s i l a n i z e d barium g l a s s coated with 1% BP powder and 72.4 percent bis(3-methacryloxy-2-hydroxypropyl) bisphenol A (BIS-GMA), 27.6 percent 1,6-hexamethylene g l y c o l dimethacrylate (1,6-HEDMA), 0.2 percent b u t y l a t e d hydroxytoluene (BHT) and v a r i o u s amines i n the l i q u i d . S e t t i n g times of the formulations v a r i e d from 1.5 to 4.0 min. The N,N-diethylaminophenylacetic a c i d (DEAPAA) was by f a r the most r e a c t i v e a c c e l e r a t o r with a 3 mmolar amine c o n c e n t r a t i o n causing a cure of 4.5 min. Fastest p o l y m e r i z a t i o n f o r the above amines occurred at 17 mmolar concena c c e l e r a t o r . A composite made from powder coated with 1 percent BP and a l i q u i d with 17 mmolar amine has a molar peroxide to amine r a t i o of 6.5 compared to a r a t i o of 1.1 to 1.5 reported as most e f f i c i e n t f o r c u r i n g u n f i l l e d r e s i n s (4, 36). T h i s much l a r g e r excess of peroxide r e q u i r e d to o b t a i n minimum s e t t i n g time should be expected s i n c e only a small p o r t i o n of the peroxide i s access i b l e to the amine. P h y s i c a l p r o p e r t i e s ( t e n s i l e strength 36-55 2 MPa, compressive strength 245-303 MPa, water s o r p t i o n 0.5-0.7/cm ) considerably exceeded the minimum requirement of the s p e c i f i c a t i o n f o r d e n t a l composite r e s i n s (37). I f low concentrations are employed i n the formulations e s p e c i a l l y with DEAPAA as a c c e l e r a t o r , the cured composites are n e a r l y c o l o r l e s s . No p e r c e p t i b l e change occurs i n the c o l o r of the specimens c o n t a i n i n g a UV absorber a f t e r 24 hours exposure to a UV l i g h t source. Because of the e x c e l l e n t o v e r a l l p h y s i c a l p r o p e r t i e s , n e a r l y c o l o r l e s s appearance and the p o t e n t i a l l y b e t t e r b i o c o m p a t i b i l i t y , compositions using these a c c e l e r a t o r s should y i e l d improved r e s t o r a t i v e s . 3

2

5

3

2

5

A c r y l i c monomers are polymerized by c a r b o x y l i c acids with t e r t i a r y aromatic amines (38), e.g., 4-N,N-(dimethylamino)pheny1a c e t i c a c i d (8), presumably v i a a charge-transfer complex. Neutral amino a c i d e s t e r a c c e l e r a t o r s should y i e l d monomer formu l a t i o n s having b e t t e r s h e l f - l i f e than the corresponding amino a c i d s . This has been e s t a b l i s h e d experimentally (39).



26.

BRAUER AND

ARGENTAR

Initiator-Accelerator Systems

367

R e a c t i v i t y of the a c c e l e r a t o r employed i s dependent on the monomer used i n the r e s p e c t i v e formulation. Whereas compositions c o n t a i n i n g DEAPAA cure bis-phenol A dimethacrylate monomer formu l a t i o n s f a s t e s t , those containing j>-(dialkylamino)phenethanol r e s u l t i n the more r a p i d polymerization of methyl methacrylate monomer-polymer s l u r r i e s (39). A d d i t i o n of Bronsted a c i d s (40) to s i m i l a r systems increases the p o l y m e r i z a t i o n r a t e but, as has been discussed above, reduces storage s t a b i l i t y . Many peroxyesters, hydroperoxides or peroxides are more storage s t a b l e than BP. However, t h e i r r e a c t i o n with t e r t i a r y amines i s g e n e r a l l y too slow to give a s u f f i c i e n t l y r a p i d cure f o r a c r y l i c r e s i n s . Composite mixes c o n t a i n i n g 2,5-dimethyl-2,5(benzoyl peroxy)hexane, _t-butyl perbenzoate, t - b u t y l hydroper oxides or dicumyl p e r o x i d e - t e r t i a r y amines do not harden f o r days. Resins cured with cumene hydroperoxide and DEAPAA harden, but y i e l d h i g h l y c o l o r e d products (39). F a s t e r cures are obtained with _t-butyl peroxymaleic a c i d (TBPM) and primary amines (j>t o l u i d i n e or £-aminophenethanol), but the shades of the r e s u l t i n g composites are unacceptable. Curing i s slower with secondary and t e r t i a r y amines. The l a t t e r compounds y i e l d m a t e r i a l s having higher strength and more d e s i r a b l e shades. Because of the l a r g e concentrations of i n i t i a t o r and t e r t i a r y amine r e q u i r e d f o r s a t i s f a c t o r y cure, the storage s t a b i l i t y of components of com p o s i t e r e s i n s containing TBPM and t e r t i a r y amines does not equal those i n i t i a t e d by the l e s s t h e r m a l l y - s t a b l e BP. One unusual and s u r p r i s i n g c h a r a c t e r i s t i c of t e r t i a r y aro matic amines i s that i n a d d i t i o n to a c t i n g as a c c e l e r a t o r s , the same compounds i n low concentration i n the presence of oxygen and an i n i t i a t o r may act as i n h i b i t o r s of polymerization (41, 42). This behavior has a l s o been a t t r i b u t e d to the a b i l i t y of the amine to engage i n charge-transfer complex formation. Oxygen a l s o i n h i b i t s r a d i c a l polymerization and r e s u l t s i n uncured f i l m s at the surface of dental sealants (42). Amine-Free Redox Systems Many i n i t i a t o r - a c c e l e r a t o r systems that contain a c c e l e r a t o r s other than amine have been suggested f o r v i n y l polymerization, but only a few have been employed i n d e n t a l r e s i n s . S u b s t i t u t i o n of j>-toluenesulfinic a c i d , a l p h a - s u b s t i t u t e d sulfones and low concentrations of h a l i d e and c u p r i c ions f o r t e r t i a r y amine a c c e l e r a t o r s , y i e l d s c o l o r l e s s products (43-48). Most of these compounds have poor s h e l f - l i f e . They r e a d i l y o x i d i z e i n a i r to s u l f o n i c a c i d s which do not a c t i v a t e polymerization. L a u r o y l peroxide, i n conjunction with a metal mercaptide (such as z i n c hexadecyl mercaptide) and a trace of copper, has been used to cure monomer-polymer s l u r r i e s c o n t a i n i n g methacrylic a c i d (4950). A d d i t i o n of Na s a l t s of saccharine to monomer containing an Ν,Ν-dialkylarylamine speeds up polymerization (51). Methacrylate monomers can be polymerized with _t-butyl-, or



368

INITIATION OF

POLYMERIZATION

cumyl hydroperoxide and thiourea d e r i v a t i v e s as reducing agent (52, 53, 54). With from 0.5 to 1 percent phenyl-, N - a c e t y l - or N - a l l y l t h i o u r e a composites with e x c e l l e n t c o l o r s t a b i l i t y have been obtained. Cumene- or J:-butyl hydroperoxide or t - b u t y l perbenzoate i n conjunction with a s c o r b i c a c i d or a s c o r b i c palmitate a l s o i n i t i a t e r a p i d polymerization, y i e l d i n g c o l o r l e s s composites with good mechanical p r o p e r t i e s (55). For commercial a p p l i c a t i o n , means must be developed f o r prevention of o x i d a t i o n of a s c o r b i c a c i d or i t s d e r i v a t i v e s on prolonged storage. T r i a l k y l b o r a n e oxides i n i t i a t e polymerization of a c r y l i c monomers at the moist dentin surface (56, 57). Bonding occurs only between the d e n t i n a l c o l l a g e n and any r e t e n t i o n to enamel i s mechanical. P h o t o i n i t i a t e d Polymerization. During the l a s t decade, a number of photochemically-cured m a t e r i a l s have been introduced to the dental p r o f e s s i o n . P r e s e n t l y , such r e s i n s are a v a i l a b l e as r e s t o r a t i v e s , p i t - a n d - f i s s u r e s e a l a n t s , bonding agents and as orthodontic bracket adhesives. The m a t e r i a l s are cured by an appropriate l i g h t source which, by means of s u i t a b l e f i l t e r s , produces r a d i a t i o n i n the near u l t r a v i o l e t region around 360 nm or u t i l i z e s v i s i b l e l i g h t of about 470 nm. For U V - i n i t i a t e d m a t e r i a l , the a c r y l i c r e s i n i s o f t e n composed of a dimethacrylate such as BIS-GMA, a polymerizable d i l u e n t , and a p h o t o i n i t i a t o r , u s u a l l y a benzoin a l k y l ether or diketone. BP (58) or phosphite e s t e r s (59) may be included to a c c e l e r a t e the cure, which should be s u b s t a n t i a l l y completed i n 30 to 60 sec. A d d i t i o n of a photoc r o s s l i n k i n g agent reduces c u r i n g time and lowers s o l u b i l i t y of UV-cured sealants (60). A s i n g l e paste comprising a urethanemethacrylate prepolymer or dimethacrylate, a monomeric d i l u e n t , an alpha diketone (camphorquinone) i n i t i a t o r , dimethylaminoethyl methacrylate (reducing agent) and s i l a n i z e d g l a s s powder y i e l d e d an experimental dental r e s t o r a t i v e with good p h y s i c a l p r o p e r t i e s (61). L i g h t cured m a t e r i a l s do not r e q u i r e mixing by the d e n t i s t and can be manipulated i n d e f i n i t e l y i n the mouth u n t i l t h e i r polymerization i s i n i t i a t e d by exposure to r a d i a t i o n . They s a t i s f y the c o n f l i c t i n g requirements of long working time and short s e t t i n g times (snap hardening) that are d i f f i c u l t to achieve with chemical i n i t i a t o r - a c c e l e r a t o r systems. The r a d i a t i o n e n t e r i n g a composite may be s c a t t e r e d at the r e s i n / p a r t i c l e i n t e r f a c e as w e l l as be absorbed by the p a r t i c l e and r e s i n . The success of r a d i a t i o n - c u r e d m a t e r i a l i s a f u n c t i o n of the l i g h t source, i t s s p e c t r a l d i s t r i b u t i o n and i n t e n s i t y , r a d i a t i o n time, the l i g h t transmission of the r e s t o r a t i v e and l i g h t absorbancy of the surrounding media (62, 63). Degree of cure decreases s l i g h t l y below the r e s t o r a t i o n surface u n t i l a depth i s reached where i t f a l l s o f f r a p i d l y . Generally, a c u r i n g c y c l e of 30 to 60 sec. ensures polymerization up to a


26.

BRAUER AND ARGENTAR


depth of at least 3 mm. Alternatively, in deeper cavities the restoration can be build in consecutive layers since the interlayer bond is satisfactory. Furthermore, curing continues after the initiating photochemical reaction is cut off as evidenced by the increase in hardness of specimens "aging" from one hour to 24 hours. Literature Cited


1. 2. 3.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Brauer, G. M. "Biomedical and Dental Applications of Polymers"; Gebelin, C. G.; Koblitz, F. F., Ed.; Plenum, New York, 1981; pp 395-409. Schnabel, E. German Patent 736,481, May 13, 1943. Blumenthal, L. "Recent German Developments in the Field of Dental Materials". FIAT field report No. 1185, Field Information Agency, Technical Office of Military Government for Germany (US), 27 May 1947. Brauer, G. M.; Davenport, R. M.; Hansen., W. C. Mod. Plastics 1956, 34, 153-168, 256. Lal, J.; Green, R. J. Polymer Sci 1955, 17, 403-409. Rose, E. E . ; Lal, J.; Green, R. J. Am. Dent. Assoc. 1958, 56, 375-381. Cornell, J. A.; Powers, C. M. J. Dent. Res. 1959, 38, 606610. Brauer, G. M.; Petrianyk, N.; Termini, D. J. J. Dent. Res. 1979, 58, 1791-1800. Pryor, W. A. "Free Radicals", McGraw-Hill: New York, 1966, p. 317-318. Walling, C. "Free Radicals in Solution" John Wiley: New York, NY, 1957, pp 189-193. Bowen, R. L.; Argentar, H. J. Am. Dent. Assoc. 1967, 75, 918-928. Bowen, R. L.; Argentar, H. J. Dent. Res. 1971, 50, 923-928. Argentar, H.; Tesk. J. A.; Parry, E. E. J. Am. Dent. Assoc. 1981, 102, 664-665. Schmitt, W.; Purrmann, R.; Jochum, P. Ger. Offen. 2, 658, 538, June 30, 1977. Bowen, R. L . ; Argentar, H. J. Dent. Res. 1972, 51, 473-482. Dnebosky, J.; Hynkova, V.; Hrabak, F. J. Dent. Res. 1975, 54 772-776. Hynkova, V . ; Hrabak, F. Makromol. Chem. 1975, 176, 16691678. Koblitz, F. F.; O'Shea, T. M.; Glenn, J. F.; DeVries, K.L. J. Dent. Res. 1977, 56B, No. 791. Antonucci, J. M.; Stansbury, J. W.; Dudderar, D. J. J. Dent. Res. Res. 1982, 61, 270. Ruyter, I. E . ; Svendsen, S. A. Odontol. Scand. 1978, 36, 75-82.


370 21. 22. 23. 24. 25. 26.


27. 28. 29. 30. 31. 32. 32a. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51.

INITIATION OF POLYMERIZATION Prior, W. A.; Hendrickson, W. H., Jr. J. Am. Chem. Soc. 1975, 97, 1582-1583. Horner, L . ; Brüggemann, H.; Knapp, K. H. Ann. 1959, 626, 119. Brown, H. C.; Okamoto, Y. J Am. Chem. Soc. 1958, 80, 49794987. Mleziva, J. Plasticke Hmoty. Kaucuk, 1964, 1, 225-231. Mleziva, J. Chem. Prumysl. 1965, 15, 80-85. Brauer, G. M.; Dulik, D.; Antonucci, J. M.; Termini, D. J.; Argentar, H. J. Dent. Res. 1979, 58, 1994-2000. Brauer, G. M.; Stansbury, J. W.; Antonucci, J. M. J. Dent. Res. 1981, 1343-1348. Argentar, H. U. S. Patent 4,243,763, Jan. 6, 1981. Argentar, H. U. S. Patent 4,284,551, Aug. 18, 1981. Kravtsov, D. N.; Faingor, D. N. Izo. Akad. Nauk. SSR, Ser. Khim 1968, 289-296; Chem. Abstr. 1968, 69, 66717. Argentar, H. J. Res. Nat. Bur. Stand. (U.S.), 80A, 173-187. Walling, C.; Indictor, N. J. Am. Chem. Soc. 1958, 80, 58145818. Argentar, H. unpublished results. Stecker, P. G., Ed. "The Merck Index," 3rd ed.: Merck: Rahway, NJ 1968, p. 60. Lapkin, V. A. Farmakol. Tokskol (Moscow), 1974, 37, 660662; Chem. Abstr. 1974, 82 68082. Jacobsen, A. H . ; Pettersen, A. H. personal communication. Bowen, R. L.; Argentar, H. J. Appl. Polymer Sci. 1973, 17 2213-2222. American Dental Association Specification No. 27 for Direct Filling Resins; J. Am. Dent. Assoc. 1977, 94, 1191-1194. Hrabak, F.; Hynkova, V. Macromol. Chem. 1981, 182, 15951603. Brauer, G. M.; Stansbury, J. W. unpublished data. Okada, Y.; Kogyo Kagaku Zasshi 1963, 66, 1317-1320; Engl. Summ. 1963, 66, A83. Yates, W. R.; Ihrig, J. L. J. Am. Chem. Soc. 1965, 87 710715. Ruyter, I. E. Acta Odontol. Scand. 1981, 39, 27-32. Hagger, O. Helv. Chim. Acta 1948, 31, 1624-1630. Hagger, O. Helv. Chim. Acta 1951, 34, 1872-1876. Castan, P.; Hagger, O. U. S. Patent 2,567,803, Sept. 11, 1951. Bredereck, H.; Bäder, E. Chem. Ber. 1954, 87, 129-139. Bredereck, H.; Bäder, E. U. S. Patent 2,846,418, Aug. 5, 1958. Brauer, G. M.; Burns, F. R. J. Polymer Sci. 1956, 19, 311321. Stern, H. J.; Shadbolt, L. E . ; Rawitzer, W. L. British Patent 721,641, Jan. 12, 1955. Shadbolt, L. E. Ger. Offen, 1,937,871, Jan. 29, 1970. Lai, J. U. S. Patent 2,833,753, May 6, 1958.


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52. Takaaki, S.; Yuji, M. J. Polymer Sci. 1966, A-1, 4, 2735-2746. 53. Sumitomo Chemical Ltd. Brit. Patent 1,177,879, Jan. 14, 1970. 54. Termin, S. C.; Richards, M. C. U. S. Patent 3,991,008, Nov. 9, 1976. 55. Antonucci, J. M.; Grams, C. L.; Termini, D. J. J. Dent. Res. 1979, 58, 1887-1889. 56. Masuhara, E. Deut. Zahnärztl. Z. 1969, 24, 620-628. 57. Nakabayashi, N.; Masuhara, E.; Michida, E.; Ohmori, I. J. Biomed. Mat. Res. 1978, 12. 149-165. 58. Waller, D. E. U. S. Patent 3,709,866, Jan. 9, 1973. 59. Schmitt, W.; Purrmann, R. Ger. Offen. 2,646,416, May 26, 1977. 60. Brauer, G. M. J Dent. Res. 1978, 57, 597-607. 61. Dart, C. E . ; Cantwell, J. B.; Traynor, J. R.; Jaworzyn, J.N. U. S. Patent 4,089,763, May 16, 1978. 62. Cook. W. D. J. Dent. Res. 1980, 59, 800-808. 63. Kilian, R. J. "Biomedical and Dental Applications of Polymers" Gebelin, C. G.; Koblitz, F. F., Ed; Plenum: New York, 1981, pp. 411-417. 64. Bier, A. Rec. Trav. Chim. 1956, 75, 866-870. RECEIVED October 5, 1982


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