Novel (meth)acrylate monomers for ultrarapid polymerization and enhanced polymer properties

7 LA-UR- 02-759 Approved for public release; distribution is unlimited. Title: A uthor(s): Submitted to: Novel (meth)acrylate monomers for ultrarapid polymerization and enhanced polymer properties Eric R. Beckel Kathryn A. Berchtold Jun Nie Hui Lu Jeffrey W. Stansbury Christopher N. Bowman Division of Polymer Chemistry (preprint) ACS National Meeting 2003 March 23-27, 2003 New Orleans, Louisiana NATIONAL LABORATORY Los Alamos National Laboratory. an affirmative actiodequal opportunity employer, is operated by the University of California for the U.S. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the US. Government retains a nonexclusive, royaltyfree license to publish or reproduce the published form of this contribution, or to allow others to do so, for U.S. Government putposes. Los Alamos National Laboratoty requests that the publisher identify this article as work performed under the auspices of the U.S. Department of Energy. Los Alamos National Laboratory strongly supports academic freedom and a researchets right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness. FORM 836 (10/96) NOVEL (METH)ACRYLATE MONOMERS FOR ULTRARAPID POLYMERIZATION AND ENHANCED POLYMER PROPERTIES Eric R. Beckel', Kurlrryn A. Berclitold'*2,JIIIINie', H ~ i LII', i Jeffrey W.Stumbury', uiid Clrristoplrer N.Bow~tnun'." 'Department of Chemical Engincering, University of Colorado, Boulder, CO 80309-0424 'Materials Science & Technology Division, MST-7, Los Alanios National Laboratory, MS ES49, Los Alanios, NM 87545 'Department of Restorative Dentistry, University of Colorado Health Sciences Center, Denver, CO 80045-0508 Introduction Ultraviolet light is known to be one of the most efficient methods to initiatc polymeric reactions in thc presence of a photonitiator. Photopolymerizations are advantageous because the chemistry of the materials can be tailored to design liquid monomers for ultrarapid polymeri7ation into a solid polymer material. One way to achieve rapid photopolyiiieri7ations is to utilize multifunctional (meth)acrylate monomers. which form highly crosslinked polymers; however, these monomers typically do not achieve complete functional group conversion.'.' Recently, Decker et a/. devcloped novel monovinyl acrylate monomers that display polyriicrization kinetics that rival those of multifunctional acrylate monomers. Thcse novel acrylate monomers incorporate secondary functionalities and end carbamate^,'".^ cyclic carbonates:'"" and groups such as o x a z o l i d o n e ~which ~ ~ promote the increased polymerization kinetics of these monomers. In addition to thc polynierization kinetics, these novel monovinyl monomers form crosslinked polymers, which are characterized by having high strength and high flexibility. Unfortunately, the exact mechanism or mechanisms responsible for the polymerization kinetics and crosslinking are not well understood. Several mechanistic theories have been proposed in an attempt to cxplain the enhanced reactivities of these novel monomers.. Decker et a/. proposed a possible hydrogen abstraction reaction due to labile hydrogens associated with the secondary functionalities and end groups.'." These labile hydrogens undergo a chain transfer reaction, creating branches and thus, radical sites for crosslinking. Recently, Jansen et ul. proposed a theory attributing increases in polymerization ratc to increases in monomer dipole riiomcnt.lZ Although such mechanisms may contribute to the enhanced rcactivity and unique material properties achieved in these polymers, they do not provide a complcte explanation. This work cxpands on Decker's novel niononier systems to try to gain an understanding of the mcchanism or niechanisnis for the polymerization kinetics and ability to crosslink. The specific goal is to elucidate the rclationship between monomer functionality and monomer structure in rclationship to polymerization kinetics and polymer network structure. A better undcrstanding of the mechanism(s) through which these materials polymerize will allow monomer design to be tailored such that specific properties will be attained. Experimental Instrumentation. Molecular weight between crosslinks (M,) was obtained from niechanical property data obtained from a Perkin-Elmer DMA 7c Dynamic Mechanical Analyzer. M, was calculated from the following cquation: Results and Discussion This work proposes and evaluates several mechanistic theories for the enhanced reactivities and material properties exhibited by these novel monomers. Specifically, hydrogen bonding, hydrogen abstraction, and electronic and resonance effects are explored. Each of these mechanisms is interrelated; thus, it is diflicult to isolate their individual contribution to the polymerization reaction. Nonetheless, experiments can be developed to cmphasize each of the above mechanisms individually to test its validity. To examine the possibility of hydrogen abstraction, the material properties of acrylate monomers were examined with experiments designed to look specifically at the molecular weight between crosslinks. Table 1 summarizes these material properties and compares the properties to those of conventional acrylate polymers. Table 1. Material properties of monomers polymerized at 25°C. Monomer T, M, #Repeat [XL] ("C) (almol) Units ImollL) 40200 160 0.03 50800 200 0.02 2 12000 100 0.09 -, 2200 18 0.50 100 -- __ __ Benzyl Carbamate Acrylate 42 Benzyl Carbonate Acrylate 18 Hydroxy Ethyl Acrylate Hydroxy Propyl Acrylate Diethylene Glycol Diacrylate The material propcrty studies outlined show above in Table 1 all utilized a photoinitiator (DMPA) concentration of 0.1 wt% = 4 x IO' mol/L. Hydrogen abstractiodchain transfer generates branching and from which crosslinks can be formed via termination by combination of these branches. Thus, if the proposed hydrogen abstraction/chain transfer mechanism is solely responsible for crosslink formation in these materials, one would expect the quantification of crosslinks to be, at a maximum, of the same order as that of the initiator concentration. However, these materials exhibit crosslink formation that exceeds the initiator concentration by almost IO-fold. Thus, traditional hydrogen abstractiodchain transfer, although a viable mechanism for the crosslinking ability of these monomers, cannot be the sole mechanism responsible for crosslink formation. Since hydrogen abstraction cannot fully account for the crosslink concentration and polynierization kinetics of these novel monomers, the possibility of electronic and resonance effects on the polymerization rate of the novel nionomers was also investigated. To accomplish the electronic and resonance studies, monomers with electron withdrawing substituents on an aromatic end group substituent were synthesized. Figure 1 shows the arrangement of electron withdrawing substituents on an aryl end group. 3 RTp E' M , =- where R is thc gas constant, p is the polymer density, E' is the storage modulus in the rubbery plateau, and T is the absolute temperature where the storage modulus was evaluated." Steady state and unstcady state kinetic data was obtained from a Nicolet Magna 760 FTlR spectrometer equipped with a horizontal transmittance apparatus. All systems were polymerized with SmW/cmz of ultraviolet light, using a filtercd (peak irradiation wavelength: 365 nm) EXFO Ultracure 100s light source and 0.1 wt% 2,2-dimethoxyphenylacetophenone (DMPA) as the photoinitiator. Figure 1. Monomer utilized for electronic and resonance effects studies. R1, R2, and R' can be F o r H. The electron withdrawing substituent studies revealed some very intriguing results. Figure 2 shows the steady state bulk polymerizations of the electron withdrawing (fluoro-substituted) nionomers. From Figure 2, it is evident that the meta substituted monomer does not have a significant polymerization rate difference from the unsubstituted monomer. However, the para, ortho, and penta substituted niononiers decreased by approximately 2-fold in polyineri7;ltion rate as compared to the nieta and unsubstituted monomers. To determine the root cause of the polytnerization rate difference in thc differing monomers, unsteady state experiments were performed to deduce the kinetic constants for each monomer. figures 3 and 4 show k, and k, versus conversion for the systems studied. k, is the propagation kinetic constant and k, is the termination kinetic constant. 67°C E .-0 l1o2 o3L e I ' I ' I ' , . , . , ' , . , . 0.0 0.1 0.2 0.3 . 0.4 0.5 0.6 0.7 0.8 0.9 9E s Conversion Figure 4. Termination kinetic constant versus conversion for the fluorosubstituted monomer. The monovinyl systems presented are: (m) Unsubstituted, ( 0 ) para, (A) meta, (V) ortho, and (e) pcnta fluorosubstituted. 0 10 20 Polymerization Time (s) Figure 2. Effect of an electron withdrawing substituent (fluoro-substitution) on steady state kinetics, The monovinyl systems presented are: (1) unsubstituted, (2) nieta, (3) ortho, (4) para, ( 5 ) penta, and (6) dinieta fluorosubstituted. Polymerizations were conducted at 67°C with 0.1 wt% DMPA at 5niW/cni2. Figurcs 3 and 4 rcvcal that the meta substituted and unsubstituted monomers have approxiniately equivalent k, and k, values over the conversion range studied. Howevcr. the para, ortho, and penta substituted monomers show drastically diffcrcnt kinetic values as compared to the meta and unsubstituted monomers. There is an approximate 2-fold increase in k, and an approxiniate IO-fold increase in k, for the para. oilho. and penta substituted monomers as compared to the meta and unsubstituted monomers. Unsteady state analysis of the fluoro-substituted niononier shows a significant electronic and resonance effect on the kinetics. These electronic effects will be influenced by several factors, including the dipole nionient of the molecule, as the choice of substituent and position on the benzene ring will affect the overall dipole moment of the molecule. Additionally, these benzene ring substituents will influence the degree of abstraction and chain transfer attainable from these monomers. The most significant observed effect is associated with the temiination kinetics, as the para and othro substitutions significantly increase the tcrniination of these monomers, thus decreasing the overall polyrncrization rate. 107j 1 4 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Conversion Figure 3. Propagation kinctic constant versus conversion for the fluorosubstituted monomer. The monovinyl systems presented are: (m) Unsubstituted, ( 0 ) para, (A) meta, (V) ortho, and (*) penta fluorosubstituted. The fluoro-substitution results verify that the electronic and resonance effects do influence the reactivity of these novel monomers. The results show that a specific substituent can negatively impact the ovcrall polymeri7;ltion rate. However, with careful selection of an appropnate substituent, a positive influence is observed and the ovcrall polymerization rate is increased. Conclusions Novel monovinyl (meth)acrylates have been developed that show ultrarapid polymerization and the ability to form crosslinked networks. The exact mechanism(s) for these enhanced polynicr properties are not wcll understood. Hydrogen abstractiodchain transfer is a possible mechanism for the ability to crosslink, but this chain transfer cannot account for the significant crosslink concentrations measured in these materials. In addition, electronic and resonance effects show a significant influence on the polymerization kinetics of these polymers. Elcctron withdrawing substituents on the benzene ring alter the reactivities of the inonoiner and display a significant increase in temiination kinctics for the para. ortho, and penta substitutions. Acknowledgements. The authors of this paper would like to thank the IUCRC for fundamentals and applications of photopolyinenzations for supporting this research. References ( 1 ) Decker, C.; Moussa, K. Macrornolecules 1989,22,4455. (2) Tryson, G.R.; Shultz, A.R. J. Poly. Sci.: Poly. Phys. 1979, 17. 1043. (3) Young, J.S.; Kannurpatti, A.R.; Bowman, C.N. Mucromol. Chem Phys. 1998,199, 1043. (4) Decker, C.; Moussa, K. h4ukron1ol.Clieni. 1991,192,507. (5) Decker, C.; Moussa, K Eur. Poly. J. 1991,27,403. (6) Decker, C.; Moussa, K. Eur. Poly. J. 1991.27, 881. (7) Moussa, K.; Decker, C. J. Poly. Scr.: Poly. Clieni. 1993, 31, 2197. (8) Brosse, J.C.; Chevalier, S.; Couvert, D.; Decker, C.; Moussa, K.; Societe Nutionale des Pourdres et Explosif~,1989. (9) Decker, C.; Moussa, K. Makroniol. Cliein. Rapid Conrin. 1990, 11, 159. (IO) Moussa, K.; Decker, C.; Brosse, J.: Chevalier, S.; Couvert, D. Societe Nutionale des Pourdres et &plosi$v, 1991. (1 1) Dccker, C. Nucl. Inst. Methods Pliys. Res. E 1999, 151, 22. (12) Jansen, J.F.G.A.; Dias, A.A.; Dorschu, M.; Cousscns, B. MucroriiolPcirles 2002. 3.5, 1529. (13) Hill, L.W.Coatrng 7ech. 1992,64,29. .