Chapter 4
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Termination Kinetics of Acrylate and Methacrylate Homo- and Copolymerizations 1,2
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Michael Buback , Mark Egorov , and Achim Feldermann
1Institutfür Physikalische Chemie der Georg-August-Universität, Tammannstrasse 6, D-37077 Goettingen, Germany 2Corresponding author: email:
[email protected] Termination kinetics of acrylate and methacrylate homo- and copolymerizations at 40°C and 1000 bar have been measured up to high conversion via the S P - P L P technique. D M P A and α-methyl-4-(methylmercapto)-α-morpholino propiophenone have been used as photoinitiators. An initial plateau region of constant k is seen which, with M M A , extends up to about 20 % monomer conversion and is followed by a reduction inktby about three orders of magnitude up to 50 % conversion. Dodecyl acrylate (DA) shows a distinctly different behavior in that constant (plateau) kt is observed up to 75 % conversion. Copolymerization kt in the plateau region is adequately described by a penultimate unit effect model. The chain-length dependence of k is investigated for several homopolymerizations using the expression: kt = k . i-α, where i is chain length. With the exception of D A , where α is about 0.4 in the entire conversion range, αis close to 0.16 in the plateau region and increases once the gel effect region is reached. t
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© 2003 American Chemical Society
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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Introduction During recent years accurate propagation rate coefficients of free-radical homo- and copolymerizations have become available by applying the pulsed laser polymerization-size-exclusion chromatography (PLP-SEC) technique (1, 2, 5). To take full advantage of these data for polymerization modeling and optimization, also other rate coefficients, such as the termination rate coefficient, k need to be precisely known. k 's are also required for describing controlled living polymerizations in which both conversion and free-radical size increase during reaction. Modeling termination rate is by far more difficult as compared to propagation rate because of diffusion control of ^ . In addition to the dependence on temperature and pressure, kt is affected by the difïusivity of macroradicals and thus depends on free-radical size and on the viscosity of the polymerizing medium. The latter quantity is determined by polymer content and by structural and dynamic properties of polymer molecules. The single pulsepulsed laser polymerization (SP-PLP) method (5, 4) allows for a point-wise probing of termination kinetics during the course of polymerization up to high conversion. In addition to providing chain-length averaged termination rate coefficients, the technique allows for measuring the chain-length dependence of k . Within a perfect S P - P L P experiment, the laser pulse instantaneously creates a significant concentration of small photoinitiator-derived radicals which immediately start to propagate and thus, unless transfer reactions come into play, provide a narrow (Poisson-type) free-radical distribution with chain length i linearily increasing with time t after applying the laser pulse. Tennination under S P - P L P conditions thus occurs between free radicals of approximately identical size.
Downloaded by NORTH CAROLINA STATE UNIV on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch004
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Actually, k /kp is the important primary kinetic parameter from S P - P L P . With kp from P L P - S E C , chain-length averaged is immediately obtained from k /kp. First such S P - P L P experiments have been carried out for a series of homopolymerizations (3, 4) and for intra-family acrylate or methacrylate systems (5, 6). These studies were extended to binary copolymerizations of inter-family acrylate-methacrylate systems. The first such inter-family system under investigation via S P - P L P was DA-dodecyl methacrylate ( D M A ) (6). This system is special in that the two homo-£ 's are almost identical. More complex situations occur with copolymerizations where both the homo-A^'s and homo-& 's are significantly different, as is the case with the systems M M A - D A and D M A methyl acrylate ( M A ) (7). Data for these systems measured at 40°C and high pressure, mostly 1000 bar, but for M M A also at 2000 bar will be presented. High-pressure conditions were selected because of the improved signal-to-noise quality of the spectroscopically measured S P - P L P conversion vs. time traces. High ρ is associated with a larger conversion per pulse due to pressure-induced propagation and pressure-retarded termination. With the weakly compressible t
t
t
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In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
44 monomer systems no reason is seen why mechanistic evidence deduced from studies at high ρ should not apply to ambient polymerization conditions. In addition to studying chain-length averaged copolymerization as a function of monomer conversion, the chain-length dependence of kt at (almost) constant degree of monomer conversion is investigated for a series of homopolymerizations.
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Experimental Part The S P - P L P experiments have been carried out as described elsewhere (5, 8). The photoinitiators a-methyl-4-(methylmercapto)-a-morpholino propiophenone ( M M M P , 98 %, Aldrich Chemie) and 2,2-dimethoxy-2phenylacetophenone ( D M P A , 99 %, Aldrich Chemie) were used as supplied at initial concentrations close to 510~ mol-L" . Methyl acrylate (>99 %, stabilized with 0.005 wt.% hydroquinone monomethylether, Fluka Chemie), butyl acrylate (>99 %, stabilized with 0.005 wt.% hydroquinone monomethylether, Fluka Chemie), dodecyl acrylate (which actually is a mixture of 55 wt.% D A and 45 wt.% tetradecyl acrylate, Fluka Chemie), methyl methacrylate (>99 %, stabilized with 0.005 wt.% hydroquinone monomethylether, Fluka Chemie), and dodecyl methacrylate ( D M A , «96 %, Aldrich Chemie) were purified by distillation under reduced pressure in the presence of K C 0 and treated by several freeze-pumpthaw cycles to remove dissolved oxygen. The samples were irradiated with X e F excimer laser pulses (at 351nm) of 2 to 3 mJ energy per pulse. Laser-induced monomer conversion is monitored via online N I R spectroscopy of the C - H modes (at the C=C double bond) at around 6170 cm" . After applying a series of excimer laser pulses, each being followed by microsecond time-resolved nearinfrared spectroscopic measurement of pulse-induced polymerization, the reaction cell is introduced into the sample chamber of an IFS 88 Fourier transform IR/NIR spectrometer (Bruker) where absolute (overall) monomer concentration is checked. During each polymerization experiment, S P - P L P measurements are carried out until the reacting system becomes inhomogeneous or the photoinitiator is consumed. 3
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Copolymerization Before addressing the chain-length dependence of the termination rate coefficient, the conversion dependence of overall (chain-length averaged) will be briefly discussed taking the inter-family copolymerization systems M M A - D A and D M A - Μ Α as examples. The pulsed laser induced monomer conversion measured (with a time resolution of microseconds) as a function of
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
45 time t after applying the laser pulse at t = 0 is fitted to eq 1 which assumes ideal kinetics in that k is constant: t
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