Pulsed-Laser Initiated Reversible Addition ... - ACS Publications

Chapter 31 Pulsed-Laser Initiated Reversible Addition Fragmen tation Chain Transfer Polymerization

Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch031

M. Buback, T. Junkers, and P. Vana Institut für Physikalische Chemie, Georg-August-Universität Göttingen, D-37077 Göttingen, Germany

The almost instantaneous production of radicals via laser pulse initiation during RAFT polymerization in conjunction with measuring the subsequent decay of the intermediate RAFT radical concentration by μs time-resolved ESR spectroscopy allows for deducing addition and fragmentation rate coeffi cients of the RAFT process. By tracing monomer concentra tion via μs time-resolved NIR spectroscopy after an initiating laser pulse during RAFT polymerization, the rate coefficient of termination between propagating radicals and its chain-length dependence may be assessed. Results obtained by the novel SP-PLP-ESR-RAFT and SP-PLP-NIR-RAFT techniques are presented for butyl acrylate polymerization.

© 2006 American Chemical Society

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

455

456


Introduction

Reversible addition-fragmentation chain transfer (RAFT) polymerization (7) has become one of the major controlled/living radical processes for generating ma terials with narrow molecular weight distribution and with complex microstructure (7-5). In contrast to other prominent controlled polymerization techniques, such as atom transfer radical polymerization (ATRP) (4) or nitroxide-mediated radical polymerization (NMP) (5), a permanent supply of initiating radicals is required for RAFT polymerization to proceed. When producing primary radicals by thermal decomposition of initiators, such as peroxides or azo compounds, elevated tem peratures are required. In order to perform RAFT polymerizations under ambient temperature conditions, continuous gamma- (6,7), plasma- (8) and UV-irradiation (photoinitiation) (9,10) were applied. Due to the intense UV/VIS absorbance of dithio compounds, decomposition of mediating RAFT agents was observed in case of photoinitiation (Ρ, 70), which thus requires judicious adjustment of excitation wavelength to the UV/VIS absorbance of the RAFT agent, in order to prevent a significant loss of mediating compounds during polymerization. A key advantage of photoinitiation by laser pulses relates to the possibility of ex ternally triggering die amount of photoinitiator-derived primary radicals, which are produced at a precisely known instant in time. This feature enables detailed kinetic analysis of individual reactions steps. Laser pulsing may, e.g., be performed by using an excimer laser with a pulse width of a few tens of nanoseconds. In the pulsed-laserpolymerization size-exclusion-chromatography (PLP-SEC) method (77), a sequence of evenly spaced laser pulses is used for accurately measuring propagation rate coeffi cients, kp. in SP-PLP experiments, photoinitiation by a laser single pulse (SP) is car ried out in conjunction with time-resolved measurement of either monomer con centration, via near-infrared spectroscopy (SP-PLP-NIR) (12,13), or of propagating radical concentration via ESR-spectroscopy (SP-PLP-ESR) (14,15). The quality of measuring termination rate coefficients, kt, in conventional radical polymerization has enormously been improved by using such SP-PLP techniques. It appears hence to be a matter of priority to apply pulsed-laser techniques also to the in-depth study of RAFT polymerization kinetics. The present article addresses three topics: (a) controlled RAFT polymerization under pulsed-laser initiation, (b) the measurement of the RAFT addition and frag mentation rate coefficients by combining a laser single pulse experiment with ESR spectroscopy, and (c) the determination of chain-length dependent ^ values by SPPLP-NIR in RAFT polymerization systems, where radical chain length is con trolled. The SP-PLP-ESR experiments were carried out at ambient pressure, whereas most of the SP-PLP-NIR experiments were run under high pressure, typically at 1000 bar, to improve signal to noise. There is no reason to assume that high pres sure changes the RAFT mechanism and a recent study into styrene RAFT polym erization demonstrated that molecular weight control is even slightly improved toward high pressure (16).


457


Experimental

Materials. Butyl acrylate (BA, 99 %, Fluka) wasfreedfromthe inhibitor by passing over a column of activated basic alumina. a-Methyl-4-(methyl-mercapto)-amorpholino-propiophenone (MMMP, 98%, Aldrich), which was employed as the photoinitiator, 2^,6,6-tetramethylpiperidine-l-oxyl (TEMPO, 99%, Aldrich), and tolu ene (99.5%, Fluka) were used as received. The RAFT agent S-S'-bis(methyl-2propionate)-trithiocarbonate (BMPT, see Figure 1) was synthesized according to the protocol described elsewhere (75). Solutions of BA, toluene, and BMPT were deoxygenized by threefreeze-pump-thawcycles. The photoinitiator was added under an argon atmosphere. SP-PLP-NIR measurements. The mixtures for laser-induced polymerization were introduced into an internal cell, consisting of two CaF windows and a cylindrical teflon tube. The internal cell wasfittedinto a high-pressure optical cell of transmission type (7 7). Polymerizations are induced at 60 °C and pressures of 5 or 1000 bar by XeF excimer laser pulses (at 351 nm) each of 2 to 3 mJ. Monomer conversion was monitored after each laser pulse via μβ time-resolved NIR spectroscopy at around 6170 cm" , that is, in thefirst-overtonerange of C-H modes in α-position to a C-C double bond. After applying a series of laser pulses, the high-pressure cell was inserted into the optical com partment of an IFS 88 FT-NIR spectrometer to determine overall monomer conversion by measuring the full C-Hfirst-overtoneNIR spectrum. SP-PLP-ESR measurements. ESR experiments were performed on a Bruker Elexsys® Ε 500 series cw-ESR spectrometer. The mixture of monomer, photoinitiator, and RAFT agent in toluene solution was filled into a quartz tube of 5 mm outer and 4 mm inner diameter and was irradiated inside a grid cavity using a Lambda Physik COMPex 102 excimer laser on the XeF line at 351 nm with a laser output energy of about 50 mJ per pulse. The ESR spectrometer and the excimer laser were triggered by a Scientific Instruments 9314 pulse generator. Radical concentration vs. time profiles were deduced from ESR intensities measured with μβ time resolution at the magnetic field that is asso ciated with the peak maximum of the ESR spectrum of the radical species under investi gation. Radical concentrations were obtained by a two-fold calibration procedure: First, die double integral of the ESR spectrum was calibrated against a TEMPO solution in ΒΑ/toluene mixtures at conditions close to the ones of the actual polymerizations. Sec ondly, the peak signal intensity atfixedmagnetic field was calibrated against the double integral of the full ESR spectrum (75). UV/VIS spectroscopy. UV/VIS spectra were recorded from 220 to 440 nm on a Bruins Omega 10 double-beam spectrometer. Simulations. Simulations were carried out using PREDICI®, version 6.4.1. SEC analyses. Molecular weight distributions were determined via size-exclusion chromatography (SEC), using a Waters 712 WISP autosampler, a Waters 515 HPLC pump, PSS-SDV columns with nominal pore sizes of ΙΟ ,10 and 10 A, a Waters 2410 refractive index detector, and THF at 35 °C as the eluent. Molecular weight distributions were evaluated according to the principle of universal calibration using reported MarkHouwink parameters (18). 2

1

5

3

2


458


RAFT polymerization under PLP conditions So far, kinetic investigations into RAFT polymerization systems were lim ited to reaction conditions with continuous initiation. In such experiments, indi vidual RAFT-specific rate coefficients can be obtained under the assumption of a steady state and by combining several independent experiments (19). Alterna tively, kinetic coefficients of the RAFT process may be estimated by quantumchemical calculations (20), which at present are restricted to small molecules, or by modeling the overall rate of polymerization (20,21), whereby kinetic informa tion is deduced from the decrease in polymerization rate upon increasing RAFT agent concentration. The rate coefficients resulting from the latter method are highly dependent on the model used for describing rate retardation. RAFT ex periments under instationary conditions after laser single pulse excitation should be suitable for deducing RAFT addition andfragmentationrate coefficients un der less stringent assumptions. Moreover, chain-length dependent ^ may be ob tained from SP-PLP experiments carried out on RAFT polymerization systems.

5S

cé

Si

•εο «ο dead polymer

(e)

Int + Int —*—> dead polymer

k


• self

Scheme 2. Reaction steps and associated rate coefficients contributing to tim dependent intermediate radical concentration in SP-PLP-ESR-RAFT experiments. The reaction steps (a), (b), and (c) are sufficient for modeling the observed change in c . More detailed estimates can be made by using the extended scheme which includes cross-termination (d) and/or self-termination (e) of Int. The chain length of the participating species needs not to be considered in mod eling time-resolved c , as radical size is not or not significantly changing during the course of one single-pulse experiment (13). Figure 5 demonstrates that the simple kinetic model given in Scheme 2 (without steps (d) and (e)) provides an adequate fit of c t vs. time plots measured by ESR spectroscopy during BMPTmediated BA polymerizations (24). Including cross-termination yields fits of similar quality as the ones shown in Figure 5. The same observation is made in case of including step (e), i.e., self-termination, which was not considered in the present study, as this reaction would lead to the formation of six-arm stars which is unlikely to occur because of steric reasons. The £ and A values deduced from modeling BMPT-mediated BA polym erization at -30°C are listed in Table 1. This relatively low temperature has been chosen for validation of the new method, as mid-chain radicals generated by transfer reactions are occurring in the BA polymerizations at elevated tempera tures. The presence of such radicals adds complexity to the data evaluation. tat

Int

to

ad

p

Table 1. Addition and fragmentation rate coefficients in BMPT-mediated BA polymerization at -30°C; C R A F T = 4.110" molL' ,k = 2.210 LmolV , and* , -k !2 0

3

1

7

1

t

kinetic model includes: no termination of intermediate cross termination

t cross

Aaa/L-mol '-s 2.0· 10 2.0· 10 5

5

1

x

1

his'

2

2.110 0.8· 10

2


463 Whether or not reactions between an intermediate and a propagating radical (cross-termination) take place (see Table 1), has no effect on £ , which agrees well with previously reported values for this system (25), and influences k$ by less than one order of magnitude. A possibly reversible termination of the inter mediates yields Λ values that lie in between those obtained by these limiting models. This is a largely reduced uncertainty in k$ compared to the difference by several orders of magnitude between A values of cumyl dithiobenzoate-mediated polymerizations, which have been derived by modeling rate of polymerization data on the basis of different model assumptions (20). ad

β


p

1.0 0.8 sir

0.6 0.4 0.2 0.0 0

3

2-10"

3

5-10- 0

0.05

0.10

0.15

t/s

Figure 5. Time evolution, after applying a single laser pulse att = 0, of the normalized ESR signal at thefieldposition that corresponds to the peak maximum of the full spectrum (see Figure 6) in BMPT-mediated BA polymerizations (C MPT 4.1Ί0~ mol-IT , C A = 2.0 mol-L' , in toluene). (Reproducedfrom ref 24. Copyright 2006 Wiley-Interscience) =

B

2

1

1

B

As a single-pulse technique, the novel method may be applied at any time and thus at any monomer conversion during RAFT polymerization. Thus, at least in principle, chain-length dependent rate parameters may be deduced and the situa tions of pre- and main RAFT equilibrium may be examined separately. Figure 6a depicts the full ESR spectrum of BMPT-derived intermediate radicals, which is obtained under quasi-stationary reaction conditions at a laser repetition rate of 20 Hz. Closer inspection of the spectrum indicates that there is an overlap of two singlet lines of different band width. The contributions of the two species change during polymerization. The SP-PLP-ESR-RAFT experiments in Figure 5 have been carried out at conditions corresponding to the ones depicted in Figure 6a. The intermediate radical in BMPT-mediated RAFT polymerization has no proton in the immediate vicinity of the radical site. Thus, no hyperfine splitting of the ESR spec-


464 trum is expected. The observation of two overlapping singlet components may be due to the fact that the intermediate RAFT radical occurs under both pre- and main equilibrium conditions. This hypothesis is in line with the observation of the sharper peak becoming weaker upon continuing laser irradiation. As illustrated in Figure 6b, a third radical species evolves after application of about 700 laser pulses. This component is assigned to the four-line spectrum of the secondary propagating radical in BA polymerization. The variation in the ratio of intermediate radical to propagating radical concentration with progressive polym erization corresponds to a change in the equilibrium constant, K, by about one or der of magnitude. This observation suggests that andfc may be different for the pre- and main equilibrium regions and, additionally, may be chain-length depend ent.


p

Figure 6. ESR spectra recorded during a quasi-stationary BMPT-mediated = 4Ί0Γ mol-L' ) BA polymerization (C A = 2 molL~ ) in solution oftoluene at -30 °C, using a laser pulse repetition rate of20 Hz, after applying approximately (a) 300, and (b) 700 laser pulses. 3

(CBMPT

1

!

B

Estimating k$fromfits of the measured CM VS. time curves to the model pre sented above, is restricted to systems where self-termination of intermediate radi cals is not significant. In case that such self-termination is included into the kinetic modeling, the resulting k$ values decrease with increasing extent of selftermination. Assuming the intermediate radicals to be stable, i.e., A = 0, the ex perimentally observed decay of the intermediate radical concentration can exclu sively be assigned to self-termination. The underlying situation is however unrea sonable in that no propagating radicals occur in such a system. In order to resolve the degree of self-termination, it appears desirable to additionally determine the concentration of propagating radicals, which may be achieved by measuring the change in intensity of the ESR components at either low or high magnetic field (see Figure 6b), or via SP-PLP experiments, in which the propagating radical concentra tion is also measured with high time resolution. Knowing the time evolution of both intermediate and propagating radical concentration enables a comprehensive ki netic analysis of RAFT kinetics. From the measured time evolution of propagating radical concentration, the chain-length dependence of ^ may unambiguously be determined, once the RAFT rate coefficients, k^ and £ , are known. p

p


465 Chain-length d e p e n d e n t termination rate c o e f f i c i e n t s d e d u c e d


from SP-PLP-NIR-RAFT e x p e r i m e n t s

In SP-PLP-NIR experiments (12), monomer conversion induced by a laser single pulse is detected by με time-resolved NIR spectroscopy, e.g., in the firstovertone region of C - H modes at the C-C double bond. In conventional SPPLP-NIR, the kinetic analysis for the termination rate coefficient, At,frommeas ured conversion vs. time traces is difficult, because of the steadily increasing chain length with time, t, after applying the laser pulse. As changes of chain length affect At, the analysis of monomer conversion vs. time traces has to be carried out by implementing models for the chain-length dependence of h As an alternative, it appears highly attractive to perform SP-PLP-NIR experiments on systems where the molecular weight is controlled by a RAFT agent (13,25). To elucidate whether such an SP-PLP-NIR-RAFT experiment is feasible, PREDICI simulations have been carried out for butyl acrylate polymerization. In these simulations, the RAFT main equilibrium, the cross-termination of the intermedi ate radical, and the chain-length dependence of k (according to At(y) kt'tt'jY^ , where i and j are the chain lengths of two terminating radicals and k? is the termination rate of radicals of chain length unity), were considered. The rate coefficients of butyl acrylate polymerization were taken from ref. (25). The transformation of the laser pulse-induced primary radicals species into radicals of the size that is characteristic for the polymerizing RAFT system, is visualized in Figure 7. The simulated (SEC-weighed) distribution of propagating radicals is given for an equilibrium constant Κ = kjk$ = 1000Lmol" for the early time interval of up to 1 ms after applying the laser pulse. Figure 7a refers to the situation directly after the generation of the primary radicals. These radicals are propagating as well as rapidly transforming into radicals of the par=

2

1

1

log (Ml g-mol" )

Figure 7. Simulated transformation of laser-induced small radicals into macroradicals of uniform size (indicated by *), which is controlled by the RAFT kinetics. The time interval coveredfrom (a) to (d) refers to the initial period of up to 1 ms after firing the laser pulse. (Reproducedfrom ref 25. Copyright 2005 American Chemical Society)



466 ticular size (indicated by the asterisk), which is characteristic of the RAFT sys tem and is given by the initial RAFT agent concentration and by the actual de gree of monomer conversion. Figure 7b and c show transitional situations. In Figure 7d, which corresponds to about 1 ms delay after firing the laser, the radi cals have nearly quantitatively been transformed into radicals exhibiting the RAFT-specific narrow chain-length distribution. The simulated variation in polydispersity of both propagating and RAFT in termediate radicals with time, that is, with increasing number of evenly separated laser pulses, is shown in Figure 8. The chosen dark time of 50 s between succes sive laser pulses allows for an almost complete decay of radical concentration after each pulse. The narrow time span, during which primary radicals are pro duced and transformed into radicals of the characteristic size, is reflected by the intense sharp peaks occurring upon application of each individual laser pulse. The negative amplitude appearing up to the third pulse is an artifact associated with PREDICI simulations for small-size species. The distribution in size of propagating radicals and of the polyRAFT species becomes narrower toward larger numbers of applied laser pulses and thus toward increasing monomer con version. This simulation result is in full agreement with the experimental obser vations made on RAFT polymerizations under PLP-conditions (see Figure 4). 3.0 polyRAFT agent

2.5 4

prop, radicals

M

?

2.0 15 H 1.0 12

number of laser pulses, η

Figure 8. Simulated time evolution ofpolydispersity index ofpolyRAFT agent and of the propagating radicals in single-pulse initiated RAFTpolymerization. The time is scaled to the number of initiating laser pulses with the dark tim between two successive laser pulses being 50 s. Influence of the stability of R A F T intermediate radicals As the RAFT equilibrium constant Κ may not exactly be known for a system that is subjected to SP-PLP-NIR-RAFT studies, the effect of variations in K, and


467 thus in the stability of the RAFT intermediate radical, on the data analysis has been estimated via simulations. The influences of cross-termination between an intermediate radical and a propagating radical as well as of decreasing fragmen tation rates of the intermediate radicals were assessed. At data was obtained from simulations of SP-PLP-NIR-RAFT experiments described above, followed by fitting the resulting monomer conversion vs. time traces to the integrated rate law, Eq.(l), for an ideal SP-PLP experiment, in which At is assumed to be chainlength /«dépendent. £^.( . Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch031

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