In the Laboratory
Reversible Addition Fragmentation Chain Transfer (RAFT) Polymerization for an Undergraduate Polymer Science Lab T. L. U. Nguyen, Francesca Bennet, Martina H. Stenzel,* and Christopher Barner-Kowollik** Centre for Advanced Macromolecular Design, School of Chemical Sciences and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia; *
[email protected] **
[email protected] Free radical polymerization (FRP) is one of the oldest and industrially most applied polymerization techniques. From approximately the early 1990s, FRP has been revolutionized by the advent of so-called living free radical polymerization (LFRP) techniques (1) that allow for a precise control over the molecular weight of the generated polymers and its polydispersity. In an earlier contribution outlining a comprehensive approach to an undergraduate polymer science curriculum (2), we have detailed that the inclusion of the novel living free radical polymerization techniques in lectures and tutorials, but especially in the laboratory practice, is a matter of priority (details on the underpinning curriculum can be found in ref 2 and will not be reiterated here). Thus far, only two reports exist where atom transfer radical polymerization (ATRP) has been proposed to be included in undergraduate polymer laboratories (3). Despite being very effective in controlling polymerization, the reversible addition fragmentation chain transfer (RAFT) (4) process requires a somewhat demanding synthesis of controlling agents, thus disallowing their use within the time confinements of an undergraduate laboratory practice. However, there exists one commercially available xanthate, ethylxanthogenacetic acid (EXGA), that is suitable for usage in undergraduate laboratories as it can be readily sourced at a reasonable price.1 EXGA, being a xanthate, falls into the class of MADIX (macromolecular design via the interchange of xanthates) agents, a subclass of RAFT agents (5).2 In addition, EXGA is capable of inducing LFRP of vinyl acetate-type monomers, a feature uncommon to NMP3 and ATRP. In the present contribution we provide an effective undergraduate laboratory learning experience in LFRP that can either be carried out in conjunction with the already described ATRP protocols or serve as a stand alone experiment. It is envisaged that the experiment we describe can be part of a dedicated polymer chemistry lab class or it could equally well be incorporated into an organic chemistry lab. Within the conceptual framework provided in our earlier contribution on polymer science curriculum development, the present laboratory experiment is ideally suited to the third-year of a chemistry or industrial chemistry degree (2).
divided into six reaction vessels, sealed with rubber septa and the mixtures are subsequently purged thoroughly with nitrogen for approximately 10 min. If the group of students carrying out the experiment is relatively large (i.e., exceeding 4) a range of experimental conditions may be studied, for example, variable RAFT agent concentrations. Ideally, the experiments should be carried out over two laboratory sessions, each lasting 4 h, preferably separated by a week. In the first session, after thorough degassing, the reaction is started by immersing the reaction vessels into a water bath at 70 °C. The reaction vessels are subsequently removed at preset times starting at a reaction time of 90 min. The mixture in the reaction vessel is then quenched with an ice bath prior to pouring the monomer–polymer mixture into a Petri dish. After recording the mass of the solution, the mixture is allowed to dry under a fumehood. The students are required to remove a reaction vessel every 20 to 30 minutes repeating the above procedure. In the second session, which should preferably be set a week later to ensure the evaporation of all monomer, the students return to determine the quantity of polymer formed and to prepare GPC (gel permeation chromatography) samples for molecular weight analysis. Details of the analysis are given in the online supplement. From the analysis, the students should obtain the time conversion correlation as well as number average molecular weight, Mn, and polydispersity index, PDI. Hazards VND is considered harmful if in contact with the skin or eyes. EXGA is also harmful if inhaled or comes into contact with skin or eyes. Exposure to the monomer and EXGA should be avoided and working in a fume hood is essential. The radical source, AIBN, is reportedly toxic and explosive. It should be kept in mind that only very small quantities are used, but AIBN should still be treated with care. After reaction, it is crucial for the sample to be kept under a fume hood for monomer evaporation. Special attention should be exercised when working with needles while degassing to avoid any injuries. Injuries can be
Experimental Methods and Materials All reagents used in this experiment are available commercially. It is recommended that the initiator 2,2′azobisisobutyronitrile (AIBN) be purified via recrystallization from methanol. Inhibitor is removed from monomer by passing the monomer over a column of basic alumina. Once purified, both chemicals must be stored in a refrigerator. A stock solution containing 20 g vinyl neodecanoate (VND), 42.0 mg (1 × 10‒2 mol L‒1) ethylxanthogenacetic acid (EXGA) (i.e., the MADIX agent),2 and 9.3 mg (2.5 × 10‒3 mol L‒1) AIBN is prepared (Figure 1). This stock solution is then
O
S
O O
O
S OH
Figure 1. Structure of (left) vinyl neodecanoate (VND) and (right) ethylxanthogenacetic acid (EXGA).
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 1 January 2008 • Journal of Chemical Education
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In the Laboratory increasing conversion
Table 1. Experimental Conversions, Number Average Molecular Weights, Mn, and Polydispersity Indices, PDI, Obtained in a EXGA-Mediated RAFT Polymerization of Vinyl Neodecanoate at 70 °C Employing AIBN as the Free Radical Initiator in Bulk 1500
2.00
7.4
4300
1.70
11.8
6700
1.65
33.0
16400
1.42
48.0
25600
1.27
54.0
29400
1.26
69.5
31900
1.30
76.1
37800
1.24
Note: The table reports selected data points given in Figure 3 and Figure 4 to illustrate a typical molecular weight and PDI evolution.
especially hazardous after withdrawing the needle from the reaction mixture. The resulting polymer is non-toxic. However, we recommend disposal using a solid waste bin for chemicals.
Normalized Response
PDI
1.6
0.8 0.6 0.4 0.2 0.0 2.5
98
Mn / (103 g mol1)
Living polymerization4 of vinyl acetate-type monomers has been investigated previously and a variety of specifically synthesized xanthates have been employed. The instructor and students are referred to the relevant literature (6). Selected experimental data from the EXGA-mediated free radical polymerization at 70 °C are collated in Table 1. The molecular weight distributions from which the data in Table 1 are derived are depicted in Figure 2. The distributions shift, as is typical for a well-performing RAFT polymerization, to higher molecular weights with increasing monomer-to-polymer conversion. The molecular growth is efficient with (almost) no low molecular weight material being left behind during the process. Inspection of the evolution of the number average molecular weight, Mn, and the polydispersity index, PDI, with reaction conversion shows a strictly linear shape with decreasing polydispersities (Figure 3). In high conversion regimes a slight increase in PDI can be observed. Such a PDI evolution has frequently been observed before and it demonstrates that the RAFT process requires a certain time to fully establish the main equilibrium and activate all initial RAFT agent. The theoretically expected molecular weight evolution departs somewhat from the experimentally observed evolution. Consultation of the literature on the RAFT process reveals several reasons that may be responsible for such a deviation and the students’ lab report should include a discussion on the deviation (see the online supplement). Higher than expected molecular weights are indicative of an inefficient activation of all RAFT agent or the subsequent deactivation of RAFT end groups. Chemical reasons for a possible RAFT end group depletion can include oxidation (exchange of C=S to C=O, thus making the C=X double bond incapable of adding radicals) as well as a continuously reducing addition rate of the propagating radicals to the C=S double bond caused
3.5
4.0
log [M / (g/mol)]
4.5
5.0
2.0 1.5 1.0
Results and Discussion
3.0
Figure 2. Full molecular weight distribution evolution as a function of monomer-to-polymer conversion in a EXGA-mediated free radical polymerization of vinyl neodecanoate at 70 °C. The exact reagent concentrations are given in the online supplement.
PDI
Mn /(g mol-1)
Conversion
1.0
0
10
20
30
40
50
60
70
80
60
70
80
40 30 20 10 0
theoretical Mn 0
10
20
30
40
50
Conversion (%)
Figure 3. Evolution of the number average molecular weight, Mn, and polydispersity index, PDI, as a function of monomer-to-polymer conversion in an EXGA-mediated free radical polymerization of vinyl neodecanoate at 70 °C. The exact reagent concentrations are given in the online supplement.
by a chain length (possibly diffusion controlled) dependence of the addition rate coefficient (7). Further, the deviation can be caused by the use of SEC5 systems that are calibrated relative to narrow polystyrene standards (such as in the present case). Similarly interesting are the first-order kinetic plots that can be constructed from the measured monomer conversion versus time evolution (Figure 4). Inspection of Figure 4 indicates that the polymerization process suffers from a so-called inhibition time, that is, a period where no effective monomer-to-polymer conversion is taking place. The origin of such inhibition periods is to date not completely understood, but possible explanations are detailed in the online supplement. A recent study (8) has compared the sample preparation techniques in the RAFT polymerization of vinyl acetate type monomers and the students should be encouraged to compare their results and technique with those discussed in the mentioned study.
Journal of Chemical Education • Vol. 85 No. 1 January 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
ln [Mo]/[M]
In the Laboratory 2.0
of an Australian Post-Graduate Award (Industry) and TLUN acknowledges receipt of an Australian Post-Graduate Award.
1.5
Notes
1.0
0.5
0.0
0
50
100
150
200
250
300
Time / min Figure 4. First-order kinetic plot from an EXGA-mediated free radical polymerization of vinyl neodecanoate at 70 °C. Mo is the initial monomer concentration at 0% conversion (before the start of the polymerization). The exact reagent concentrations are given in the online supplement.
Further evaluation of Figure 4 also shows that once the polymerization process has commenced, the reaction proceeds with a constant concentration of radicals as indicated by the non-changing slope of the graph. It must be stressed at this point, as often wrongly reported in the literature, that a linear first-order kinetic plot is not a criterion for the livingness of a polymerization. However, the data scatter is relatively large; this is due to the graph being composed of data from two independent experimental runs the students performed. Although the initial reagent concentrations were identical for each experiment, the high susceptibility of vinyl acetate-type polymerization to impurities significantly lowers the kinetic reproducibility of each run. It is important to note that the Mn versus conversion evolution is not affected by these kinetic effects, as in Figure 3 all time dependent effects are eliminated. Inspection of Figure 3 clearly demonstrates that the scatter in the data is considerably less than that observed in Figure 4. Conclusions In the present contribution we have detailed an undergraduate laboratory class experiment that is simple to implement and demonstrates how the RAFT process can impart living characteristics to a free radical polymerization. The learning outcomes from this laboratory practice are well-defined and include (i) providing hands-on experience for the undergraduate student in the preparation of well-defined polymers of variable molecular weight and low polydispersity, (ii) gaining a basic understanding of the RAFT mechanism, and (iii) being able to qualitatively judge which RAFT agent has to be employed to achieve living free radical polymerization for a certain monomer. Acknowledgments CB-K and MHS would like to thank the School of Chemical Sciences and Engineering at the University of New South Wales for the academic freedom provided to explore novel concepts in undergraduate classes. FB acknowledges receipt
1. Ethylxanthogenacetic acid (EXGA, CAS 25554-84-1) can be obtained via Sigma-Aldrich (Product Number S390321-1EA) and was available within 3 weeks from placing the order in sufficient quantity for the present laboratory practice. Reordering was unproblematic as well. 2. The terms MADIX and RAFT can be used interchangeably as they essentially denote the same mechanistic process in cases where xanthates are employed as controlling agents. EXGA is the specific reagent name. 3. NMP is nitroxide-mediated polymerization. 4. Living polymerization is also referred to as controlled polymerization. 5. SEC is size-exclusion chromatography.
Literature Cited 1. Matyjaszewski. K. Controlled/Living Radical Polymerization: Progress in ATRP, NMP and RAFT; American Chemical Society: Washington, DC, 2000; Vol. 768. 2. Stenzel, M. H.; Barner-Kowollik, C. J. Chem. Educ. 2006, 83, 1521–1530. 3. Beers, K. L.; Woodworth, B.; Matyjaszewski, K. J. Chem. Educ. 2001, 78, 544–547. Matyjaszewski, K.; Beers, K. L.; Woodworth, B.; Metzner, Z. J. Chem. Educ. 2001, 78, 547–550. 4. (a) Mayadunne, R. T. A.; Rizzardo, E.; Chiefari, J.; Chong, Y. K.; Moad, G.; Thang, S. H. Macromolecules 1999, 32, 6977. (b) Chiefari, J.; Chong, Y.-K.; Ercole, F.; Krstina, J.; Jeffrey, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C.; Moad, G.; Rizzardo, E. Thang, S. H. Macromolecules 1998, 31, 5559–5562. (c) Chong, Y. K.; Le, T. P .T; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1999, 32, 2071–2074. (d) Barner-Kowollik, C.; Davis, T. P.; Heuts, J. P. A.; Stenzel, M. H.; Vana, P.; Whittaker, M. J. J. Polym. Sci. Polym. Chem. 2002, 41, 365. 5. (a) Taton, D.; Wilczewska, A. Z.; Destrac, M. Macromol. Rapid Commun. 2001, 22, 1497–1503. (b) Destrac, M.; Bzducha, W.; Taton, D.; Gauthier-Gillaizeau, I.; Zard, S. Z. Macromol Rapid Commun. 2002, 23, 1049–1054. 6. Stenzel, M. H.; Cummins, L.; Roberts, G. E.; Davis, T. P.; Vana, P.; Barner-Kowollik, C. Macromol. Chem. Phys. 2003, 204, 1160. 7. Barner-Kowollik, C.; Buback, M.; Charleux, B.; Coote, M. L.; Drache, M.; Fukuda, T.; Goto, A.; Klumpermann, B.; Lowe, A. B.; McLeary, J.; Moad, G.; Monteiro, M. J.; Sanderson, R. D.; Tonge, M. P.; Vana, P. J. Poly. Sci., Part A 2006, 44, 5809–5831. 8. Favier, A.; Barner-Kowollik, C.; Davis, T. P.; Stenzel, M. H. Macromol. Chem. Phys. 2004, 205, 925–936.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Jan/abs97.html Abstract and keywords Full text (PDF) Links to cited JCE articles Supplement Student handouts
Instructor notes
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