MEASUREMENT OF THE CHAIN TRANSFER CONSTANT FOR THE POLYMERIZATION OF STYRENE USING D-LIMONENE AS RENEWABLE CTA

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1 MEASUREMENT OF THE CHAIN TRANSFER CONSTANT FOR THE POLYMERIZATION OF STYRENE USING D-LIMONENE AS RENEWABLE CTA Rodrigo Schlischting 1, Ricardo A. F. Machado 2, Pedro H. H. de Araújo 2, Johan P. A. Heuts 3 and Alex M. van Herk 3* 1 Federal University of Technology UTFPR, Campus Francisco Beltrão, Francisco Beltrão-PR 2 Federal University of Santa Catarina UFSC, Campus Florianópolis, Florianópolis-SC 3* Eindhoven University of Technology TU/e, Netherlands. a.m.v.herk@tue.nl D-limonene is an essential oil obtained as byproduct in the food industry that can be used in free radical polymerization in order to decrease polymer molecular weight. For this work styrene was bulk polymerized in the presence of d- limonene at different temperatures and d-limonene concentrations using 2,2 -azobis(isobutyronitrile) (AIBN) as initiator. The chain transfer constant was evaluated by Mayo procedure using both the number-average molecular weight (Mn) and the weight-average molecular weight (Mw). Results showed that Mayo equation presented essentially the same chain transfer constant value independent of the initiator concentration and the reaction temperature. Keywords: D-limonene, Styrene Polymerization, Chain Transfer Constant. Introduction d-limonene is an optical active compound that may be found in citric fruits peel and has been used in a wide range of industrial process (food, pharmaceutical, cosmetic, polymeric, etc). Being available in large quantities, requiring no synthetic modification and a negligible toxicity then compared with some chain transfer agents like mercaptans, giving to the d-limonene a wide-ranging potential for it application. 1,2 Low molecular weight polymers have a great importance for chemical industry in the production of detergents, dispersants and paints, for instance. This reduction in the molecular weight involves different approaches that may be applied in a combination way for a practical situation. Increasing the initiator amount until a high concentration is one way to promote a decrease in the polymer molecular weight but this procedure is uneconomic and often unapplied in the industry. A second method and more useful is to add a chain transfer agent to reaction. 3 There are several chain transfer agents that may be used in the free-radical polymerization; such as mercaptans, but the toxicity and the malodorous nature are a significant disadvantage. 4 Another method to decrease the molecular weight is to carry the reaction in solution medium as done by Mathers and co-workers. 1

2 They used limonene as solvent and chain transfer agent at same time for the ring-opening metathesis polymerization of alkenes. The chain transfer constant may be evaluated by Mayo Equation or the Chain Length Distribution (CLD) procedure The general chain transfer constant, C CTA, is defined as the ratio of the chain transfer to CTA and propagation rate constants, k tr /k p. For example, C CTA is the ratio of the rate constant for chain transfer to chain transfer agent (CTA) and the rate constant for propagation, and it is a measure of the reactivity of a chain transfer agent. The chain transfer constant measured by the Mayo equation show the number-average degree of polymerization (DP n ) as a function of the rates of chain growth and chain stopping: 13,18 n * ( 1 ) [ ] p [ ] [ ] 1 + λ R kt CTA = + CM + CCTA (1) DP k M M In this expression, λ is the fraction of termination by disproportionation, k t the average termination rate constant, [R * ] the overall radical concentration, [M] the monomer concentration, in our case styrene, C M the chain transfer constant to monomer and [CTA] the concentration of the chain transfer agent, that is limonene in this system. Chain transfer constant evaluation based on number-average molecular weight (Mn) should be done carefully since Mn may be affected by errors in the size exclusion chromatography (SEC) as well as systems where there is an often occurrence of backbiting, 6 which is independent of the chain transfer presence, limiting the effect of chain transfer agent on Mn. A solution for both cases is the use of the weight-average molecular weight (Mw) in the equation 1. In a chain transfer dominated system Mw is equal to 2 Mn, except for very low molecular weight, hence DPn = Mw/(2m 0 ) for this system, where m 0 is the monomer mass. Mw is less likely affected by the presence of low molecular weight molecules and errors done during the SEC analysis, such as peak and base-line selection, 5,9 resulting in a better evaluation of the chain transfer constant. 12 For this work styrene was bulk polymerized in the presence of limonene at different temperatures and limonene concentrations using 2,2 -azobis(isobutyronitrile) (AIBN) as initiator. The chain transfer constant was evaluated by Mayo procedure using both the number-average molecular weight (Mn) and the weight-average molecular weight (Mw) and compared to the literature values of some different chain transfer agents used in the styrene polymerization. Experimental Materials AIBN (2,2 Azobis(isobutyronitrile) was recrystallized twice from methanol and used as initiator. Styrene (Sigma-Aldrich Inc.) and Limonene (Fluka, 99%) were previously distilled.

3 Polymerization Procedure Limonene and styrene were previously purged with argon during 20 minutes. For the experiments were used two different initiator molar ratios, 1.46 x 10-3 and 3 x 10-3 (mol of AIBN/mol of limonene plus styrene). Samples were prepared in small vials (1mL) with different styrene molar fractions being placed in a thermal plate at temperatures of 40, 60 and 90ºC for a pre-determined time, with reactions being quenched by rapid cooling followed by a gravimetric procedure in order to determine the final conversion (around 1% for all reactions). Molecular Weight Analyses Molecular weight distributions were determined by size exclusion chromatography using a Viscotek Triple-SEC differential refractive index detector, a Gynkotec pump and four mixed-b columns (Polymer Laboratories). Tetrahydrofuran (Biosolve, air-stabilized, 99.8%) was used as eluent at 1 ml/min. Calibration curve was obtained by standard polystyrene with narrow polydispersity index. Results and Discussion The usual procedure to evaluate the chain transfer constant (C CTA ) involves the determination of the polymerization average degree for different [CTA]/[M] molar ratios, where C CTA is obtained by the straight-line slope for the DPn -1 vs. [CTA]/[M] data plotting. This procedure assumes that the product k t [R*] is constant for different [CTA]/[M] values. As previously discussed Mw is less likely affected by errors like base line and peak selections during the SEC analysis than Mn. 19,20 The present work used both Mn and Mw for the evaluation of the chain transfer constant to d-limonene, C CTA. Table 1 shows the Mn and Mw obtained for different temperatures, molar fraction of initiator and [CTA]/[M] molar ratios used during the styrene polymerization, as well as the chain transfer constant determinated by equation 1. The Mayo plot has one weakness; when termination rate is chain length dependent, this implies in a non linear behavior by the first term in the eq. 1. This term can be controlled by the overall radical concentration, which in turn is controlled by the initiator concentration and temperature. 5 In the present work the experiments were carried out at 40, 60 and 90ºC, corresponding to a decomposition rate constant for the AIBN around 5 x 10-7, 1 x 10-5 and 5 x 10-4 s -1, respectively. 21 The initiator molar ratios used in this work were 1.46 x 10-3 and 3 x 10-3 ([AIBN]/[M]+[CTA]), which represent a concentration around (1-2.6) x 10-2 (mol/l).

4 Table 1 Chain transfer constant for the styrene/limonene system at different temperatures and molar fraction of AIBN. T(ºC) [L]/[M] a [AIBN] b =1.46 x 10-3 [AIBN] b = 3x 10-3 Mn c Mw d Mn c Mw d C L = C L = C L = C L = C L = C L = C L = C L = C L = C L = C L = C L = In Figures 1 and 2, Mayo plots based on Mn and Mw, respectively, are shown for different temperatures of polymerization reaction and using a molar fraction of AIBN of 1.46 x Comparing these figures it is possible to see that the experimental data show the same scattered behavior and by an analysis of the Table 1 is verified that the chain transfer constants obtained are very close.

5 Figure 1. Mayo plot for the determination of with [AIBN] = 1.46 x 10-3 based on DPn = Mn/ Where: ( ) T = 40 ºC; ( ) 60 ºC e ( ) 90 ºC. Figure 2. Mayo plot for the determination of with [AIBN] = 1.46 x 10-3 based on DPn = [Mw/(2 x )]. Where: ( ) T = 40 ºC; ( ) 60 ºC e ( ) 90 ºC. Figure 3. Mayo plot for the determination of with [AIBN] = 3 x 10-3 based on DPn = Mn/ Where: ( ) T = 40 ºC; ( ) 60 ºC e ( ) 90 ºC. Figure 4. Mayo plot for the determination of with [AIBN] = 3 x 10-3 based on DPn = [Mw/ (2 x )]. Where: ( ) T = 40 ºC; ( ) 60 ºC e ( ) 90 ºC. In the Table 1 presents the results for a initiator molar ratio of 3 x 10-3 under different temperatures and the Mayo plot based on Mn and Mw may be seen in Figures 3 and 4, respectively. When comparing these figures it may be seen that the data in Figure 3 are slightly more scattered than those in Figure 4 but the chain transfer values are close to the ones presented in Figure 4, as may be seen in Table 1. The results presented in Table 1 and Figures 1 to 4 showed that temperature and initiator concentration used in this work have no influence over the chain transfer constant for d-limonene during styrene bulk polymerization. Values obtained using the Mn and Mw approaches showed good agreement.

6 The chain transfer constant to d-limonene estimated in this work was compared to values for some different chain transfer agents evaluable in the literature, Table 2. Chain transfer constant to limonene showed a value close to the dibutyl disulfide and dibutyl sulfide values at 60ºC. When compared to a regular chain transfer agent like 1-butanethiol or to some catalytic chain transfer agent like bis(methanol) complex of bis[(difluoroboryl)-diphenylglyoximato]cobalt (II) COPhBF, the value to the chain transfer constant to limonene was rather lower. Table 2 Chain transfer constant to different compounds in the styrene polymerization. Temp. (ºC) Styrene a 1-butanethiol a Dibutyl disulfide a Dibutyl sulfide a COPhBF b Limonene c x 10-5 a Conclusions In this work the chain transfer constant to d-limonene during the styrene bulk polymerization was evaluated for different temperatures and limonene concentration by Mayo equation, showing no influence for the temperature range or the initiator concentration used, being this one just a function of the limonene concentration. The chain transfer constant found for the experiments suggest that the limonene is not so effective chain transfer agent to be used in the styrene polymerization if compared to COPhBF or 1- butanethiol at 60ºC, but his lower toxicity and the renewable characteristic are a good reason to be used, mainly whether a high chain transfer agent is not necessary or in cases where the limonene may be used as dispersant phase and chain transfer agent as done by Mathers and co-workers. 3 Acknowledgment The authors thank the financial support from the Alban Program, the European Union Program of High Level Scholarships for Latin America, scholarship no. E06D101421BR. References 1. R.T. Mathers; K.C. McMahon; K. Damodaran, K.; C.J. Retarides; D.J. Kelley. Macromolecules 2006, 39, S.G. Gaynor. Macromolecules 2003, 36, A. Vadebenito; M.V. Encinas. Journal of Photochemistry and Photobiology A: Chemistry 2008, 194, D.J. Forster; J.P.A. Heuts; F.P. Lucien; T.P. Davis. Macromolecules 1999, 32, J.P.A. Heuts; T.P. Davis; G.T. Russel. Macromolecules 1999, 32, S. Harrisson; T.P Davis; R.A. Evans; E. Rizzardo. Macromolecules 2000, 33, D.A. Morrison; L. Eadie; T.P Davis. Macromolecules 2001, 34, D. Kukulj; T.P Davis; R.G. Gilbert. Macromolecules 1998, 31, 994.

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