RAFT-Mediated ab Initio Emulsion Copolymerization of 1,3-Butadiene

Article pubs.acs.org/Macromolecules

RAFT-Mediated ab Initio Emulsion Copolymerization of 1,3Butadiene with Acrylonitrile Lebohang Hlalele,†,‡ Dagmar R. D’hooge,§ Christoph J. Dürr,†,‡ Andreas Kaiser,∥ Sven Brandau,∥ and Christopher Barner-Kowollik*,†,‡ †

Preparative Macromolecular Chemistry, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstr. 18, 76128 Karlsruhe, Germany ‡ Institut für Biologische Grenzflächen, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany § Laboratory for Chemical Technology (LCT), Department of Chemical Engineering and Technical Chemistry, Ghent University, Technologiepark 914, 9052 Zwijnaarde (Gent), Belgium ∥ Lanxess Emulsion Rubber, BP 7-Z.I. Rue du Ried, 67610 La Wantzenau, France S Supporting Information *

ABSTRACT: The successful RAFT-mediated ab initio emulsion copolymerization of acrylonitrile and 1,3-butadiene using 2(((dodecylsulfanyl)carbonothioyl)sulfanyl)propanoic acid (DoPAT) is reported at 45−55 °C. The number-average molecular weight exhibits a linear evolution as a function of monomer conversion (5000 ≤ Mn (g mol−1) ≤ 41 000, 1.3 ≤ Đ (−) ≤ 3.3). Relatively good control (e.g., Đ ≈ 1.2 for selected conditions) over the polymerization up to moderate monomer conversion (50−60%) was attained when the employed initial molar ratio of RAFT agent to initiator was 2.5 or higher. Good ω-end-group functionality is evidenced by chain extension of NBR with a polystyrene block, with both 1H NMR and SEC showing the average fraction of the NBR block as ca. 75 mol%. A kinetic model implemented via the PREDICI software package confirms the experimental findings, including a semiempirical approach to account for branch formation. The onset of the loss in control over the copolymerization at conversions >40% was tentatively attributed to branch formation. The current study evidences that RAFT mediated ab initio emulsion polymerization of 1,3-butadiene and acrylonitrile is a viable polymerization protocol for the synthesis of well-defined next generation nitrile−butadiene rubbers including in industrial context.

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INTRODUCTION

Finely controlling the copolymerization kinetics of ACN with 1,3-butadiene is a challenging task due to the nature of the produced polymer (Figure S1, Supporting Information): the vinyl bonds (pendant and internal) in the backbone of the polymer can partake in the polymerization process in side reactions that may result in a branched structure. For the industrial production of NBR with minimal branching, emulsion polymerization is therefore conducted under “cold” polymerization conditions and only allowed to proceed to

Nitrile−butadiene rubber (NBR) is an important elastomer that finds application in a variety of fields, e.g., automotive and aeronautical applications. Industrially, NBR is produced via free radical emulsion polymerization of acrylonitrile (ACN) and 1,3butadiene under so-called “cold” or “hot” polymerization conditions depending on the required polymer grade. These NBR grades vary for instance in topology (branching degree), incorporated ACN amount, and Mooney viscosity. The industrial NBR polymerization process, however, suffers from a lack of control over the molecular weight distribution (MWD), branching amount, and end-group functionality. © 2014 American Chemical Society

Received: January 7, 2014 Revised: April 8, 2014 Published: April 23, 2014 2820

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For n-butyl acrylate, Chenal et al. recently reported ab initio RAFT-mediated emulsion polymerizations in which a poly(acrylic acid)trithiocarbonate macro-RAFT agent was employed as both a mediator and a stabilizer, leading to block copolymer formation.16 Rieger et al. also reported the selfstabilized RAFT-mediated emulsion polymerization of n-butyl acrylate and styrene using poly(N,N-dimethylacrylamide)trithiocarbonate as macro-RAFT agent.10 In addition, Xu and co-workers previously reported ab initio RAFT-mediated emulsion polymerization of 1,3-butadiene utilizing amphiphilic macro-RAFT agents.22 The amphiphilic macro-RAFT agents employed were a series of poly(acrylic acid-b-styrene)trithiocarbonates, and the effect of the hydrophilic block size on coagulation was also assessed. However, for RAFT emulsion copolymerization involving ACN and 1,3-butadiene, no such studies have been conducted. Interestingly, an important recent RAFT contribution for NBR products is the successful implementation of RAFTmediated copolymerization of acrylonitrile and 1,3-butadiene in solution.23,24 Moreover, Dürr et al. illustrated the use of functional end-groups to prepare NBR-based block and miktoarm copolymers for polymers prepared under solution polymerization conditions.25 For industrial applications, it is critical to transmit this controlling feature to emulsion systems; the advantages of emulsion polymerization and its practicality under industrial conditions in comparison to solution polymerization have been well documented in the literature.26 To date, the adaptation of CRP techniques to large scale industrial processes has been limited to mainstream commodity polymers. With a variety of postpolymerization protocols already developed for solution-based copolymerization of acrylonitrile with 1,3-butadiene,25,27 the end-group functional NBR obtained through ab initio RAFT-mediated emulsion polymerization may further be used for any of the already developed postpolymerization modifications. The current study presents, for the first time, a viable ab initio RAFT-mediated emulsion copolymerization of acrylonitrile with 1,3-butadiene under azeotropic feed composition. Specifically, the effect of the initial molar ratio of RAFT to initiator on the level of control over the copolymerization has been assessed in detail. Furthermore, the PREDICI software package is used to simulate the ab initio RAFT-mediated emulsion polymerization process and to obtain insights into the copolymerization process in the absence and presence of branch formation.

moderate monomer conversions, typically less than 60%. Beyond a monomer conversion of 60%, significant branching occurs as manifested in increasing dispersity (Đ) values. The recent advent of controlled radical polymerization (CRP) (also known as reversible deactivation radical polymerization) has allowed for the improved synthesis of polymers with predefined average molecular weights, narrow MWDs, functionality, and controlled branching.1−7 In these CRP techniques, macroradicals are temporarily deactivated into dormant macrospecies via incorporation of end-group functionality, thus significantly extending their lifetime. Ideally, a concurrent growth of these dormant species is obtained by consecutive activation−growth−deactivation cycles. In order to conduct the copolymerization of 1,3-butadiene and acrylonitrile to conversion values exceeding 60% while maintaining the average linear microstructure, use of CRP mediating agents is necessary. Furthermore, the presence of CRP chain-end functionality in emulsion polymerization produced NBR can provide a platform for the development of new postpolymerization protocols for tuning structure−property relationships in different NBR grades. For industrial applications, an important CRP technique is reversible addition−fragmentation chain transfer (RAFT) polymerization. In particular, RAFT-mediated emulsion polymerization systems have enjoyed substantial interest, however with a varying degree of success.1,8−13 The RAFT-mediated emulsion polymerization can be conducted under either seeded or ab initio conditions.1 Prescott et al.9 reported seeded RAFTmediated emulsion polymerization in which a water-insoluble RAFT agent, 2-phenylprop-2-yl phenyldithioacetate, was employed. To solve the problem of RAFT agent transport to the seed particles, Prescott et al.9 used acetone. The acetone amount was evaporated, and the seed particles were swollen with monomer before initiating the polymerization process. The use of xanthates in seeded emulsion polymerization of styrene was reported by Smulders et al.14 Furthermore, Peklak et al.15 reported a kinetic modeling study for the seeded RAFTmediated emulsion polymerization of styrene. The comparison of modeling and experimental data allowed for a better understanding of the inhibition and retardation phenomena that are typical for RAFT in emulsion polymerization processes. In the case of ab initio RAFT-mediated emulsion polymerization, two approaches exist via which the process can be conducted. The first involves the use of a RAFT agent in addition to a surfactant that stabilizes the emulsion while the second route, in contrast, involves the use of macro-RAFT agent that acts as both mediator and stabilizer. The employed macro-RAFT agents are typically based on amphiphilic block copolymers and hydrophilic homopolymers.10,13,16−22 Luo et al. reported ab initio RAFT-mediated emulsion polymerization of styrene employing three different surfactants, namely sodium dodecyl sulfate, poly(acrylic acid-b-styrene)trithiocarbonate macro-RAFT, and poly(acrylic acid-b-styrene).19 The small molecule RAFT agent used in their study to mediate the polymerization was 2-(((dodecylsulfanyl)carbonothioyl)sulfanyl)propanoic acid. In the instance where sodium dodecyl sulfate was used as the surfactant in conjunction with the small molecule RAFT agent, the ab initio RAFT-mediated emulsion polymerization suffered lack of control, with Đ > 2.4. For the ab initio RAFT-mediated processes with poly(acrylic acid-bstyrene)trithiocarbonate macro-RAFT or poly(acrylic acid-bstyrene) as stabilizers, a higher level of control was reported with Đ < 1.5.

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EXPERIMENTAL SECTION

Materials. Acrylonitrile (Acros, >99%) and 1,3-butadiene (>99.5%, Air Liquide) were used as received. Styrene (99% extra pure, stabilized, Acros Organics) was purified by passing through a column of basic alumina, 2,2-azobis(isobutyronitrile) (AIBN) (98%, Sigma-Aldrich) was recrystallized twice from methanol, triethylamine (Acros, 99%), sodium carbonate and potassium hydroxide (VWR), potassium persulfate (Alfa Aesar, 97%), and chlorobenzene (Acros, 99+%). 2(((Dodecylsulfanyl)carbonothioyl)sulfanyl)propanoic acid (DoPAT) and oleic acid (only surfactant) were obtained from Lanxess Deutschland GmbH. Instrumentation. A standard pressure stable reactor with an overhead stirrer, utilizing a 300 mL quartz glass pot, was used for the RAFT polymerizations. Size exclusion chromatography (SEC) measurements were performed on a Polymer Laboratories/Varian PL50 modular system comprising an autoinjector and a Polymer Laboratories 5.0 μm bead-size guard column (50 × 7.5 mm2), followed by three linear PL columns (105, 104, and 103 Å) and a differential 2821

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refractive index detector using THF as the eluent at 35 °C with a flow rate of 1.0 mL min−1. The gel permeation chromatography (GPC) system was initially calibrated using narrow polystyrene standards ranging from 540 to 2 × 106 g mol−1. The average molecular weights were subsequently assessed using the recently determined Mark− Houwink−Sakurada parameters of NBR obtained for azeotropic composition which are based on NBRs featuring a mainly nonbranched architecture.28 RAFT-Mediated Emulsion Copolymerization. Table 1 collates the quantities of a typical RAFT-mediated emulsion copolymerization

Samples were withdrawn periodically for the gravimetrical determination of monomer conversion, and samples used for the determination of molecular weight were precipitated in chilled ethanol. The precipitated polymer was dried under vacuum at ambient temperature prior to SEC analysis. The pressure stable reactor utilized an internal temperature probe for direct determination of the reaction mixture temperature profile as a function of reaction time. NBR Chain Extension in Solution. Trithiocarbonate ω-functional NBR (2.0 g), as obtained via the emulsion RAFT polymerization, was dissolved in chlorobenzene, then styrene (48 mmol) and AIBN (0.015 mmol) were added to the solution. The resulting mixture was degassed by four freeze−pump−thaw cycles and backfilled with nitrogen gas. The flask was subsequently immersed in a preheated oil bath at 55 °C, and the reaction was allowed to proceed for 12 h. The resulting polymer was precipitated in chilled ethanol and dried under vacuum at ambient temperature for 24 h. For the evaluation of the chain extension experiments, an SEC calibration with narrow polystyrene standards was employed. Kinetic Modeling. Kinetic modeling tools have previously been employed to study mostly seeded emulsion polymerization kinetics. On the other hand, for ab initio emulsion polymerization, the number of modeling studies is still relatively scarce. For example, Altarawneh et al. reported the mathematical modeling and model validation of ab initio RAFT-mediated emulsion polymerization of styrene.32 For industrially important monomers, such as acrylonitrile and 1,3butadiene, the modeling studies to date have, however, generally been confined to various reactor operations for conventional emulsion radical polymerization processes; extensive resources have previously been employed to understand and improve the emulsion-based free radical copolymerization of acrylonitrile with 1,3-butadiene under different reaction conditions, yet not in the presence of RAFT mediating agents.33−38 In the current work, PREDICI simulations including a Smith− Ewart based modeling approach are performed to explore the potential of RAFT polymerization to afford a controlled emulsion polymerization of butadiene/acrylonitrile systems. For simplicity, compartmentalization is only considered for radicals according to the so-called “zero-one system” approach. Leaving group radicals originating from the small RAFT molecule are assumed to be transferred to the aqueous phase, whereas initiator-derived radicals are assumed to take up a limited number of monomer units (3) before they enter the micelles/polymer particles. It should be stressed that the kinetic modeling efforts do not aim at a qualitative simulation of all characteristics of the RAFT emulsion polymerization process, given the aforementioned approximate SEC analysis, which strictly only leads to apparent Mn values. However, the followed modeling approach suffices to compare different experimental protocols on a relative basis and to identify optimal polymerization conditions and trends within the system.

Table 1. Typical Reaction Recipe for the RAFT-Mediated Emulsion Copolymerization of Acrylonitrile and 1,3Butadiene (Azeotropic Feed Composition)a,b component

mass/g

component

mass/g

oleic acid Na2CO3 KOH (6.25 M) H2O

2.16 0.08 1.26 135.38

butadiene

47.00

TEAd H2O

0.35 3.76

28.21 0.77

KPS H2O

0.24 3.76

acrylonitrile DoPATc a

Note that ingredients grouped together in the table represent mixtures that are separately added to the reaction mixture. Polymerizations were conducted until ∼70% monomer conversion at which the solid content ranged between 20 and 23% with an average particle diameter close to 50 nm. bThe NBR latexes obtained exhibited stability for several weeks. All the copolymerization were conducted at 50 °C, except for cases where a different polymerization temperature was used, as highlighted in the respective captions. The pH of the system was approximately 10 while the pressure of the reaction varied between 2.5 and 6 bar depending on the stage of the polymerization. c For the effect of initial RAFT agent concentration on the level of control, [DoPAT]0 was varied accordingly. dThe activation of KPS was assessed for different TEA feed patterns, with the optimum conditions being TEA shown in the table as the initial amount and feeding 100% and 200% of [TEA]0 at 2 and 4 h marks, respectively.

(azeotropic feed composition) and the resulting colloidal properties. While the industrially employed reaction mixture includes additional components which cannot be disclosed, the emulsion polymerization can be conducted with the mixture composition provided in Table 1. The additional components only serve a secondary purpose and thus have no bearing on the emulsion process or the stability of the resulting latex. In a typical copolymerization, the surfactant solution was purged with nitrogen gas for 15 min and subsequently transferred to a pressure stable reactor. Next, the surfactant solution was subjected to three nitrogen/vacuum cycles. A purged solution of DoPAT in acrylonitrile was introduced into the reactor under inert conditions followed by addition of 1,3-butadiene to the reactor via a metal buret, which had been degassed by three nitrogen/vacuum cycles prior to the addition of 1,3-butadiene. The mixture was stirred at 600 rpm and heated to the polymerization temperature (e.g., 50 °C) prior to addition of triethylamine (TEA) base and potassium persulfate (KPS) initiator. The surfactant has been added in such a way that its concentration exceeds the critical micelle concentration. For simplicity, it is assumed that a potential micellar behavior of DoPAT can be ignored. The azeotropic feed composition (38/62) has been confirmed experimentally, indicative of a limited effect of partitioning. It should be stressed that in addition to the temperature the decomposition rate of persulfates is affected by pH and basicity with an increased basicity leading to an increased rate of decomposition.29−31 The use of base activation of KPS allows for moderate temperatures (45−55 °C) to be employed for the decomposition of the initiator, explaining the aforementioned initiation procedure.

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RESULTS AND DISCUSSION The RAFT-mediated ab initio emulsion copolymerization of acrylonitrile with 1,3-butadiene is studied under isothermal and nonisothermal conditions (i.e., using a temperature profile) employing 2-(((dodecylsulfanyl)carbonothioyl)sulfanyl)propanoic acid (DoPAT) as chain transfer agent and baseactivated potassium persulfate (KPS) as the initiation system, with triethylamine (TEA) as the employed base. Industrially relevant polymerization conditions are reported, allowing a high conversion and relatively good control over polymer properties based on a combined experimental and modeling analysis. It is shown that the use of a temperature profile leads to optimum results. Isothermal Case Studies. Figure 1 illustrates at 50 °C and below conversions of 30% the effect of the initial DoPAT concentration on the evolution of conversion with time (symbols ∗ and ●) including an additional case (symbol ○) in which an additional injection of base is carried out. For the 2822

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the preparation of NBR is marked by a significant extent of noncontrolled and at times undesirable degrees of chain branching that can lead to cross-links causing high dispersities at high monomer conversions (Đ > 3.0). In contrast, it can be expected that the presence of RAFT mediating agent may allow overcoming this issue. In Figure 2a, the corresponding evolution of the MWDs ([monomer]0:[DoPAT]0:[KPS]0 ratio of 1594:5:1 at 47 °C) as a function of conversion is depicted, showing a clear shift to lower retention times (higher molecular weights) with increasing overall monomer conversion. The resulting evolution of Mn and Đ with overall monomer conversion is illustrated in Figure 2b. The Mn is observed to increase linearly with monomer conversion, which is characteristic of a CRP process, while the Đ values initially decrease with conversion before increasing at higher conversion values. However, the values of Đ remain still relatively low ( 50 °C as opposed to T < 50 °C. We therefore developed a two-stage temperature variation, in which the polymerization is conducted at a higher temperature (55 °C) at the initial stages of the RAFT-mediated emulsion polymerization process, before lowering the temperature to 45 °C for the second stage of the polymerization reaction: [monomer]0:[DoPAT]0:[KPS]0 of 1594:2.5:1. Figure 11 illustrates the evolution of the overall monomer conversion and polymerization temperature with polymerization time for a typical two-stage temperature process. The polymerization is allowed to proceed for 4 h at 55 ± 1 °C, after which the temperature is lowered to 46 ± 1 °C for the remainder of the polymerization. At the point where the temperature is lowered to 46 ± 1 °C, triethylamine is also

Figure 9. Evolution of experimental and simulated number-average molecular weight with conversion for the ab initio DoPAT-mediated emulsion copolymerization of 1,3-butadiene with acrylonitrile, with a [monomer]:[DoPAT]:[KPS] ratio of 1594:2.5:1. Solid points (■) represents the experimental data, while the lines represent simulated Mn for varying degrees of chain branching (x in eq 1), solid line x = 0, dashed line x = 0.025, dotted line x = 0.045, and dash-dotted line x = 0.065 (azeotropic feed conditions).

the simulation resulted in a better correlation between the simulated and experimentally observed evolution of Mn with conversion. This is, however, still a qualitative assessment of the extent of the effect of chain branching on the molecular weight data for the ab initio RAFT-mediated emulsion copolymerization of acrylonitrile with 1,3-butadiene. Finally, Figure 10 shows a comparison between the experimental evolution of the number-average molecular weight

Figure 10. Experimental vs simulated evolution of Mn with conversion for ab initio RAFT-mediated copolymerization of acrylonitrile with 1,3butadiene, with a chain branching included in the prediction of the molecular weight based on eq 1. Dashed and solid lines correspond to simulations carried out at a [DoPAT]0 = 10.8 and 18.0 mM, respectively, under azeotropic feed conditions. The value of x (according to eq 1) used in both simulations was 0.065.

with conversion and the retuned simulation output (eq 1) for two initial RAFT agent concentrations. In the calculation of simulated Mn, to account for the effect of branching on the overall molecular weight, the exponential factor determined in fitting the molecular weight data in Figure 9 was again used.

Figure 11. Evolution of overall monomer conversion with time and the corresponding temperature profile for the ab initio DoPATmediated emulsion copolymerization of acrylonitrile and 1,3-butadiene at a [monomer]0:[DoPAT]0:[KPS]0 of 1594:2.5:1 ratio under azeotropic feed conditions. 2827

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Notes

added to the polymerization system to further enhance activation of the decomposition of potassium persulfate, increasing the radical flux and thus offsetting the expected rate decrease due to a temperature decrease. The corresponding dispersity values are close to 2.0. It can thus be concluded that the proposed approach allows the ab initio RAFT-mediated polymerization of acrylonitrile with 1,3-butadiene to achieve conversion values close to 70% with Đ ∼ 2.0 within a 15 h time frame.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Lanxess Deutschland GmbH for financial support for the current project and the excellent collaboration. C.B.-K. acknowledges additional funding from the Karlsruhe Institute of Technology (KIT) within the framework of the Helmholtz STN and BioInterfaces programs as well as the German Research Council and the ministry of arts and science of the state of Baden-Württemberg. D.R.D. acknowledges financial support from the Long Term Structural Methusalem Funding by the Flemish Government, the Interuniversity Attraction Poles Programme−Belgian State−Belgian Science Policy, and the Fund for Scientific Research Flanders (FWO). D.R.D. also acknowledges the Fund for Scientific Research Flanders (FWO) for a postdoctoral fellowship.

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CONCLUSIONS The RAFT-mediated ab initio emulsion copolymerization of acrylonitrile with 1,3-butadiene using 2-(((dodecylsulfanyl)carbonothioyl)sulfanyl)propanoic acid (DoPAT) as RAFT mediating agent and oleic acid as surfactant was successfully carried out under isothermal and nonisothermal conditions with base activated potassium persulfate (KPS) initiation, aiming at reduced dispersity (Đ) values. The copolymerizations were performed in the 45−55 °C temperature regime unconventional of a KPS initiation system with Mn values up to 41 000 g mol−1 and Đ values lower than 2.5. The lowest dispersities are achieved for relatively low molecular weights. The NBR produced via the RAFT-mediated emulsion polymerization showed good ω-functional end-group fidelity of the trithiocarbonate, as illustrated through a chain extension with styrene in solution. Analysis of NBR-b-PS with 1H NMR and SEC evidenced a NBR to polystyrene average block ratio of 3:1. For [DoPAT]0:[initiator]0 > 2.5 the copolymerization proceeded to relatively high conversions, (>60%) with Đ ∼ 2.0 at the highest conversions. The dispersity decreased with conversion up to monomer conversions of ∼30%, beyond which Đ gradually increased, an effect most pronounced for a low [DoPAT]0:[initiator]0 ratio, where conventional free radical behavior with significant branching is again observed. A kinetic model for the copolymerization representative of the experimental conditions was successfully implemented in the PREDICI program package and combined with a semiempirical approach to further highlight the significance of chain branching on the observed number-average molecular weight data as a function of monomer conversion. The branching was assessed semiempirically by a single parameter based on different reaction conditions. The model can be extended to further probe the relation between the initial RAFT concentration, conversion, and Đ. In summary, the ab initio RAFT-mediated emulsion polymerization has been illustrated as a viable tool for producing well-controlled end-functional NBRs in an industrial setting.

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ASSOCIATED CONTENT

S Supporting Information *

Copolymerization kinetic model implemented in PREDICI, rate coefficients employed in the simulation, detailed description of simulation conditions and block copolymer analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Fax +49 721 608 45740; e-mail christopher.barner-kowollik@ kit.edu (C.B.-K.). 2828

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