Mechanistic Insights into the UV-Induced Radical Copolymerization of

Article pubs.acs.org/Macromolecules

Mechanistic Insights into the UV-Induced Radical Copolymerization of 1,3-Butadiene with Acrylonitrile Lebohang Hlalele,† Christoph J. Dürr,† Paul Lederhose,† Andreas Kaiser,‡ Stefan Hüsgen,§ Sven Brandau,‡ and Christopher Barner-Kowollik†,* †

Preparative Macromolecular Chemistry, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstrasse 18, 76128 Karlsruhe, Germany ‡ Lanxess Emulsion Rubber, BP 7Z.I. Rue du Ried, 67610 LaWantzenau, France § Lanxess Deutschland GmbH, 51967 Leverkusen, Germany S Supporting Information *

ABSTRACT: An in-depth mechanistic study into the solution based initiator-free UV-induced radical copolymerization of 1,3-butadiene with acrylonitrile is reported. The light induced constant radical flux leads to moderate monomer conversions within 4 to 24 h. The number-average molecular weights of the prepared nitrile butadiene rubber (NBR) range from 2500 to 50 000 g mol−1 (1.7 ≤ PDI ≤ 2.4), while the achievable monomer conversion ranged from close to 7 up to 31% depending on the polymerization temperature, reaction time and UV light intensity. The rate coefficient for the generation of primary radicals, determined as the coupled parameter k*1 k3, showed a dependence on the UV light intensity with values between 6.0 s−2 and 34.6 s−2 deduced for the UV light intensity range of 280 to 700 W. The estimated values of the lower limit average termination rate coefficient displayed no dependence on the UV light intensity, with lower limit values between 2.6 × 108 L mol−1 s−1 and 6.3 × 108 L mol−1 s−1 for the UV light intensity range of 280 to 700 W. The deduced values for the average termination rate coefficient were above the expected values for comparable average termination rate coefficients.

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INTRODUCTION Copolymers of acrylonitrile and 1,3-butadiene (nitrile rubber, NBR) have a wide range of applications, with an annual industrial production output of around 500 000 metric tonnes. With capabilities to withstand temperatures in the range −40 to +125 °C, NBR finds applications from the automotive to the aeronautical industry. Industrial production of NBR to date has been solely carried out via emulsion polymerization processes. Advantages associated with emulsion polymerization have been well documented in the literature.1−5 However, unlike solution polymerization processes, reaction recipes are normally complex in emulsion processes. Recently, successful RAFT mediated solution copolymerization of acrylonitrile and 1,3butadiene with good control over functionality, molecular weight and its distribution has been reported.6,7 In addition to thermal based initiation systems, irradiation based initiation methods have previously been reported for the polymerization of 1,3-dienes and other monomer classes. For example, Stannett and co-workers reported the use of γ-radiation in emulsion copolymerization of 1,3-butadiene with acrylonitrile.8 The use of radiation-induced polymerization is of particular interest, as no external initiator is employed. Some monomers canunder irradiation with light of appropriate wavelength © 2013 American Chemical Society

undergo photochemical processes resulting in the generation of primary radicals. Direct initiation by irradiation of styrene in copolymerization with acrylonitrile and fumaronitrile have also been reported to proceed via initiation by a 1,2-diradical of styrene.9−11 In the case of 1,3-butadiene and its derivatives, UV light in the range of ca. 220−240 nm has been reported to produce primary radicals capable of initiating the polymerization. Wang et al. reported UV-induced radical copolymerization of 2,3-dimethyl-1,3-butadiene and acrylonitrile.12 Upon irradiation with UV light of appropriate wavelength, 1,3butadiene is excited from the ground state (S0) to the singlet excited state (S1) (Scheme 1). The lifetime of 1,3-butadiene in the S1 state has been reported to be in the order of 35 fs,13 a value significantly shorter compared to a typical lifetime of a singlet excited state in the order of 10−9 s. The depopulation of the S1 state occurs readily in a non-radiative heat dissipating process, such that only a limited number of 1,3-butadiene in the S1 state will undergo intersystem crossing (ISC) to the triplet excited state (T1). A photoinduced polymerization of this type Received: January 14, 2013 Revised: February 14, 2013 Published: March 5, 2013 2109

dx.doi.org/10.1021/ma4000896 | Macromolecules 2013, 46, 2109−2117

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540 to 2 × 106 g mol−1. The molecular weight of NBR was determined relative to polystyrene standards. Materials. Acrylonitrile (Acros, >99%), 1,3-butadiene (>99.5%, Air Liquide), and chlorobenzene (Acros, 99+%) were used as received. UV-Induced Copolymerization. In a typical polymerization, a mixture of acrylonitrile (20 mL, 303 mmol) in chlorobenzene (51.5 mL) was purged with nitrogen gas for 10 min. The purged mixture was transferred into a pressure quartz reactor and further degassed by four nitrogen/vacuum cycles. 1,3-Butadiene (42 mL, 505 mmol) was subsequently added to the reactor via a metal buret which was degassed by three nitrogen/vacuum cycles prior to the addition of 1,3butadiene. The reaction mixture was then irradiated for a prescribed time with periodic sampling, with approximately 100 mL of the reaction mixture exposed to the UV light. Monomer conversion was determined gravimetrically after sample drying at 85 °C under vacuum while samples for SEC analysis were precipitated in chilled methanol and dried under vacuum at ambient temperature. In all copolymerization experiments, the azeotropic monomer feed composition ( f 0BD = 0.62) was employed. A schematic representation of the employed set− up can be found in the Supporting Information (refer to Scheme SI-2). Modeling. The simulation of the light induced copolymerization process was carried out using the PREDICI software package (version 6.81.1). Residual oriented parameter estimation was employed to fit the experimental data. All simulations were carried out in distribution mode. In all simulations carried out in PREDICI, monomer concentrations employed were identical to those described in the experimental conditions of the UV-induced copolymerization.

Scheme 1. UV Excitation of 1,3-Butadiene (B) from the Ground State (S0) into B1* in the Singlet Excited State (S1) That Undergoes Either Non-Radiative Relaxation Back to the S0 or Intersystem Crossing (ISC) To Yield B3* in the Triplet Excited State (T1) from Which the Resulting DiRadical Can Initiate the Polymerization

supplies a constant concentration of primary radicals throughout the entire polymerization when targeting moderate monomer conversions. In comparison to a monomer such as ethylene, the kinetic and mechanistic aspects of (co)polymerization processes associated with 1,3-butadiene have not received a similar degree of attention over the last 2−3 decades. For ethylene based (co)polymerization processes, both experimental and theoretical approaches have extensively been employed to study the polymerization.14−23 In the current contribution, we report the UV-induced radical copolymerization of 1,3-butadiene with acrylonitrile at the azeotropic feed composition. The effects of UV light intensity, temperature and polymerization time (residence time) on the attainable monomer conversion and molecular weight are assessed. The PREDICI software package was used to deduce the coupled parameter k*1 k3 (refer to Scheme 2) and

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RESULTS AND DISCUSSION The UV-induced radical copolymerization reactions of acrylonitrile with 1,3-butadiene were carried out at temperatures ranging from 25 to 100 °C. Two approaches were employed to regulate the polymerization temperature, i.e. internal and external temperature control. Internal temperature control made use of the equilibrium temperature from heat dissipated from the relaxation of B1*. Depending on the preset lamp intensity, the system equilibrated at a specific temperature at which the polymerization occurred. At 280 W, the reaction temperature equilibrated at 50 ± 2 °C, while an equilibrium temperature of 70 ± 2 °C was achieved for 350 W. The control of the polymerization via the internal approach was employed for polymerizations carried out between 50 and 100 °C. External temperature control made use of a water bath in which the temperature was regulated with the aid of a cryostat to a desired polymerization temperature. The relaxation of 1,3butadiene from the S1 to the S0 state is a non-radiative relaxation process accompanied by dissipation of heat and depending on the lamp intensityvariable reactor temperatures ranging from 50 to 100 °C could be well maintained (Figure SI-3, Supporting Information). A typical evolution of the overall monomer conversion as a function of time for a photoinduced radical copolymerization of 1,3-butadiene and acrylonitrile conducted at 100 °C is illustrated in Figure 1. Contrary to a typical free radical polymerization utilizing azoor peroxy- based initiators, the concentration of primary radicals (i.e., monomer-derived-radicals) in a UV-induced polymerization at moderate conversion remains constant throughout the entire polymerization. The concentration of these primary radicals in a UV-induced polymerization can be controlled by the intensity of the UV light. For a specific temperature, a decrease in light intensity produces a lower concentration of monomer derived primary radicals, favoring lower conversion and higher molecular weights. The effects of temperature and UV light intensity on the attainable molecular weight and overall monomer conversion are addressed in detail

Scheme 2. Excitation by UV Light of 1,3-Butadiene (B) from the Ground State to the Single Excited State at a Rate Governed by k1* and the Corresponding Relaxation Process at a Rate Governed by k*−1 Are Illustrated by Reaction 1, While Reaction 2 Illustrates the Subsequent Transformation to the Triplet Excited State Governed by the Rate Coefficient k3

average termination rate coefficient ktc. The coupled parameter k1*k3 governs the rate of generation of primary radicals from 1,3-butadine through which the copolymerization is initiated.

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EXPERIMENTAL SECTION

Instrumentation. A standard pressure reactor with an overhead stirrer, utilizing a 300 mL quartz glass pot was used for the polymerizations. The mercury lamp used was a power-tunable TQ718 medium pressure mercury arc lamp (700W) operated with a solid state AC-power supply (P-EVG 10). The emission spectrum of the UV lamp is shown in the Supporting Information (refer to Figure SI-1). Size exclusion chromatography (SEC) measurements were performed on a Polymer Laboratories/Varian PL50 modular system comprising an autoinjector, 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 refractive index detector using THF as the eluent at 35 °C with a flow rate of 1.0 mL min−1. The GPC system was calibrated using polystyrene standards ranging from 2110


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Taking into consideration the reactions illustrated in Scheme 1 for the initiation process, a kinetic model (refer to Scheme 3) Scheme 3. Kinetic Model for the UV-Induced Radical Copolymerization of 1,3-Butadiene (B) and Acrylonitrile (A) Implemented into PREDICI with the Employed Values of the Rate Coefficients Shown in Tables 2 and 3

Figure 1. Evolution of overall monomer conversion with time for UVinduced radical copolymerization of acrylonitrile and 1,3-butadiene at the azeotropic feed composition at 100 °C with UV light intensity set at 700 W, employing internal temperature control.

in the later sections. In experiments conducted at temperatures between 50 and 100 °C, no dependence of Mn on temperature is observed (refer to Table 1). The lack of change in Mn with Table 1. Conversion and Molecular Weight Data for the UVInduced Copolymerization of 1,3-Butadiene and Acrylonitrile (Azeotropic Conditions) Carried out at Different Light Intensities Employing Internal Temperature Control entry power/W 1-i 1-ii 1-iii 1-iv

280 350 420 700

temp/ °C

conversion/%

Mna/g mol−1

dispersity

time/h

53 70 80 100

15.6 23.7 31.5 31.0

41 800 51 700 44 600 40 100

1.7 2.0 2.4 2.2

20 14 14 4

was constructed to probe the kinetics of the UV-induced radical copolymerization. The model illustrated in Scheme 3 was implemented into PREDICI to explore the initiation and termination kinetics of the process as a function of both temperature and UV light intensity. Simulations. According to Scheme 1, photoexcitation of 1,3-butadiene (B) results in its transformation to a singlet excited state S1 (Scheme 3, reaction 1, forward) from which B1* readily reverts back to ground state S0 (Scheme 3, Reaction 1, reverse) in a heat dissipating non-radiative relaxation process. However, a fraction of 1,3-butadiene in S1 will undergo intersystem crossing (ISC) to yield B3* in the triplet excited state, T1 (Scheme 3, Reaction 2). The resulting diradical (R•) of 1,3-butadiene can add to either acrylonitrile or another 1,3butadiene initiating the copolymerization process. The rate coefficients used in the model illustrated in Scheme 3 are summarized in Tables 2 and 3. Because of a significantly short lifetime of B1* in S1 (35 fs), the value of k*−1 was estimated from the reciprocal of the lifetime (τ) in S1 (eq 2). 1 τ≈ k −*1 (2)

a

Molecular weights were determined by SEC relative to polystyrene standards.

increasing UV light intensity and temperature is a result of a counter-balancing effect between the radical flux and the average rate coefficient of propagation (kav p ). An increase in UV light intensity results in an increased polymerization temperature for systems in which internal temperature control is employed. The result is an increase in kav p which favors an increase in molecular weight of the copolymer. Such an expected increase in molecular weight is offset by an increase in the radical concentration which is also a result of increasing UV light intensity (see eq 1, assuming no chain transfer processes). Experiments conducted without an external regulation of temperature illustrate a compounded effect of UV light intensity (effect on radical concentration) and temperature (effect on kav p ). These coupled parameters (UV light intensity and temperature) in copolymerizations carried out employing internal temperature control were decoupled in the experiments where external temperature control was employed, isolating the effect of UV light intensity from temperature. DPn =

The termination rate coefficient for reactions, k0t , involving primary radicals was set to 1.0 × 109 L mol−1 s−1.25 To the best of our knowledge, there has been no report in literature for the values of k1* and k3, i.e. the rate coefficients governing the generation of primary radical from 1,3-butadiene upon irradiation with UV light of appropriate wavelength. The rate coefficients k*1 and k3 are correlated parameters in the proposed model presented in Scheme 3 and estimation of a unique solution for the independent rate coefficients k1* and k3 from

kpav[M] ktc[P•]

(1) 2111


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the MWD − conversion data is not possible. Therefore, the coupled parameter k*1 k3 as well as ktc were estimated via the parameter estimation tool of PREDICI from simultaneous fitting of both conversion and molecular weight distributions. The values of k1*k3 and ktc estimated from the fitting of both molecular weight distributions and conversion simultaneously are shown in Table 3, illustrating a clear dependence of the estimated k1*k3 coupled parameter on the UV light intensity in the range 280−700 W (see Figure SI-4, Supporting Information). Figures 2 and 3 depict the fitted molecular weight distributions and the corresponding error space mappings, respectively, for polymerizations conducted at 53 °C (280 W), 70 °C (350 W), 80 °C (420 W) and 100 °C (700 W) employing internal temperature control. From the error space mappings illustrated in Figure 3, it can clearly be seen that the values deduced for the average termination rate coefficient, ktc, and k*1 k3 are unique point solutions as determined from the global minimum within a search range of 1.0 × 10−5 ≤ k*1 k3 (s−2) ≤ 1.0 × 105 and 1.0 × 106 ≤ ktc (L mol−1 s−1) ≤ 1.0 × 1011. It is worth noting that the termination rate coefficient displays a dependence on temperature, however such dependence is weak (i.e., the associated activation energies are low) and was therefore regarded as insignificant for the treatment of the data.27

Table 2. Rate Coefficients Employed in the Modeling of the UV-Induced B/A Copolymerization coefficient

coefficient value

reference

k1*k3 k−1 *a rA rB k0t ktc

refer to Table 3 2.86 × 1013 s−1 0.0158 0.4083 1.0 × 109 L mol−1 s−1 refer to Table 3

determined in this work 13 24 24 25 determined in this work

a The value k−1 * was estimated from the reciprocal of the lifetime of 1,3butadiene in the singlet excited state;13 The Arrhenius parameters for 26 BB 24 6 −1 −1 s , the determination of kAA p , and kp , are A = 1.79 × 10 L mol Ea = 15.40 kJ mol−1 and A = 8.05 × 107 L mol−1 s−1, Ea = 35.70 kJ mol−1, respectively.

Table 3. Estimated Values of k*1 k3 and ktc as a Function of UV Light Intensity for UV-Induced B/A Copolymerization Conducted Employing Internal Temperature Control conversion/% entry

power/W

temp/°C

3-i 3-ii 3-iii 3-iv

700 420 350 280

100 80 70 53

k*1 k3/s 34.6 15.1 7.2 6.0

−2

−1

ktc/L mol 6.3 6.0 3.4 2.6

× × × ×

−1

s

108 108 108 108

sim

expt

28.5 29.6 19.9 14.2

31.0 31.5 23.7 15.6

Figure 2. Simulated (solid line) and experimental (dotted line) molecular weight distributions for B/A copolymerizations carried out at 53 °C/280 W (a), 70 °C/350 W (b), 80 °C/420 W (c), and 100 °C/700 W (d) employing internal temperature control, with the deduced values of k*1 k3 and ktc collated in Table 3. 2112


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Figure 3. Error space mapping for the simultaneous fitting of monomer conversion and molecular weight distribution data for B/A copolymerizations at 53 °C/280 W (a), 70 °C/350 W (b), 80 °C/420 W (c) and 100 °C/700 W (d), with the deduced values of k1*k3 and ktc shown in Table 3. The color coded scale (top right in figures) in the error space mappings represents the residual error.

Table 4. Effect of Temperature on Conversion and Molecular Weight Data for the UV-Induced B/A Copolymerization Conducted at 700 W with the Kinetic Parameters Employed in the Simulation Shown in Tables 3 Mna/g mol−1

conversion/% entry 4-i 4-ii 4-iii

temp/°C 100 50 26

sim 28.5 21.3 7.0

expt 31.0 26.6 9.1

sim 43 600 11 300 5000

expt 40 100 22 000 9100

app rateb/s−1 expt −5

2.58 × 10 3.58 × 10−6 1.10 × 10−6

time/h 4 24 24

a

Molecular weights were determined by SEC relative to polystyrene standards. bThe apparent rate is calculated as illustrated by eqs SI-1 and SI-2 in the Supporting Information.

Effect of Temperature. The effect of temperature on the polymerization kinetics was assessed by carrying out experiments with all parameters held constant at variable temperatures. Assuming a linear evolution on conversiona fair approximation based on the data provided in Figure 1the final conversion can be used to estimate the apparent rate of the polymerization. The assumption of a linear evolution of the overall monomer conversion with time is a valid approximation, taking into account the rate of generation of primary radical remains constant up to moderate conversions. From Figure 1, the loss in linearity at higher monomer conversion is primarily

For a specific UV light intensity, the estimated average rate coefficients reported in Table 3 should hold for the particular UV light intensity at any polymerization temperature. That is, at a constant UV light intensity an increase or decrease in conversion and molecular weight can solely be attributed to the average rate coefficient of propagation which is temperature dependent, as the radical concentration is constant at a constant UV light intensity. A validation of the effect of constant intensity and variable temperature on conversion and molecular weight is carried out in the next section. 2113


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Table 5. Effect of the UV Light Intensity on the Attainable Conversion and Molecular Weight for Copolymerizations Conducted at 25 °C for 24 h with the Kinetic Parameters Employed in the Simulation Shown in Table 3 Mna/g mol−1

conversion/% entry

temp/°C

power/W

sim

expt

sim

expt

app rateb/s−1 expt

5-i 5-ii 5-iii

26 25 25

700 420 350

7.0 4.5 4.1

9.1 7.3 6.3

5000 7400 14 100

9100 7200 9000

1.10 × 10−6 8.77 × 10−7 7.53 × 10−7

a Molecular weights were determined by SEC relative to polystyrene standards. bThe apparent rate is calculated as illustrated by eqs SI-1 and SI-2 in the Supporting Information section.

It is worth noting that for entries 5-ii and 5-iii in Table 5, the simulated molecular weights are greater than the experimentally determined values, with the phenomenon reversed for conversion values. This observation is attributable to the underestimation in the estimated value of the coupled parameter k1*k3, leading to an overestimation in simulated molecular weights and a corresponding underestimation in simulated monomer conversions. The underestimations in the coupled parameter imply that the simulated propagating radical concentration will be lower in the simulated cases, leading to relatively higher molecular weights and lower conversion values. A complication in the current study is the difficulty to obtain exclusively linear chains that could enable a linear correlation between molecular weight data and conversion values to be drawn. In addition to the effect on the kinetics of the copolymerization process, the UV light intensity has the potential to affect the microstructure of the polymer, as will be illustrated in the next section. Polymer Resident Time (Reaction Time). The effect of UV exposure time on the molecular weight distribution for a copolymerization at 700 W (100 °C) is depicted in Figure 4.

due to reduced UV light intensity reaching the reaction medium. Such a loss is due to the partial depositing of crosslinked polymer on the reactor wall and thus a change in the transparency of the quartz reactor. The evolution of the apparent rate of polymerization for the photoinduced radical polymerization as a function of temperature at a constant UV light intensity (700 W) is summarized in Table 4. For polymerizations conducted at a UV light intensity of 700 W, yet at different temperatures, the radical concentration should in principle be similar for all experiments. Thus, the observed increase in the apparent rate of polymerization is attributable to an increase in the average rate coefficient of propagation. The simulated values of conversion and numberaverage molecular weight reported for entries 4-ii (50 °C) and 4-iii (26 °C) in Table 4 were predicted from estimated k1*k3 and ktc values from entry 3-i (700 W, 100 °C) in Table 3. For the copolymerization conducted at 50 and 26 °C (entry 4-ii and 4iii, Table 4), the discrepancy between the simulated and experimental molecular weights is attributable to cross-linking reactions due to the extended reaction times relative to entry 4i, Table 4. Interestingly, for entries 4-ii and 4-iii in Table 4, a common factor of approximately two exists between simulated and experimental molecular weights, from which it can be inferred that the extent of cross-linking reactions (or polymer chain branching) is comparable for similar reaction times (residence time) and UV lamp intensity. A relatively good correlation between simulated and experimental conversion is observed in Table 4, while the differences in molecular weights is attributable to cross-linking reactions under the given experimental conditions. With the effect of temperature on the UV-induced B/A copolymerization assessed, the next section will focus on the effect of UV light intensity on the copolymerization system. Effect of UV Light Intensity. Since the B/A copolymerization is initiated through UV light irradiation, it is important to fully understand the effect of UV light intensity on the copolymerization system. The effects of UV light intensity on the kinetics of photoinduced B/A copolymerization at a constant temperature are summarized in Table 5 for polymerizations conducted at 25 ± 1 °C. The calculated apparent rate of polymerization is observed to decrease with the reduction in UV light intensity, attributable to a decrease in the propagating radical concentration. To further evaluate the applicability of the estimated values of k1*k3 and ktc from Table 3, predicted conversion values and number-average molecular weights at 25 ± 1 °C and variable power are shown in Table 5. A relatively good correlation between experimental and simulated data is observed, further validating the estimated parameters. Even though the polydispersity index values have not been shown for experimental and simulated data in both Tables 4 and 5, comparable values in the range between 1.7 and 2.4 for the experimental and 1.4 and 1.5 for the simulation were obtained.

Figure 4. Experimental and simulated molecular weight distributions of NBR for a UV-induced radical copolymerization conducted at 700 W and 100 °C for 6 h employing internal temperature control with the rate coefficients employed for the simulation shown in Tables 2 and 3.

The molecular weight distribution broadens (on the high molecular weight side) with polymerization time. A significant deviation between simulated and experimental molecular weight distributions is observed on the higher molecular weight side of the distributions. A plausible explanation for broadening of the molecular weight distribution is UV-induced cross-linking reactions that 2114


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become more pronounced with increasing polymer concentration. The broadening effect increases with polymerization time (i.e., monomer conversion), a parameter directly correlated with the polymer concentration. The simulated molecular weight distribution in Figure 4 is narrower than the experimentally measured distributions since cross-linking reactions are not accounted for in the copolymerization model (refer to Scheme 3). The cross-linking reactions were not included in the polymerization model due to the nonuniform nature of such reactions involved in the UV induced copolymerization. As the primary intent of the current study was to gain kinetic and mechanistic insight of the copolymerization process under UV irradiation, no measurements on the gel fraction or cross-linking density were carried out. To further investigate the effect of resident time of the polymer on the observed molecular weight distribution (MWD), UV-induced radical polymerization were conducted at 700 W and 100 °C to relatively low monomer conversion of up to 12% (refer to Figure 5). The resulting evolution of

Figure 6. Experimental (dotted line) and simulated (solid line) molecular weight distributions of NBR prepared via UV-induced radical copolymerization conducted at 700 W for 90 min employing internal temperature control, with simulation carried out with rate coefficients shown in entry 3-i in Table 3.

distribution illustrated in Figures 5 and 6, respectively. It should, however, be noted that in distribution mode of PREDICI, such calculations are computationally expensive. For relatively long reaction times, the initial time period before reaching the equilibrium temperature (refer to Figure SI-3, Supporting Information, for a typical profile) accounts for a fraction (of the total reaction time) that can be regarded qualitatively negligible allowing for the use of isothermal conditions to minimize the required computational time. Simulation of the B/A copolymerization process showed comparable values of monomer conversion for simulations carried out using experimental temperature profiles and isothermal conditions. It should, however, be noted that the temperature gradient at early stages of the polymerization will influence the molecular weight distributions, as illustrated by the tailing behavior on the low molecular weight side of the molecular weight distribution. From the estimated values of k*1 k3 and ktc at variable UV light intensities, the dependence of k1*k3 on UV light intensity is illustrated in Figure SI-4, Supporting Information. However, no apparent trend can directly be deduced for the termination rate coefficient ktc. For theoretical parameter estimation studies carried out in the present work, values of ktc in all cases exceeded typical average value of 1.0 × 108 L mol−1 s−1. Interestingly, for the UV-induced radical copolymerization experiments, the estimated average termination rate coefficients are two to six-times higher than the typical average value of 108 L mol−1 s−1 for low conversion methyl methacrylate/styrene copolymerization.28,29 Verdurmen et al. previously reported a value for the termination rate coefficient for a seeded emulsion polymerization of 1,3-butadiene as 7 × 109 L mol−1 s−1, in a study in which the rate coefficient of propagation was determined as 320 L mol−1 s−1 at 60 °C.30 Washington et al. provided a comprehensive overview on rate coefficients of termination and propagation for both acrylonitrile and 1,3butadiene.5 The estimated values of the average termination rate coefficient in the current work are significantly smaller than the values reported in the work of Washington et al. and references there in.5 The molecular weight distributions employed in the parameter estimation tool of PREDICI were

Figure 5. Experimental (●) and simulated (solid line) monomer conversion for B/A copolymerization at carried out at 700 W for 90 min employing internal temperature control, with the simulation carried out employing the rate coefficient shown in Table 2 and entry 3-i in Table 3.

monomer conversion and molecular weight distribution illustrated in Figures 5 and 6, respectively, were fitted to deduce the values k1*k3 and ktc. The estimated values of k1*k3 and ktc from high (Table 3, entry 3-i) and low conversion experiments showed no significant differences. The improved correlation in Figure 6 relative to Figure 4 is indicative of the effect of cross-linking due to a prolonged resident time of the polymer. The deviation from linearity in the evolution of conversion at early reaction times (refer to Figure 5) is a result of a temperature profile during the initial heating period before the equilibrium temperature (100 °C) is reached, with a comparable case illustrated in Figure SI-3, Supporting Information. Because of short reaction times in low conversion experiments, the fitting in PREDICI during parameter estimation was carried out using a recorded experimental temperature profile, instead of assuming an isothermal process based on the final equilibrium temperature. The use of such a gradient temperature profile prior to equilibrium temperature allowed for an improved correlation between both experimental and simulated conversion−time plots and molecular weight 2115


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for the estimation of k*1 k3 and ktc as illustrated by the error space mapping for the range of the rate coefficients assessed. The values estimated for the UV-induced copolymerization reactions were in the ranges 6.0 ≤ k1*k3 (s−2) ≤ 34.6 and 2.6 × 108 ≤ ktc (L mol−1 s−1) ≤ 6.0 × 108 for UV light intensities ranging from 280 to 700 W.

determined relative to polystyrene standards. Dürr et al. have recently determined the Mark−Houwink parameters of NBR with azeotropic copolymer composition, illustrating that the molecular weights of NBR determined relative to polystyrene standards are overestimated.31 The preliminary results demonstrate that the molecular weights of NBR determined relative to polystyrene standards are overestimated by up to a factor of about 2, in agreement with a previous comparison of NBR molecular weight data determined via SEC and NMR.32 As such, the average termination rate coefficient determined in the current study constitutes a lower limit, due to the overestimation of the molecular weight. The actual average termination rate coefficient is higher than herein reported values, with the values reported in the present study regarded as the lower limit. Thus, the difference between typically expected average termination rate coefficients at the diffusion limit and the values determined herein is even larger, exceeding a factor of 6, making this observation even more interesting. For elevated temperature experiments, the initial temperature gradient before the equilibrium temperature is reached results in molecular weight distribution broadening that in turn introduces uncertainties into the fitting procedure to deduce the values of k1*k3 and ktc. Additionally, the cross-linking reactions during the course of the polymerization contribute to the uncertainty in the estimated values. Of the two estimated parameters, the molecular weight distributions will mostly be affected by the average termination rate coefficient. Thus, the error associated with ktc is greater than that associated with the estimated value of k*1 k3. In RAFT mediated solution B/A copolymerization, it was established that 85−90% of RAFT terminated chains (ω-chain end functionality) had B as the terminal monomer unit.33 Taking into considerations the terminal monomer units of active chains and the ratio between the 1,2- and 1,4-insertion of B, the prevalent termination is between two chains bearing B as the terminal unit. Despite these limitations, the developed copolymerization model and the values of the rate coefficients deduced are valuable for obtaining an understanding of the reaction kinetics of a UVinduced B/A copolymerization. Depending on the desired molecular weight of the polymer and monomer conversion, both temperature and UV light intensity can be manipulated to reach the desired monomer conversion/molecular weight distribution goal.

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ASSOCIATED CONTENT

S Supporting Information *

Temperature heating profiles, UV lamp emission spectrum, experimental setup, and apparent rate of polymerization. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

* Fax: +49 721 608 45740; E-mail: [email protected]. Notes

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 Excellence Initiative for leading German universities as well as the German Research Council and the ministry of arts and science of the state of Baden-Württemberg.

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ABBREVIATIONS A, acrylonitrile; B, 1,3-butadiene; NBR, nitrile−butadiene− rubber; SEC, size exclusion chromatography; MWD, molecular weight distribution; ISC, intersystem crossing

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REFERENCES

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CONCLUSIONS The UV-induced copolymerization of 1,3-butadiene with acrylonitrile was successfully carried out and studied in detail. The effects of both temperature and UV light intensity on conversion and molecular weight were assessed. For a specific temperature, relatively high UV light intensities favored higher conversion values at the expense of molecular weight due to an increase in radical concentration. For copolymerization reactions with relatively long reaction times (e.g., time ≥ 4 h at 700 W), cross-linking reactions resulted in a broadening of molecular weight distributions. For the preparation of NBR with minimal cross-linking, short reaction times and moderate lamp intensities are preferable. A kinetic model for the copolymerization was developed in PREDICI andthrough its parameter estimation toolthe average coupled rate coefficient for radical generation (k*1 k3) and average termination rate coefficient (ktc) were estimated by simultaneous fitting of the monomer conversions and full molecular weight distributions. Unique solutions were obtained in all instances 2116


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Mechanistic Insights into the UV-Induced Radical Copolymerization of

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