- Top of page
- 1. Introduction
- 2. Kinetic Model
- 3. Results and Discussion
- 4. Conclusion
- Supporting Information
Industrial production of polymers is often carried out by free radical polymerization (FRP) since this polymerization technique is applicable to a wide range of monomers, tolerant to impurities, and economically beneficial compared to other chain-growth polymerization techniques.1, 2 In particular, acrylate-based polymers contribute significantly to the polymer market as evidenced by their use in a wide variety of applications covering mainly paints, adhesives, textiles, and coatings.3 Typically, polyacrylates are produced via emulsion polymerization, but depending on the required production scale and the area of application, solution and bulk FRP are also of practical importance.3, 4
For some applications of polyacrylates, the production process could benefit from a better control over end-group functionality (EGF). For instance, in the coating industry, the controlled incorporation of EGF in the polymer could avoid the use of expensive functional monomers. Moreover, advanced well-defined macromolecular architectures, such as linear gradient copolymers and star-shaped polymers with controlled arm length, cannot by obtained by FRP due to the inherent difficulty to control EGF and chain length. Therefore, in the last decades a variety of so-called controlled radical polymerization (CRP) techniques5–11 have been developed at laboratory scale in which a mediating agent (e.g., a nitroxide or a transition metal complex) is added allowing control over the polymer microstructure and thus the synthesis of complex polymer topologies and compositions, involving polyacrylate segments.
Since polyacrylates are manufactured almost exclusively by radical polymerization (RP)3 a thorough understanding of the interplay of the radical reactions involved in the production process is of paramount importance. Of special interest are chain transfer to polymer and βC-scission reactions,12–15 which influence polymer properties, such as the average chain length and branching content. These properties can be directly manipulated in view of the desired application of the final polymer product, e.g., by promoting or inhibiting the occurrence of side reactions.16–18 Short chain branches (SCBs) originate after propagation of tertiary macroradicals that are generated via intramolecular chain transfer to polymer, i.e., backbiting (left reaction path in Scheme 1), which consists of self-abstraction of a hydrogen atom from the backbone of a secondary macroradical, mainly involving a cyclic six-membered transition state.16 In addition, due to the higher stability of the tertiary macroradicals a rate retardation takes place in case backbiting is sufficiently important. On the other hand, long chain branches (LCBs) typically result after propagation of tertiary macroradicals formed by intermolecular chain transfer to polymer (right reaction path in Scheme 1), in which a secondary macroradical abstracts a hydrogen atom from another polymer chain generating a tertiary macroradical and a dead polymer chain. Alternatively, at elevated temperatures LCBs can be obtained after addition of macroradicals to macromonomers that are formed by βC-scission. However, several kinetic studies have indicated that the contribution of LCBs to the total amount of branches is very low, especially at low to intermediate conversions.15, 19–23
Scheme 1. Mechanism of chain transfer to polymer reactions in acrylate polymerization. End denotes the ATRP initiator fragment or a hydrogen atom. For nBuA polymerization, R corresponds to an n-butyl group; M stands for monomer.
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In contrast to RP of ethylene and vinyl acetate in which the occurrence of chain transfer to polymer reactions is a long-standing fact that has been well documented since its discovery more than a half century ago,24–28 the importance of branching in acrylate RP has only emerged in the early nineties by the detection of quaternary carbons via 13C NMR spectroscopy.29–31 Interestingly, Ahmad et al.32 have recently reported that the branching level of poly(n-butyl acrylate) can be reduced significantly by performing a CRP instead of an FRP.
One of the most frequently used CRP techniques is (normal) atom transfer radical polymerization (ATRP),33, 34 the principle of which is given in Scheme 2a. During the polymerization macroradicals (Ri; i: chain length) are temporarily and catalytically deactivated by a transition metal complex () to a dormant form (RiX), which contains EGF (X). Typically, the ATRP is started with ATRP initiator (R0X) and activator in absence of deactivator (). For sufficiently high deactivation rates, termination reactions can be suppressed resulting in a high EGF, i.e., almost no dead polymer molecules are formed. If the ATRP initiation is fast, a polymer characterized by a low polydispersity index (PDI) can be obtained as well.
Scheme 2. (a) Principle of normal ATRP and (b) principle of ICAR ATRP; ka, kda, kp, and kt are the rate coefficients for activation, deactivation, propagation, and termination; ka,IX, kda,I, and kp,I are the rate coefficients for activation, deactivation, and propagation related to the conventional initiator; f and kdis correspond to the conventional radical initiator efficiency and the corresponding rate coefficient; i = 0 corresponds to ATRP initiator; n(+1) is the oxidation number of the transition metal complex (de)activator; X corresponds to a halogen atom, and L to ligand.
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Crucial for control of the branching level in polyacrylates is the selection of the ATRP catalyst. Based on simulations, Reyes and Asua35 indicated that ATRP catalysts which can strongly deactivate macroradicals are necessary to obtain a lower branching level than in FRP, in which no mediating agent is present. Later on, Konkolewicz et al.23, 36 showed that the branching level is also influenced by tertiary activation and is to a large extent determined by the relative importance of the rate of backbiting and tertiary propagation. Only if these two rates are well-balanced from low conversion onwards, ATRP provides branching levels as high as FRP. Moreover, these authors pointed out that the branching level can be increased by increasing the targeted chain length (TCL), i.e., the initial molar ratio of monomer to ATRP initiator.
Even though selection of the appropriate ATRP catalyst enables the synthesis of well-defined polyacrylates and purification methods are available for its removal,37–41 the amount of catalyst in a normal ATRP process is too high to obtain an economic profitable process.1, 42 Therefore, in the last years, ATRP modified techniques5, 43–54 have been developed in which a low catalyst concentration (ppm level with respect to monomer) is employed. Importantly, these techniques can be applied using commercially available ligands, are more environmentally friendly and avoid the oxygen sensitivity of the activator upon storage. Furthermore, they can be carried out within polymerization times similar to industrially applied RPs (≈ ≤8 h). In contrast, normal ATRP processes typically take longer than 1 d when using low catalyst amounts.
One of the most important techniques to reduce the amount of ATRP catalyst is initiators for continuous activator regeneration (ICAR) ATRP,43 in which the polymerization is started in the presence of a conventional radical initiator (I2), ATRP initiator (R0X) and deactivator (). Thermal dissociation of I2 provides a source of free radicals from which activator molecules (), are continuously generated, allowing activation of the ATRP initiator and thus the occurrence of the same reactions as in normal ATRP (Scheme 2b).
Recent simulations55 have shown that in ICAR ATRP the control over polymerization rate and polymer properties can be directly manipulated by adjusting the polymerization conditions. For instance, it was demonstrated that, depending on the ATRP catalyst reactivity, step-wise addition of conventional radical initiator in the ICAR ATRP of methyl methacrylate and styrene is needed to reach high conversion while still obtaining a good livingness and control over chain length with ppm levels of ATRP catalyst. Moreover, Konkolewicz et al.56 recently reported for the first time the successful ICAR ATRP of oligo(ethylene oxide) methyl ether acrylate in water using a low (<100) ppm level of ATRP catalyst further proving the importance of this modified ATRP technique for a broad range of monomers and in particular acrylates. Additionally, the potential of ICAR ATRP for the controlled production of polyacrylates can be inferred from the new polymeric materials prepared via normal ATRP. For example, Auschra et al.4 reported the development of new pigment dispersants for the formulation of high solids and waterborne coatings using n-butyl acrylate (nBuA) and dimethylaminoethyl acrylate (DMAEA) as monomers.
Alternatively, well-defined polyacrylates can be synthesized while using low catalyst amounts through activators regenerated by electron transfer (ARGET) ATRP47, 57 or electrochemically mediated ATRP (eATRP)48 in which the activator is regenerated from a reducing agent or by reduction at an electrode. In the presence of metallic copper, a so-called supplementary activator and reducing agent (SARA) ATRP or single electron transfer living radical polymerization (SET-LRP) can also be obtained depending on comproportionation or disproportionation being the dominant side reaction path for the involved catalytic species.58–60 In particular for methyl acrylate, Chan et al.61 and Kwak et al.57 have recently successfully combined ARGET ATRP and the use of a copper wire reducing significantly the residual catalyst amount up to 10 ppm for a TCL of ca. 100.
However, both for ICAR ATRP and these alternative techniques, only a limited number of kinetic studies are available in which the influence of the TCL and catalyst amount on the polymerization rate and control over polymer properties is mapped in detail.56–58, 62–64 Such information is crucial for a comparison of these techniques to evaluate their potential industrial application, since the ultimate goal of a CRP technique is to produce a wide range of average chain lengths at acceptable polymerization rates while preserving control over PDI and EGF. In this work, such detailed kinetic modeling study is presented for the bulk ICAR ATRP of nBuA using the commercially available N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) as ligand and copper (II) bromide (CuBr2) as transition metal salt. A similar modeling approach can be followed for other related CRP techniques avoiding time consuming experimental screening procedures for a given catalyst.
The activation/deactivation kinetic parameters are assessed based on the experimental data of Ahmad et al.32 for the normal bulk ATRP of nBuA taking into account the reactivity difference between secondary and tertiary macrospecies. The kinetic model also considers the possible influence of diffusional limitations on termination and deactivation. For TCLs up to thousand, it is shown that ICAR ATRP can be successfully applied to synthesize well-defined poly(nBuA) with ppm levels of ATRP catalyst within reasonable polymerization time and with limited loss of EGF. The simulation results confirm in particular the potential of CRP techniques using low amounts of copper for the controlled incorporation of EGF in polymer chains. Diffusional limitations are shown to be most important on secondary deactivation leading to a rate acceleration at high conversion.
2. Kinetic Model
- Top of page
- 1. Introduction
- 2. Kinetic Model
- 3. Results and Discussion
- 4. Conclusion
- Supporting Information
The reaction scheme used in the kinetic modeling of the bulk ATRP of nBuA and the corresponding intrinsic kinetic parameters are summarized in Table 1. Activation, deactivation, backbiting and βC-scission are included next to typical radical polymerization (RP) reactions, i.e., propagation, chain transfer to monomer and termination. A distinction is further made between the reactivity of secondary (s) and tertiary (t) macrospecies. For simplicity, intermolecular chain transfer to polymer is neglected since literature reports indicate that LCBs barely contribute to the total branching content.15, 19–23 Similarly, termination by disproportionation65, 66 and addition reactions involving macromonomers are neglected in a first approximation.67
Table 1. Reactions involved in the bulk normal/ICAR ATRP of nBuA and their intrinsic kinetic parameters (kl,chem); i,j = chain length; Ri,t and Ri,s: tertiary and secondary macroradicals; : apparent rate coefficient for the reaction step l; R0 and I: ATRP initiator and conventional initiator derived radical; T = 378 K; MM: macromonomer.
| ||Elementary reaction||Equation||kl, chem||Ref.|
|Normal ATRP||Activation (a)||3.1 [L · mol−1 · s−1]||72|
|1.6 [L · mol−1 · s−1]||This work|
|2.6 × 101 [L · mol−1 · s−1]||This work|
|Deactivation (da)||8.0 × 108 [L · mol−1 · s−1]||This work|
|4.0 × 108 [L · mol−1 · s−1]||This work|
|4.0 × 107 [L · mol−1 · s−1]||This work|
|Propagation (p)||7.4 × 104 [L · mol−1 · s−1]||68|
|7.4 × 104 [L · mol−1 · s−1]||68|
|1.5 × 102 [L · mol−1 · s−1]||69|
|Chain transfer to monomer (trM)||9.0 [L · mol−1 · s−1]||70|
|9.0 [L · mol−1 · s−1]||70|
|8.5 × 10−2 [L · mol−1 · s−1]||70|
|Backbiting (bb)||2.0 × 103 [s−1]||69|
|βC-scission (βC-sc)||2.2 [s−1]||71|
|Termination by recombination (tc)||2.3 × 108 [L · mol−1 · s−1]||69|
|4.6 × 107 [L · mol−1 · s−1]||69|
|3.0 × 106 [L · mol−1 · s−1]||69|
|Extra for ICAR ATRP||Dissociation (dis)||8 × 10−2 [s−1] with f = 0.75 [–]||55, 73|
|Activation (a)||1.6 × 101 [L · mol−1 · s−1]||Based on55|
|Deactivation (da)||4.0 × 107 [L · mol−1 · s−1]||Based on55|
|Propagation (p)||7.4 × 105 [L · mol−1 · s−1]||Based on55|
The ATRP catalyst and initiator are the same as those used by Ahmad et al.32 in their experimental study of the bulk normal ATRP of nBuA at 378 K, i.e., CuBr/PMDETA and methyl 2-bromopropionate (MBrP). For the reactions common with FRP, intrinsic rate coefficients reported in literature are used.68–71 For activation of the ATRP initiator a value of 3.1 L · mol−1 · s−1 is considered based on the experimental study of Seeliger and Matyjaszewski.72 The remaining secondary and tertiary activation and deactivation intrinsic rate coefficients are adjusted according to the experimental data of Ahmad et al.32 As discussed below, the obtained values are consistent with literature values and confirm the higher stability of tertiary macrospecies.
For ICAR ATRP, dissociation of the conventional radical initiator and activation, deactivation and propagation involving conventional radical initiator fragments are also considered (Table 1). tert-Butyl peroxy-2-ethylhexanoate (Trigonox21s) is employed as conventional radical initiator, since it is particularly suited for the polymerization of acrylates in the range of 353–423 K.73, 74 For simplicity, a typical constant initiator efficiency of 0.75 is used and the intrinsic activation and deactivation rate coefficients related to conventional radical initiator fragments are taken equal to those of the secondary macrospecies. The latter approach has been selected since simulations have revealed that a typical tenfold reactivity difference has no significant influence on the results.55 The conversion profile and the evolution of the polymer properties with time are simulated using the methodology developed by D'hooge et al.,67, 75 which is based on an extension of the method of moments coupled with an application of the quasi-steady state approximation for the calculation of population weighted apparent rate coefficients using a convergence test.
CRP kinetic studies75–79 have indicated that diffusional limitations can result in a lowering of the apparent termination reactivity during the polymerization and that deactivation can become diffusion controlled at sufficiently high conversion. Therefore, in this work the possible influence of diffusional limitations on termination and deactivation is considered in agreement with literature reports.75, 80–84 For more details, the reader is referred to the Supporting Information.