Article pubs.acs.org/Macromolecules
Investigation of the End Group Fidelity at High Conversion during Nitroxide-Mediated Acrylate Polymerizations Yohann Guillaneuf,†,* Didier Gigmes,† and Thomas Junkers‡,* †
Aix-Marseille Univ., UMR CNRS 7273: Institut de Chimie Radicalaire, Equipe CROPS, Av. Escadrille Normandie Niemen, Case 542, Marseille 13397, Cedex 20, France ‡ Polymer Reaction Design Group, Institute for Materials Research, Universiteit Hasselt, Agoralaan, Gebouw D, B-3590-Diepenbeek, Belgium S Supporting Information *
ABSTRACT: The impact of formation of midchain radicals and more specifically the follow up reactions of β-scission and macromonomer addition to propagating macroradicals stemming from this processon the nitroxide-mediated polymerization of acrylates have been studied via kinetic modeling with the software package Predici on the example of butyl acrylate polymerization at 120 °C. Only small influences of the midchain radical formation on the livingness of the process is observed, however, large effects must be envisaged by the (reverse) scission reaction at high monomer-to-polymer conversions. A significant loss of livingness, depending on the temperature, monomer and initiator concentration must be expected at elevated stages of polymerizations. For a polymerization at 120 °C and a target chain length of 100, less than 75% livingness of the polymer product can be expected at 80% conversion. From this point of polymerization on, significant broadening of the overall polymer product is predicted in accordance to literature data and eventually the chain-length− conversion relation is lost at the end of reaction.
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INTRODUCTION The up-rise of controlled synthesis of polymers via radical polymerization has undoubtedly led to a large variety of materials and products that are accessible via relatively easy synthesis pathways.1 Methods such as atom transfer radical polymerization,2−5 ATRP, reversible addition−fragmentation transfer polymerization,6−9 RAFT, or nitroxide-mediated polymerization,10,11 NMP, are highly effective in controlling molecular weight, polydispersity and functionality of polymers. While research on these living/controlled methods is mostly concerned with the ability to control molecular weight and polydispersity, focus has changed in recent years to also address the functional fidelity in a more detailed way of the obtained polymers and more specifically, the potential to convert these functionalities into usable moieties.12,13 In principle, a living polymerization method should yield uniform polymers with respect to their chain ends, but unlike in anionic polymerization, radical processes always undergo side processes such as irreversible termination to some extend and it is the synthetic challenge to minimize these reaction channels. How the reaction conditions affect the end group fidelity (and hence the potential to use the obtained polymers for further reactions, be it generation of block copolymers or click chemistry approaches) has been addressed often in the past. In most living polymerizations, one observes a loss of functionality with increasing conversion due to the cumulative effect of termination. Also, the initiation efficiency often plays a pivotal © XXXX American Chemical Society
role. Less often addressed is, however, that specific side reactions that may occur for example in acrylate polymerization systems can have a much more disturbing effect on the polymer product. Acrylate monomers are known to undergo extensive transfer to polymer reactions (also referred to as backbiting in some cases) during polymerization, in which the secondary propagating chain ends of a macroradical are transformed into more stabilized tertiary radicals that are located on the polymer backbone.14 These radicals are usually referred to as midchain radicals, MCR. Under common reaction conditions for radical polymerizations, a majority of radicals is found to be in the MCR state. These radicals accumulate due to their decreased reactivity compared to their secondary counterparts. This does, however, nonetheless mean that they do not play a significant role in the reaction. As a consequence of their reduced reactivity, the overall polymerization rate of an acrylate polymerization is slightly retarded, leading to nonideal polymerization kinetics.15 Second, the follow-up reactions of a MCR, that is monomer addition or termination, leads to the formation of chain branches. Thus, polyacrylates made via radical polymerization are usually branched to a significant extends, making the characterization of such products generally Received: May 11, 2012 Revised: June 7, 2012
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Figure 1. Mn and Mw/Mn (dispersity) versus conversion for the miniemulsion polymerization of BA carried out at 125 °C initiated by MONAMS (3.07 × 10−2 M) and with (2.6 (●), 5.01 (⧫), and 10.1 (■) % of excess free SG1 nitroxide. Reproduced from ref 29 with permission. Copyright Wiley 2002.
more difficult.16 While branching occurs at low to intermediate reaction temperatures, MCRs lead to even more complex sidereactions at elevated temperatures, that is above ca. 80 °C. In this temperature regime, β-scission reactions become progressively more important.17,18 Thereby, the MCRs fragment to either side of the radical center, forming a secondary propagating radical and an unsaturated polymer species. As was shown, these unsaturated macromonomer species are not at all inactive. In a reverse scission step, they can be added to growing radical chains in an addition−fragmentation transfer process, once again forming MCRs.19−21 The influence of these acrylate specific reactions on controlled radical polymerizations has partly been investigated before (or rather vice versa, the effect of living polymerization on acrylate side reactions was elucidated).22 Rate retardation effects have not been focus of such studies as living polymerizations inherently cause rate changes with the exact of close-to-ideal RAFT systems.23 Previous investigations were mostly concerned with the branching level. It was shown recently in a joint effort from many research laboratories, that living polymerization most likely leads to a reduction in the number of branches per polymerized chain units, the reason for this effect was however not entirely understood.22 Recently, Matyjaszewski and co-workers24 studied theoretically the origin of such difference using either Predici and kinetic Monte Carlo simulations and conclude that competitive processes, i.e., the occurrence of two distinct equilibria and the interconversion reaction between secondary and tertiary macroradicals could in certain case lead to a decrease of the branching level. This reduction might also be caused by the low comparability of experimental data. Quantitative information on chain branching in polyacrylates is not easily deduced, and truly reliable data are scarce.16 A clear reduction of the degree of branching of polybutyl acrylate samples made in presence of a potent chain transfer agent could however be shown, which supports that living polymerization could, at least partially, have similar effects.25 A very interesting case of branching from the viewpoint of living polymerization is random intermolecular transfer. Such reaction leads eventually to the formation of a second controlling group on a single chain (since a new radical center is formed on a dormant macroalkoxyamine), which of course has a large impact on the polymerization and any subsequent reactions involving these chains. Interestingly, this kind of reactions is the only access so far to gather kinetic information on the random transfer step; Vana and co-workers determined the number of RAFT end groups on polymer chains to quantify
the transfer rate in butyl acrylate polymerization.26 In a similar study, Charleux and co-workers tried to monitor the number of alkoxyamine moieties via matrix assisted laser desorption ionization−time-of-flight spectrometry.27 They could not detect more than one active end group per chain, however, as they claimed themselves, intermolecular chain transfer cannot be excluded even if this reaction seems to be negligible compared to the intramolecular chain transfer. The nitroxide mediated polymerization is an attractive process for production of well-defined polymers on a larger scale.28 Therefore, in order to enhance the efficiency of the process it is important to understand how the control systems react at high monomer to polymer conversions. As detailed above, the reactions of acrylate monomers are complex, especially at high conversions. Unfortunately, clear experimental evidence is difficult to gather for such reaction regimes since the determination of branching levels and the quantification of macromonomers evolving in such reactions are not gathered via routine characterization. From Charleux and co-workers29 it is nevertheless known that dispersity in N-tert-butyl-N-1-diethylphosphono-2,2-dimethylpropyl-0,1-methoxycarbonyl ethylhydroxylamine (MONAMS)/N-tert-butyl-N-(1-diethylphosphono-2,2 -dimethylpropyl) nitroxide (SG1) mediated butyl acrylate polymerization at 125 °C increases from normal low dispersity levels to values exceeding 2 at high conversions, an effect which can only partially be explained by branching (Figure 1). Thus, we have adapted a theoretical approach by simulating the polymerization up to high conversions under variation of reaction conditions and certain kinetic parameters via the program package Predici30 to elucidate the influence of the acrylate-specific reactions on the outcome of the polymerization. Special attention is given there to the end group fidelity rather than molecular weight as this determines in how far the obtained polymers are able to be used as macroinitiators for subsequent reactions steps, or as substrates for click-type conjugation reactions. It should be mentioned that we have separated the description of the kinetic simulations from the discussion of the results to make the manuscript more readable to synthetic polymer chemists who are not familiar with kinetic modeling of radical polymerizations. Detailed information on the employed model, kinetic rate coefficients for all individual reaction steps as well as figures for an in-depth understanding of certain modeling steps can be found in the Supporting Information section. In what follows, statements will be made based on the preliminary simulation results outlined there and B
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Scheme 1. Reaction Equilibria in NMP of Acrylate Monomers
At the same time, however, also a macromonomer species that can take further part in the reactions is produced. Homopropagation of the macromonomer species can be neglected, a copolymerization (essentially the reverse scission step) does however occur.19,20 Indeed, a relative high reaction rate coefficient can be envisaged for this reaction and as soon as significant amounts of macromonomer are accumulated (hence at higher conversions), a change in reaction kinetics must be observed and MCRs are also obtained via reverse scission. Last, of course also MCRs can undergo any combination/ disproportionation reactions in bimolecular termination. In an NMP system, these reactions will, however, not play a large role as it can safely be assumed that such reactions are slower than conventional termination processes and thus suppressed by the persistent radical effect. Rate coefficients for the individual reaction steps are partially available. Propagation,32 backbiting (or intramolecular transfer), monomer addition to MCRs, and termination rate coefficients are known to relatively high accuracy.33−35 Some data is available on the β-scission reaction,36 accuracy is however limited. For the reverse scission rate, reasonable estimates were made so far,19,37 and it can be safely assumed that the rate coefficient is in the order of the coefficient of secondary radical propagation. The specific rate coefficients of the NMP reaction are known with a rather good accuracy since the rate coefficient could be determined directly without the need of a polymerization kinetic analysis.38−41 Recently, in situ NMR was used to determine MCR-specific kc and kd values.42 It was concluded that there is a difference between the data obtained in this study compared to data reported in earlier literature, however solvent effects (i.e., DMSO)−which are known to have a strong influence on both the dissociation and recombination reactionwere not taken into account.43−46 For the bulk acrylate polymerization at 120 °C that is herein studied, the recombination rate coefficient kc that describes spin annihilation between free nitroxide and propagating chains is estimated to be 2.8 × 107 L·mol−1·s−1.47 The associated dissociation rate kd is assumed to be 1.55 × 10−3 s−1 (Ea = 129.3 kJ·mol−1).41 For the specific rate coefficients of the NMP equilibrium involving MCRs, one can estimate that combination will happen at a slower rate than for its secondary counterpart due to the increased steric demand of the radical site. At the same time, the dissociation rate will increase, also due to steric effects, but also because of the tertiary nature of the MCR and hence higher stability of the dissociation
the interested reader is referred to there if more background information is sought.
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RESULTS AND DISCUSSION Description of the Reaction Scheme. NMP polymerizations are usually performed at elevated temperatures (i) to speed up the reaction and (ii) to allow a favorable equilibrium between active and dormant chains. Thus, when acrylate monomers are polymerized under NMP conditions, one is exactly in the temperature regime where one would expect very nonideal polymerization behavior of the acrylate. In Scheme 1 the most important reactions that can occur in hightemperature NMP of acrylates are summarized. The growing chains with a secondary radical terminus are in a conventional NMP equilibrium as is shown at the top of the scheme. However, whenever the radical is in its active state, it can undergo backbiting reactions to form MCR species.14 These MCR species can be retransformed into secondary radicals (in the scheme summarized in a single kinetic coefficient ktrans. As described above, a clear distinction must be made between intra- and intermolecular transfer. The first leads to a MCR with a proton end group while the latter leads to formation of a dead polymer chain and a polymer that carries next to the MCR functionality an intact alkoxyamine function at its chain end). After a few monomer addition steps to the MCR, the chain behaves like a “normal” macroradical again and can enter the NMP equilibrium as usual. However, the MCR can also directly enter an equilibrium with the nitroxide by formation of a tertiary alkoxyamine. The occurrence of such reaction was recently envisioned by Klumpermann and co-workers.31 Nevertheless, in this study, the hypothesis′ were only based on 1H NMR prediction using a NMR predictor software and a thorough analysis of the 1H NMR spectrum led us to the conclusion that it is not possible to discriminate the two structures (see Supporting Information for more details). The effect of this second NMP-equilibrium will be discussed below in detail, but it can already be said that such alkoxyamine will be less stable than its chain-end counterpart due to steric demand and relative stability of the MCR, thus reducing its impact on the overall polymerization. As shown in Scheme 1, the other important pathway for a MCR is β-scission. In this step, a macromonomer is formed and a secondary radical, identical in its structure to the original propagating chain. Thus, the scission reaction converts the MCR back in its original (propagating radical) state, even if the chain length is reduced. C
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products. Hence, in total the MCR specific NMP equilibrium will be beset with a larger equilibrium constant KNMP compared to the main equilibrium. To what extend KNMP is increased, is however unclear and cannot easily be determined. The kd value for the model methyl propionyl SG1-based alkoxyamine that mimic the polyacrylates chain end has been determined to be 1.0/3.0 × 10−3 s−1 for the two diastereoisomers at 120 °C (i.e., activation energy of 128.4−130.8 kJ·mol−1)48 and the one concerning the 1-carboxymethyl-1-methyl−ethyl fragment that mimic a tertiary alkoxyamine has been determined to be 0.8 s−1 at 120 °C (i.e., activation energy of 108.9 kJ·mol−1).49 We can expect that the steric hindrance of two polymer chains apart the aminoxyl moiety will lead to a further increase of the kd value as we already observed when a bulky poly(methyl methacrylate) chain induced a strong penultimate effect50 that increased the kd value by a factor close to 30 compared to the model alkoxyamine. The same phenomenon occurred for the recombination rate coefficient since the polymeric macroradicals always recombine at a rate 1 order of magnitude lower than the model radical, reaching 2 orders of magnitude51 in the case of a bulky PMMA macroradical. The combination of all these parameters could lead to an increase of the K value between 2 and at least 4 orders of magnitude. Influence of the MCR−NMP Equilibrium. It can be shown via simulation that a larger equilibrium constant has no overall effect on the polymerization (see Supporting Information). The pure presence of MCRs is inconsequential with respect to the obtained molecular weights and end group fidelities, it even has a beneficiary effect on the control over the polymerization due to the decreased average termination rate. Thus, for further simulations, we adopted rather high kc and low kd values for the specific equilibrium of the MCRs with the nitroxide to ensure that the impact of MCRs on the equilibrium is not underestimated and to cover a as broad as possible range of kinetic conditions. Interesting is, however, that the NMP equilibrium has a profound influence on the fraction of midchain radicals present during polymerization. If no NMP reactions are assumed at all, 80 or more percent of radicals at any given time consist of the MCR type in a free radical polymerization of an acrylate.52 When a reversible deactivation mechanism via reversible termination is active, this number is decreased. Indeed, simulations with literature kd and kc values show that a reduction is achieved to a fraction of below 70%, even at 120 °C for low conversion (it should be noted that with increasing conversion, the fraction increases to over 90% due to the decreasing amount of monomer). This reduction of midchain radical concentration results in largely reduced degrees of branching that have been observed for NMP reactions of acrylates.22 If the simulation model is extended to also consider a MCR-specific NMP equilibrium, a very different situation is seen with respect to the fraction of midchain radicals. If the MCR-related equilibrium constant KNMPMCR lies within 2 orders of magnitude to KNMP, then a significant further reduction of the MCR levels must be expected. For KNMPMCR = KNMP a reduction to almost 0% MCR-type radicals would be reached, with KNMPMCR = 10KNMP a reduction to about 30% is expected. Nevertheless, even under such assumptions, the simulation shows that at high monomer conversions the fraction of midchain radicals would get close to the number that is expected in a free-radical polymerization due to the lack of monomer units to convert MCRs into secondary branched macroradicals. A scenario where KNMPMCR is this low is however highly unlikely. It is known from studies on low conversion
Figure 2. Combined product distributions for NMP reactions of butyl acrylate under assumption of ideal radical polymerization, reaction with backbiting and with random scission events.
radical polymerization that the termination rate of MCRs is significantly reduced.48,53 A similar effect could be expected for kc. Also, it is known that the dissociation rate is largely dependent on steric factors. MCRs are in fact much less accessible, thus a larger kdMCR compared to kd must be expected. Exact values for the MCR−NMP equilibrium are hard to identify, but from a simulation point of view (based on the fraction of MCRs during polymerization, only the size of KNMPMCR is important rather than the absolute level of kcMCR and kdMCR. Thus, for further simulations, kcMCR was set to equal kc. For the dissociation rate, a value of kdMCR = 10kd appears to be a reasonable choice and further simulations have been carried out with this assumption. While a profound influence on xMCR is observed with variation of KNMPMCR, surprisingly no large effect on the degree of branching is connected. Irrespective of the size of the equilibrium constant (as long as it is not lower than KNMP) no change in the overall observable degree of branching can be seen in the simulations. It must be noted, however, that the employed Predici model may not allow for a very precise modeling of branching levels. Influence of the Intermolecular Transfer. Another factor that can influence the amount of midchain radicals and the degree of branching in the polymer is intermolecular transfer. In the literature, a value of 0.33r L·mol−1·s−1 (where r is the chain length of the polymer where the radical center is transferred upon) is given for 60 °C.26 If this value is extrapolated to 120 °C under assumption of an activation energy of 30 kJ·mol−1 (which is roughly in the order of the backbiting reaction),14 about a 10-fold increase in this rate might be expected. Still, the number of chains that have taken part in such transfer reaction is still below 1% of the total polymer product and the reaction might be deemed inconsequential for most parts. Still, intermolecular transfer was in the following considered with a rate coefficient of 3.3r L·mol−1·s−1. Influence of the β-Scission Reaction. With the model being established with respect to the transfer reactions, one can then study the effect of the remaining secondary reactions of the MCRs. Termination events can be largely excluded to play a major role due to the persistent radical effect and even though such reactions like self-termination and cross-termination of MCRs are included in the model, they have only very little D
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effect on the outcome of the simulation. A large effect on the polymer product in both end group fidelity and product distribution is observed when the scission (as well as reverse scission) reaction is activated in the model with literature βscission and macromonomer addition rate coefficients. A summary of these effects is depicted in Figure 2. In there, the product distributions (as would be measured by SEC) for high conversion NMP reactions at 120 °C is compared for the different model assumptions. The full line depicts the outcome of an ideal NMP process without transfer to polymer reactions, the dotted line represents the resulting distribution under assumption of random backbiting events and the dashed line gives the result when also the β-scission reaction is put under consideration. As described above, no significant change in the product distribution is found when only backbiting and midchain radical monomer addition are assumed. With the βscission reaction, however, a largely changed product distribution is seen. This demonstrates the major impact this reaction has on the outcome of the polymerization. The average molecular weight is reduced and a much broader distribution is obtained, with a polydispersity of roughly 2. It should be noted here that the broadness of the distribution is not only given by the scission reaction itself. A larger contribution comes also from the reverse scission reaction that allows for mixing of chain lengths and from which consequently definition in the dispersity is lost, too. The situation is however not as severe as Figure 2 might indicate. In there, the product distributions for very high conversions are shown, that is 98−99%. As seen before in the simulations, the fraction of midchain radicals increases rapidly with increasing conversion due to the decrease of the monomer conversion and the rate reduction of monomer addition to MCRs that goes along with that. Hence, the impact of this disturbing effect becomes more severe with increasing conversion and only reaches significant levels at high conversion. Figure 3 shows a series of molecular weight distributions under consideration of transfer and β-scission at different stages of the polymerization. As can be nicely seen, at first a (linear) increase in molecular weight can be observed. From a certain stage on, however, in the high conversion regime, progress in average molecular weight is lost and the distributions start to broaden and as a result of this, the average molecular weight even decreases and the PDI increases from 1.25 at 65% conversion to 1.65 at 95% conversion and 2 to 99% conversion (Figure 3a). The trend represented in Figure 3a is similar to the one described in Figure 1, showing that qualitatively the simulations are in good agreement with experimental data. Nevertheless our purpose is not to try to extract rate coefficients from these data since the experimental conditions are not the same. Additionally, a meaningful quantitative comparison of experimental and simulation data is with the current set of kinetic rate coefficients only very difficult to achieve. As already noted above, this process is mostly due to the interchange of chains from addition of macromonomers that were previously formed in polymerization and that can react with secondary macroradicals. Alongside the molecular weight distributions (it should be noted that full product distributions are depicted, thus a combination of the various polymer fractions in the simulation in order to obtain a picture that can be compared with experiments), also the fraction of chains carrying the alkoxyamine end group is shown.
Figure 3. (a) Fraction of living chains and overall polydispersity of product distributions at different conversions and (b) full product distributions for NMP reactions of butyl acrylate under assumption of transfer and scission reactions at different conversions and c0(initiator) = 0.07 mol·L−1. For further details on the simulation parameters see text of the table in the Supporting Information.
Polymerization at High Monomer Conversions. Not surprisingly, a loss of functionality is clearly seen at conversions above 50%. At 60% monomer-to-polymer conversion, already less than 80% of chains constitute the desired living polymer product. At 80% conversion, and thus at stages where still a reasonable increase in molecular weight is obtained, already only around 60% of chains are living, a value that further decreases to close to only 30% at full conversion. Interestingly, the simulation predicts a minimum in polydispersity of the living fraction at conversions of around 60% (with a value of around 1.20), thus at a stage where already a non-negligible proportion of chains is nonliving. Generally, the polydispersity follows the general trend in NMP with initial slightly elevated dispersities, which reduce quickly with increasing molecular weight. From the minimum on, however, a rapid increase in polydispersity is observed, reaching 1.5 at 95% conversion and finally 2 at practically full conversion, which is in good agreement with literature data.29 It must be noted though that the above-described behavior is only typical for the stated initiator concentration of 0.07 mol·L−1. Figure 4 depicts the change in the living fraction and the average molecular weight with conversion of BA NMP at three different initiator concentrations under otherwise identical conditions. When more initiator is employed, then the polymerization becomes more ideal due to the reduction in midchain radical formation (see above). At 0.7 mol·L−1, practically no disturbances from MCR formation must be envisaged. It should be noted, however, that under such conditions only oligomers are formed with a maximum chain length of about 10. In contrast, when much smaller alkoxyamine concentrations are used, a breakdown in the controlled polymerization becomes well visible. With 0.007 mol·L−1 initiator concentration and thus a target molecular weight from above 105 g·mol−1) the simulation predicts a low amount of living chains even from the start of the polymerization on and a complete loss of the conversion-molecular E
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Figure 4. Fraction of living chains (dashed) and evolution of molecular weight of the living fraction in NMP polymerization at different initial initiator concentrations (as indicated). For further details on the simulation parameters see text of the table in the Supporting Information.
weight relation above roughly 50% of conversion. When comparing the different scenarios, it is noteworthy to add that in each simulation a short induction period is seen. This period coincides with the initialization time of polymerization in which the initial alkoxyamine initiator is consumed and transformed into pBA-SG1 chains. This time can be partially guided by the addition or subtraction of free SG1 in the reaction recipe, but has little to no impact on the outcome of the simulations as discussed. Generally, the above-presented data should not be seen as a worse-case scenario. Since in all simulations, kc = kc,MCR and kd = 10kd,MCR was chosen, already reduced fractions of MCR species can be envisaged in these simulations as noted above. Lower MCR-specific combination rate coefficients and even faster decomposition of the adducts are a likely scenario and thus the fraction of living chains in actual polymerizations could easily be lower than given. However, a crucial number in the simulations is clearly the rate of β-scission, which is to date not known with sufficient precision (in contrast to the reverse scission reaction, that can now be considered to be relatively well assessed, at least at lower temperatures).19,37 While the order of magnitude of the kinetic rate coefficient can be safely assumed to be correct, larger variations may occur within that limit. Thus, the exact results from the simulations for specific reaction conditions must be treated with care due to the large impact of this rate parameter and mismatches with experiments are somewhat likely to occur. Nevertheless, trends should be well represented and it can safely be concluded that β scission has a very disturbing effect on NMP polymerizations. MCRs themselves only slightly interfere with the livingness of the
polymerization, their specific follow-up reactions, however, do. Since NMP is usually carried out at temperatures where scission reactions play a significant role (in contrast to ATRP and RAFT where lower temperatures are most often employed), it seems to be advisible in future studies on the impact of transfer-to-polymer reactions on the branching level of the polymer from living polymerization to also take βscission into the considerations, at least when polymers from the high-conversion regime are discussed. It should be noted, that also for some particular reactions in RAFT systems or atom transfer radical coupling reactions54 macromonomer formation is likely to occur making reaction products partially complex.55 β-scission is generally beset with a very high activation energy and thus reduction of the temperature by only 10−20 °C should have already a significant impact. Because of the lack of precise data on some of the rate coefficients, we refrained from performing such simulations, but preliminary results indicate that for example the performance of the polymerization with an initiator concentration of 0.007 mol·L−1 can be brought to levels comparable to the situation depicted in Figure 3a and thus to a regime that yields acceptable results in terms of end group functionality and molecular weight control when the reaction temperature is reduced to 100 °C.
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RECOMMENDATIONS From the outcome of the simulations, a few practical recommendations can be derived for carrying out acrylate NMP. Generally, less interference from MCRs and their followup reactions must be envisaged when increasing amounts of initiator are employed. As a rule of the thumb, smaller target F
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molecular weights result in higher fractions of living chains. This rule seems also to hold when reactions with less initiator are stopped at premature stage, thus at low conversions. Obviously, even though not demonstrated in full detail in the present work, a reduction in polymerization temperature has certainly beneficial influence and as low as possible temperatures should be employed. Also, when performing polymerizations, differentiations between end group fidelity and molecular weight control must be made and reaction conditions be chosen according to which is more important. A more or less efficient molecular weight control with increasing Mn can still be achieved also in regimes where the end group fidelity is already heavily impacted. Conclusively, it is also recommendable to account for the formation of macromonomers with respect to block copolymerizations. Presence of MM species in the reaction mixture can lead to copolymerization of these MMs with any propagating radical.19 Depending on which strategy is used in block copolymer formation and which monomers are polymerized, chain grafts may be introduced not only on the acrylate block, but also on the nonacrylate block, resulting in highly complex macromolecular structures and product mixtures.
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ASSOCIATED CONTENT
S Supporting Information *
Details on the simulation model and more detailed simulation results. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*(Y.G.) E-mail:
[email protected]. Fax: +33 4 9128 8758. (T.J.) E-mail:
[email protected]. Telephone: +32 (11) 26 8318. Fax: +32 (11) 26 8399. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are also grateful to the University of Provence for an invited professor position for T.J. REFERENCES
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