Synthesis of Macromonomers from High-Temperature Activation of

Article pubs.acs.org/Macromolecules

Synthesis of Macromonomers from High-Temperature Activation of Nitroxide Mediated Polymerization (NMP)-made Polyacrylates Joke Vandenbergh and Thomas Junkers* Polymer Reaction Design Group, Institute for Materials Research (imo-imomec), Universiteit Hasselt, Agoralaan Building D, B-3590 Diepenbeek, Belgium S Supporting Information *

ABSTRACT: Macromonomers are synthesized from thermal reactivation of three different precursors, Poly(n-butyl acrylate) P(nBuA), Poly(tert-butyl acrylate) P(tBuA), and Poly(2-ethylhexyl acrylate) P(EHA), synthesized via nitroxide mediated polymerization (NMP). Reactivation of the polymer chains is carried out at 140 °C, whereby the polyacrylate macroradicals undergo chain transfer reactions forming midchain radicals, MCR, followed by β-scission reactions leading to unsaturated macromonomers. Soft-ionization mass spectrometry of product samples reveals that in all cases predominantly macromonomers that carry a hydrogen end group on the other chain end are formed, which is also accompanied by small reductions in molecular weight (−200 g·mol−1) and slight increases in polydispersity (+0.2). Furthermore, we demonstrate that macromonomers under these reaction conditions are not only formed through simple backbiting/β-scission via six-membered ring transition structures but also that complex addition−fragmentation equilibria must play a considerable role. As observed before for macromonomers made from activation of atom transfer radical polymerization (ATRP)-made precursors, a size-selective reaction pathway is observed that favors the generation of macromonomers with only odd numbers of monomer units on the backbone, supporting an MCR migration mechanism, which allows MCRs to move along the backbone.

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INTRODUCTION Macromonomers (MMs)1 are oligomeric or polymeric species with a polymerizable vinyl terminus. In general, they constitute very efficient polymeric building blocks in order to synthesize complex polymeric architectures such as branched polymers, block-, graft-, comb-, or star-polymers with unique physical properties and self-assembly abilities.2,3 MMs are accessible via several pathways.4−7 One formation pathway for MMs is the conventional free radical polymerization (FRP) of acrylates at high temperatures whereby backbiting/β-scission reactions lead to the desired structures.8−10 During the FRP of acrylates, midchain radicals (MCRs) are formed at moderate temperatures via chain transfer-to-polymer reactions.11,12 In a conventional polymerization at 60 °C, close to 80% of all radicals present are already of the MCR type, which can undergo termination and propagation reactions, hence resulting in chain branches. At higher temperatures, well above 80 °C, also β-scission reactions may occur, giving rise to secondary radicals and unsaturated MMs (see Scheme 1). Three independent transfer reactions can lead to the formation of MCRs. First, through backbiting reactions MCRs are formed at the chain end via a six-membered ring transition structure (compare Scheme 2 for details). Second, also random intramolecular transfer to any arbitrary position on the backbone can take place. Finally, random intermolecular transfer to neighboring chains may in principle occur. Often, these three processes are very difficult to distinguish from one © 2013 American Chemical Society

another, although each reaction is associated with a distinct reaction rate coefficient. Generally, backbiting is believed to be the dominating transfer pathway, and rate coefficients have been determined for that reaction.12 The contribution of the other reactions is less clear and sometimes debated. β-Scission has complex consequences for any acrylate polymerization. Short-chain radicals are produced, and information on the end groups is lost, which poses a significant problem for high-conversion controlled radical polymerization.13 Also, scission is in principle a reversible process. Growing macroradicals can add to MMs in addition− fragmentation equilibria with addition rates in the order of the monomer propagation. The synthesis of MMs from acrylate polymerization at high temperatures was first demonstrated by Chiefari et al. in a onepot synthetic route.14,15 While this study marked a turning point in the understanding of acrylate MCR reactions, the detailed mechanism was only later clarified via systematic Electrospray Ionization-Mass Spectrometry (ESI-MS) studies and kinetic simulations.16−21 Indeed, as mentioned above, it could be shown that high temperature acrylate polymerizations resemble (reversible) addition−fragmentation reaction systems,22 thereby generating highly pure MMs when conditions Received: March 5, 2013 Revised: April 13, 2013 Published: April 24, 2013 3324

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Scheme 1. General Mechanism of MCR Formation/β-Scission Occurring at High Temperature Acrylate Polymerization

Scheme 2. Addition−Fragmentation Equilibrium Leading to the Accumulation of MMH

were chosen accordingly (low monomer concentration, slow radical generation). In principle, all acrylate monomers show the same distinct behavior and undergo the backbiting-scission cycles.23,24 In order to improve MM synthesis, we recently introduced a method to form MMs directly from atom transfer radical polymerization (ATRP)-made precursors.25 When such chains were activated at high temperatures by the reaction with CuBr2 and tin octanoate in the absence of monomer, backbiting followed by β-scission occurred as the primary reaction pathway, ultimately forming MMs. The advantage of such an approach is that, in this way, a preselection of chain length and dispersity of the material could be made in the ATRP process, a choice that is not given in the FRP approaches used before. With controlled radical polymerization (CRP) methods, (co)polymer composition, topology, microstructure, and end-group functionalities can be tuned. This makes such polymers extremely suitable for further reaction which may include radical coupling26 and click-chemistry approaches27−29 whereby very well-defined highly complex polymer architectures can be obtained. In FRP, the molecular weight of the MMs depends on the monomer concentration and polydispersities are generally broad.16 Therefore, combining controlled radical polymerization techniques in general with the acrylate-typical synthesis of MMs could in principle lead to very well-defined and uniform MM species. While such an approach was largely successful in our previous study, some lack in end group fidelity was observed in the ATRP-based system and considerable side products were formed in the reaction. Interestingly, it was not the radical combination that caused the loss of end group functionality, but seemingly intermolecular transfer to polymer reactions, as was evidenced by a relatively high abundance of proton-terminated polymer chains, which can only stem from such a reaction. In agreement with that observation, only at very low polymer concentrations MMs with reasonable purity could be formed. Investigation of product samples with SEC and ESI-MS further revealed some remarkable observations. First, although backbiting via the six-membered ring transition structure was the favored process of transfer, yielding MMs with more or less unchanged average sizes compared to the starting ATRP polymer, a size-selective reaction pathwaymore precisely a MCR migration mechanismwas identified, which allowed MCRs to move along the backbone of the main chain. Migration of the MCR appeared to occur at any time via sixmembered ring structures, which is why certain chain lengths were favored over others as was evidenced by ESI-MS. Such a

migration reaction had been proposed before based on ESR studies30,31 but had not been directly experimentally observed. A recent quantum mechanical investigation on secondary reactions occurring in FRP of n-BuA also suggested that such an MCR migration mechanism could be feasible.32 In our current study, we focus now on the MM formation from different polyacrylates in order to investigate to what extent disparate reactivities might exist with respect to the migration reaction between various monomers. Also, we switched the precursors from ATRP polymers to polymers made from nitroxide mediated polymerization, NMP.33,34 Such a change in precursor allows the number of reactants in the system to be minimized, which eliminates potential side reactions. As we will show, indeed the random transfer events, which were dominating in the AGET-ATRP approach, could be diminished by this strategy, allowing reactions at higher polymer concentrations and more pure products. Additionally, also an end group selectivity could be observed, which was not present in the ATRP-based system. Three polyacrylates, namely poly(n-butyl acrylate), P(nBuA), poly(tert-butyl acrylate), P(tBuA), PEHA and poly(ethylhexyl acrylate) were investigated. Previous ESR studies suggested a slower radical migration for acrylates with spherically bulky side chains such as tBuA and faster migration rates for acrylates with longer alkyl side chains.31 We thus decided to test this hypothesis by using tBuA as a bulky side group, nBuA as a reference monomer, and EHA as a long-side-chain acrylate. In principle, a linear alkyl acrylate might be better suited for that purpose; however, monomers such as dodecyl acrylate (or longer analogues) are common mixtures of monomers with different alkyl chain lengths, thus rendering the mass spectrometric evaluation of the data impossible. In our study, reaction products from the hightemperature treatment of the materials are again followed by ESI-MS, giving interesting insights into the complex dynamic equilibria taking place in the addition−fragmentation equilibrium between MMs and secondary radicals.

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EXPERIMENTAL SECTION

Materials. The monomers n-butyl acrylate (n-BuA, Acros, 99%), tert-butyl acrylate (t-BuA, Acros, 99%), and 2-ethylhexyl acrylate (EHA, Acros, 99%) were deinhibited over a column of activated basic alumina, prior to use. N-(2-Methyl-2-propyl)-N-(1-diethylphosphono2,2-dimethylpropyl)-O-(2-carboxyprop-2-yl)hydroxylamide (MAMASG1, BlockBuilder MA, 96%, Arkema) and N-tert-butyl-N-[1diethylphosphono-(2,2-dimethylpropyl) nitroxide] (SG1, 86.5%, Arkema) were used as received. All solvents used are obtained from commercial sources (Acros and Sigma-Aldrich) and used without further purification. 3325

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reaction mixture in the Schlenk was heated up to 112 °C in an oil bath in order to start the polymerization. After a reaction time of 100 min, the polymerization was stopped by cooling in liquid nitrogen and an NMR sample was taken to determine a conversion of 14%. The polymer/monomer mixture was transferred into an aluminum pan which was placed in a vacuum oven at 40 °C overnight. With the oven the excess of the residual monomer was evaporated, yielding 0.454 g of P(EHA) (3) polymer with Mn = 2000 g mol−1 and PDI = 1.27 (by THF-SEC, K = 9.85 × 10−5 dL·g−1, α = 0.719).36 ESI-MS analysis reveals an SG1 end group functionality of 95%. Macromonomer Formation. In a typical experiment, NMP polyacrylate 1, 2, or 3 (53 μmol, 1 equiv) was dissolved in 2 mL of anisole in a Schlenk tube (5 wt % solution). The Schlenk tube was sealed and subjected to three freeze−pump−thaw cycles, after which it was filled with a nitrogen atmosphere. The reaction mixture in the Schlenk was heated up to 140 °C in an oil bath to start the reaction. After a reaction time of 7 h, the reaction was stopped by cooling in liquid nitrogen. The product mixture was transferred into an aluminum pan, and the excess of solvents was evaporated, yielding the MM product mixture (1MM, 2MM, 3MM).

Characterization. 1H NMR spectra were recorded in deuterated chloroform with a Varian Inova 300 spectrometer at 300 MHz applying a pulse delay of 12 s using a Varian probe (5 mm-4-nucleus AutoSWPFG). Analysis of the MWDs of the polymer samples were performed on a Tosoh EcoSEC operated by PSS WinGPC software, equipped with a PLgel 5.0 μm guard column (50 mm × 8 mm), followed by three PLgel 5 μm Mixed-C columns (300 mm × 8 mm) and a differential refractive index detector using THF as the eluent at 40 °C with a flow rate of 1 mL·min−1. The SEC system was calibrated using linear narrow polystyrene standards ranging from 474 to 7.5 × 106 g·mol−1 (PS (K = 14.1 × 10−5 dL·g−1 and α = 0.70) and toluene as a flow marker. ESI-MS of the NMP precursors was performed using an LTQ Orbitrap Velos Pro mass spectrometer (ThermoFischer Scientific) equipped with an atmospheric pressure ionization source operating in the nebulizer assisted electrospray mode. The instrument was calibrated in the m/z range 220−2000 using a standard solution containing caffeine, MRFA, and Ultramark 1621. A constant spray voltage of 5 kV was used, and nitrogen at a dimensionless auxiliary gas flowrate of 5 and a dimensionless sheath gas flowrate of 10 were applied. The S-lens RF level, the gate lens voltage, the front lens voltage, and the capillary temperature were set to 50%, −90 V, −8.5 V, and 275 °C respectively. ESI-MS of the MM samples was performed using an LCQ Fleet mass spectrometer (ThermoFischer Scientific) equipped with an atmospheric pressure ionization source operating in the nebulizer assisted electrospray mode. The instrument was calibrated in the m/z range 220−2000 using a standard solution containing caffeine, MRFA, and Ultramark 1621. A constant spray voltage of 5 kV was used, and nitrogen at a dimensionless auxiliary gas flowrate of 3 and a dimensionless sheath gas flowrate of 3 were applied. The capillary voltage, the tube lens offset voltage, and the capillary temperature were set to 25 V, 120 V, and 275 °C respectively. A 250 μL aliquot of a polymer solution with a concentration of 10 μg·mL−1 was injected. A mixture of THF and methanol (THF/MeOH = 3:2), both HPLC grade, was used as solvent. NMP Polymerization of nBuA. 0.390 mmol of MAMA-SG1 (149 mg, 1 equiv) was added together with 0.039 mol (5 g, 100 equiv) of the monomer nBuA and 0.059 mmol (0.017 g, 0.15 equiv) of SG-1 nitroxide under an inert atmosphere into a sealed Schlenk tube. The Schlenk tube was subjected to three freeze−pump−thaw cycles to remove residual oxygen, after which it was filled with nitrogen. The reaction mixture in the Schlenk was heated to 112 °C in an oil bath in order to start the polymerization. After a reaction time of 100 min, the polymerization was stopped by cooling in liquid nitrogen and an NMR sample was taken to determine a conversion of 16%. The polymer/ monomer mixture was transferred into an aluminum pan where the excess of the residual monomer was evaporated, yielding 0.750 g of P(nBuA) (1) polymer with Mn = 1800 g mol−1 and PDI = 1.30 (by THF-SEC, K = 12 × 10−5 dL·g−1, α = 0.7). ESI-MS analysis reveals an SG1 end group functionality of 95%. NMP Polymerization of tBuA. 0.390 mmol of MAMA-SG1 (149 mg, 1 equiv) was added together with 0.039 mol (5 g, 100 equiv) of the monomer tBuA and 0.059 mmol (0.017 g, 0.15 equiv) of SG-1 nitroxide under an inert atmosphere into a sealed Schlenk tube. The Schlenk tube was subjected to three freeze−pump−thaw cycles to remove residual oxygen, after which it was filled with nitrogen. The reaction mixture in the Schlenk was heated up to 112 °C in an oil bath in order to start the polymerization. After a reaction time of 100 min, the polymerization was stopped by cooling in liquid nitrogen and an NMR sample was taken to determine a conversion of 11%. The polymer/monomer mixture was transferred into an aluminum pan where the excess of the residual monomer was evaporated, yielding 0.700 g of P(tBuA) (2) polymer with Mn = 1500 g mol−1 and PDI = 1.36 (by THF-SEC, K = 19.7 × 10−5 dL·g−1, α = 0.66).35 ESI-MS analysis reveals an SG1 end group functionality of 95%. NMP Polymerization of EHA. 0.217 mmol of MAMA-SG1 (83 mg, 1 equiv) was added together with 0.022 mol (4 g, 100 equiv) of the monomer EHA and 0.033 mmol (0.010 g, 0.15 equiv) of SG-1 nitroxide under an inert atmosphere into a sealed Schlenk tube. The Schlenk tube was subjected to three freeze−pump−thaw cycles to remove residual oxygen, after which it was filled with nitrogen. The

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RESULTS AND DISCUSSION Via conventional NMP, three different polyacrylate polymers were obtained with high end group fidelities (≥95%), as was confirmed via ESI-MS (see figures). Subsequently, the polymers were dissolved in anisole (5 wt % polymer/solvent ratio) and reactivated at a reaction temperature of 140 °C for 7 h with the intention to produce uniform, low polydispersity MMs. The reaction temperature was chosen based on the optimization that had been carried out on the AGET-ATRP system in order to provide comparable data. It should, however, be noted that choosing somewhat lower or higher temperatures will only lead to small shifts in the product spectrum. The obtained molecular weights, polydispersities, and major species observed in ESI-MS for the different NMP precursors and MM mixtures are summarized in Table 1. Table 1. SEC and ESI-MS Results for Polyacrylate NMP Precursors and MM Synthesis (Reaction Temperature = 140 °C, Reaction Time = 7 h) species 1 1MM 2 2MM 3 3MM

Mn monomer (g·mol−1) nBuA nBuA tBuA tBuA EHA EHA

1800 1500 1500 1400 2000 1700

Mp (g·mol−1)

PDI

predominant species in ESI-MS

2400 2300 2000 2100 2700 2200

1.30 1.50 1.36 1.56 1.27 1.37

NMP P(nBuA) MMH NMP P(tBuA) MMH NMP P(EHA) MMH

Before the results from these attempts to form MMs are discussed, it is useful to compare the previously investigated AGET-ATRP system to extrapolate what results could be expected. With the AGET-ATRP reactivation system, we observed a reduction in overall molecular weight of the MMs compared to its precursors, which we could attribute to the MCR migration as well as to random transfer events taking place. Also, MMs with two different end groups were observed, one with a proton end group (showing the size-selection stemming from migration) and one with the original ATRP initiator groups still attached. Both species occurred roughly in similar abundances, indicating a more or less random MCR scission reaction that favored no particular side. Thus, for the NMP system, a similar outcome should be observed, that is, a 3326

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al.31 However, when compared to the ATRP system where a significant reduction in the average molecular weight was observed, this time the shift appears to be only minor25 and one must be careful not to overinterpret the shifts observed in Table 1. It should be noted that in the present case only small molecular weight polymers were investigated to allow for a comprehensive ESI-MS analysis. The reactions outlined below, however, will also be applicable to chains with a higher molecular weight, where then the overall shift of Mn of a few hundred daltons will not have a very significant effect and where also the PDI can be assumed to be less affected due to the overall higher Mn. More insights into the transfer process can be gained via softionization mass spectrometry of the samples (see Figures 3, 5,, and 6). In Figure 3, the product spectrum of the precursor material is shown (upper part) as well as the spectrum for the MM product that is formed after the reaction (lower part). In fact, the precursor P(nBuA) is of high quality and end group fidelity (>95% end group fidelity). Two distinct peak series are observed (in the form of either single charged, circles, or double charged species, triangles), which can be assigned to the structures as given in Figure 4. Both correspond to the polymer carrying the SG1-typical alkoxyamine on one side of the chain and the MAMA-SG1 typical end group on the other. The two peak series result from deprotonation of the carboxylic group, which results in a shift of roughly 22 m/z. After thermal treatment, only one main product is observed among some other less abundant species. The main product is assigned to MMH (see Figure 4 for structure). As is nicely apparent in the overview spectrum on the left side of the figure, again size selection is observed, as was the case for the MMH species in the AGET-ATRP system (for a discussion on why this size selection occurs, please see our previous study25). The random distribution of the precursor is converted into a distinct alternation of peaks. Remarkably in Figure 3 though is the practically complete absence of the MM that would still carry the initiator end group. In the ATRP system, more or less the same concentration was observed for that species, then referred to as MMX, compared to MMH. The lack of the presence of MMX is at first glance difficult to understand, but the complex addition−fragmentation equilibria might account for the disappearance of its distribution (see below for a detailed discussion). Another significant deviation from the previous study is the disappearance of the transfer-to-polymer peak, which was very abundant in the MM formation at the same overall polymer concentration in the AGET system, giving rise to a much cleaner product spectrum (however, if polymer concentrations are increased to 17 wt %, also in the NMP approach a transfer peak becomes detectable in significant amounts; spectra can be found in the Supporting Information). Further (minor) peaks that can be assigned in the spectrum belong to disproportionation products, then carrying either the initiator end group (black) or a proton (purple). Last, also one peak is observed in the spectrum (marked with green circle), which cannot be assigned to any species, neither to a combination of any of the confirmed radicals being present during the reaction nor to any product stemming from a transfer-to-polymer reaction to one of the observed products. The origin of this peak currently thus cannot be explained. Regardless, no starting polymer material carrying the SG1 group remains in the product spectrum, indicating the completion of the reaction (if no alkoxyamine species is present in the reaction mixture, then no new macroradicals can

mixture of MMs with two different end groups (initiator group or proton) as well as a reduction in molecular weight. Figure 1 depicts the NMR spectrum of the resulting material obtained after thermal treatment. The resonances at 6.2 and 5.5

Figure 1. 1H NMR of P(nBuA) polymer mixture 1MM after thermal reactivation. Resonances at 6.15 and 5.50 ppm indicate the presence of MM species. 1H NMR spectra of 2MM and 3MM can be found in the Supporting Information.

ppm (indicated by dashed box) correspond to the chemical shifts of the vinyl end groups demonstrating the presence of MM species. The molecular weight distributions of the P(nBuA) NMP precursor 1 and the corresponding MM product 1MM are depicted in Figure 2 (full molecular weight

Figure 2. SEC chromatograms of NMP precursor P(nBuA) 1 and of MM mixture 1MM synthesized at 140 °C via thermal reactivation.

distributions for 2, 2MM and 3, 3MM can be found in the Supporting Information). In general, somewhat higher PDI values are obtained after thermal reactivation into MMs. The PDI increased from about 1.3 to 1.5 after the reaction (see Table 1 for values), and a small shift of the chromatogram toward lower masses can be observed. For both P(nBuA) 1MM and P(EHA) 2MM, a decrease in Mn of around 300 Da was observed, while for P(tBuA) 3MM only a decrease of 100 Da was observed. These results indicate that radical migration is faster for P(nBuA) and P(EHA) and slower for P(tBuA), which is in agreement with the previous ESR observations from Kajiwara et 3327

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Figure 3. ESI-MS spectra of NMP precursor P(nBuA) 1 and of MM mixture 1MM synthesized via thermal reactivation at 140 °C for 7 h. Left: Full MS spectrum. Right: Spectrum of single monomer repeating unit (circles represent single charged species; triangles represent double charged species).

Due to this persistent radical effect, it is worthwhile to discuss the influence of the free nitroxide on the reaction itself. At 140 °C, secondary radicals and SG1 nitroxide moieties will be in equilibrium with dormant alkoxyamines. Tertiary MCR radicals will also undergo such an equilibrium, whereby, however, the equilibrium in this specific case will be much more on the side of the free radicals due to the higher stability of the MCRs and the significantly higher steric demand of the alkoxyamine adduct.13 Thus, the presence of larger concentrations of SG1 will lead to a reduction of the overall radical concentration and average time in which an individual radical is in the active state. The effect on secondary radical concentrations will thereby be larger than that on the MCRs, explaining why MCR migration can still take place. Why MCR formation and β-scission is seemingly unaffected by the persistent radical effect during transfer to the polymer remains to this point unclear and will be the subject of future modeling studies. It can only be speculated that the overall lifetime of the radical species is responsible for the practically quantitative reduction of random transfer-to-polymer products since this is the only distinct difference between the ATRP and NMP systems. On the other hand, when increasing the copper concentration in the AGET system, thus decreasing the radical lifetime, no significant change in the product spectrum could be observed. Nevertheless, since all other components in the AGET reaction exhibit no transfer activity and the same type of polymer is employed, the radical lifetime remains the only difference.

Figure 4. Structures of species observed in ESI-MS spectra of NMP precursor polyacrylates and of MM mixtures synthesized via thermal reactivation at 140 °C for 7 h (circles represent single charged species, triangles represent double charged species).

be formed at high temperatures and no further reaction may consequently occur). It must in this context, however, be noted that the fate of the free nitroxide is somewhat unclear. Portions of its population will be depleted in the reaction with small radical fragments (which would be largely invisible in ESI-MS) or will simply remain in the reaction mixture until precipitation of the product, causing a significant persistent radical effect. 3328

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Figure 5. ESI-MS spectra of NMP precursor P(tBuA) 2 and of MM mixture 2MM synthesized via thermal reactivation at 140 °C for 7 h. Left: Full MS spectrum. Right: Spectrum of single monomer repeating unit (circles represent single charged species; triangles represent double charged species).

in the mass spectrum.17 Moreover, MMX is via reverse scission in equilibrium with MMH, which reinforces the accumulation. This effect had also been confirmed in Predici simulations.21 In consequence, the same mechanism will be in place here. One should thereby, however, also note the interplay of MCR migration in this mechanism, since in the above-mentioned simulations this was not accounted for. Size selectivity favoring distinct chain lengths is still operational, if not even further enforced. Both MMH and the secondary radical with a proton end group as given on the left side of Scheme 2 will be of the favored chain lengths. The MMH species formed will always be of an odd number of chain segments, while the protonterminated secondary radical that is formed on the other side of the equilibrium will always be of an even number of segments. Reaction of the two with each will always result in an MCR where the radical sits on a position favoring the same chain segment selectivity as MCRs generated from primary backbiting/migration events. At 140 °C, SG1 will to some degree decay over the course of one experiment; half-life times measured at 120 °C by Fischer and co-workers (15 h) indicate,37 however, that the stable radical concentration will remain high at all times. Thus, it may be assumed that a persistent radical effect may dominate the process and radical concentrations might be lowered. At the same time the overall lifetime of radicals would increase (as a sum of lifetimes from all activation−deactivation cycles), giving the system more time to equilibrate compared to the AGETATRP activation system, where radical (and dormant) species deplete significantly faster. As a consequence of this

The same trend as is seen for n-butyl acrylate precursors in Figure 3 can also be found for the two other polymers under investigation, tBuA and EHA. Kajiwara had suggested that with P(tBuA) less MCR migration should be observable.30 As can be seen from Figures 5 and 6, however, no significant difference in the migration behavior is seen. The same typical pattern as in Figure 3 is observed, unambiguously proving for all cases that the migration step took place several times per chain. With P(EHA), a small variation in the spectrum is observed, which is the occurrence of several low-abundant side products. This indeed indicates a slight reactivity change when going from the two butyl acrylates to the more bulky EHA. Overall, however, also for P(EHA) the general trend is confirmed and at least no major difference in the reactivity must be taken into account. The disappearance of MM species containing the original initiator moiety at the chain end can be understood on the basis of the addition−fragmentation equilibria that are taking place throughout the reaction. Already for the MM formation from the high-temperature polymerization of low-concentrated acrylates, the same effect was observed in that only MMH was present in the product. This selectivity could be explained by an accumulation of MMH from initiator-containing secondary radicals. Each of these macroradicals can undergo several backbiting/scission cycles, each time generating an MMH species (see Scheme 2). Effectively, the system proceeds via a kind of pre-equilibrium and main equilibrium, not unlike the RAFT process, until the concentration of the protonterminated species outweighs the concentration of the initiatorcontaining species to the extent that these are no longer visible 3329

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Figure 6. ESI-MS spectra of NMP precursor P(EHA) 3 and of MM mixture 3MM synthesized via thermal reactivation at 140 °C for 7 h. Left: Full MS spectrum. Right: Spectrum of single monomer repeating unit (circles represent single charged species; triangles represent double charged species. The peak marked with an asterisk results from background impurity).

prolongation of lifetime, equilibration toward MMH can occur more efficiently for the NMP system and thus explain why the MMX species is still seen in the (faster) AGET-ATRP system. This is also reflected in the overall longer time that is required for the transformation. In the AGET system, pure product spectra were obtained after 3 h, while for the NMP system at least 7 h were required to convert all SG1-terminated species. It should be noted, however, that this hypothesis is only one possible explanation for the differences between the two activation procedures. More in-depth kinetic modeling exercises will be required to elucidate to what extent the persistent radical effect is the differentiating factor between both methods.

employment of significantly higher polymer concentrations than the ATRP system, where an extensive transfer to the polymer resulting in saturated product species was observed. Thus, with the NMP system, cleaner products are obtained while at the same time allowing for the synthesis of larger amounts of product within one reaction (5 wt % of polymer in solution for NMP precursors vs 0.1 wt % for the AGET ATRP precursor). The results from the mass spectrometric analysis of the MM product allows for significant information on radical (migration) pathways being operational during reaction, a fact which has previously not been acknowledged widely. At the same time, the process is a useful tool for synthetic purposes. Polyacrylate MMs have in this way become available with predefined molecular weight and comparatively low dispersity without requiring the use of specific initiators.

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CONCLUSIONS MMs have been synthesized from NMP precursor polymers made of nBuA, tBuA, and EHA. In all three cases, comparatively pure MMs have been obtained. Midchain radical migration can be confirmed in all three cases by the occurrence of a specific size selection in the product spectrum. Only MMs with uneven numbers of chain segments are formed, as must be expected when the radical pathway proceeds via several migration steps following upon each other. In contrast to our previously investigated system, where activation of macroradicals from the precursor was initiated via an AGET-ATRP process, only MMs carrying a proton end group on the chain terminus are observed, which can be explained via the complex addition−fragmentation chain equilibria being in place between MMs and secondary radicals. Also, the NMP system allows for

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information includes 1H NMR spectra of MM mixtures 2MM and 3MM, SEC overlay chromatograms of 2 and 2MM and of 3 and 3MM, a table of masses for species identified with ESI-MS and some additional ESI-MS spectra for samples obtained under different reaction concentrations. This material is available free of charge via the Internet at http://pubs.acs.org. 3330

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +32(11) 268318. Fax: +32(11)268399. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the award of the Odysseus project “Precise Polymer Design for the Development of New Materials” by the Fund for Scientific Research-Flanders (FWO).

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dx.doi.org/10.1021/ma400477t | Macromolecules 2013, 46, 3324−3331