Free Radical Propagation Rate Coefficients of N-Containing

Article pubs.acs.org/Macromolecules

Free Radical Propagation Rate Coefficients of N‑Containing Methacrylates: Are We Family? Katrin B. Kockler,†,‡ Friederike Fleischhaker,§ and Christopher Barner-Kowollik*,†,‡,∥ †

Preparative Macromolecular Chemistry, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstr. 18, 76128 Karlsruhe, Germany ‡ Institut für Biologische Grenzflächen, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Karlsruhe, Germany § BASF SE, Registered Office, 67056 Ludwigshafen, Germany ∥ School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George Street, Brisbane, QLD 4000, Australia S Supporting Information *

ABSTRACT: Nitrogen-containing methacrylates are a highly interesting class of monomers, yet only very limited data exist describing their propagation rate coefficients, kp. Herein, we investigate the propagation behavior of three N-containing monomers, namely 2-(N,N-diethylamino)ethyl methacrylate (DEAEMA), 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA), and 3-(N,N-dimethylamino)propyl methacrylate (DMAPMAE), where we systematically vary the ester side chain with respect to spacer and branching length with the aim of establishing if all so far investigated N-containing methacrylates display family type behavior with regard to kp. Thus, the Mark−Houwink−Kuhn−Sakurada parameters alongside the Arrhenius parameters of kp were determined for these monomers via triple detection SEC and pulsed laser polymerization−size exclusion chromatography (PLPSEC). The obtained data result in Arrhenius parameters for DEAEMA of A = 2.07 (−0.79 to +3.98) × 106 L mol−1 s−1 and EA = 20.45 (−2.02 to +2.28) kJ mol−1, for DMAEMA of A = 2.64 (−0.79 to +1.98) × 106 L mol−1 s−1 and EA = 20.71 (−1.31 to +1.32) kJ mol−1, and for DMAPMAE of A = 1.22 (−0.54 to +8.02) × 106 L mol−1 s−1 and EA = 19.59 (−2.74 to +3.83) kJ mol−1. The data of the herein investigated monomers are critically compared to the previously published data of 2-(N-ethylanilino)ethyl methacrylate (NEAEMA), 2-(1-piperidyl)ethyl methacrylate (PipEMA), and 2-morpholinoethyl methacrylate (MOMA). It is found that DEAEMA and the previously investigated monomers can be described by one family, leading to a joint Arrhenius description for the four monomers NEAEMA, MOMA, PipEMA, and DEAEMA, best described by A = 1.55 (−0.57 to +3.88) × 106 L mol−1 s−1 and EA = 19.68 (−1.76 to +2.60) kJ mol−1.

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INTRODUCTION

reaction kinetics. Extensive investigations in bulk as well as organic solution have been carried out on acrylates and methacrylates with linear alkyl ester side chains,5,8,10,13−17 acrylates and methacrylates with branched alkyl ester side chains,14,16−21 and methacrylates with cyclic ester side chains.7,22 In many instances, overarching trends and family type behaviors could be detected among the different monomer groups.7,16,17 In addition to the noted alkyl monomers, monomers with more complex ester side chains became attractive for investigation via PLP-SEC. The existing data array of propagation rate coefficients and Arrhenius parameters has been augmented by several monomers with urethane or hydroxyl functionalized ester side chains, respectively (2(phenylcarbamoyloxy)ethyl acrylate (PhCEA), 2-(phenylcarbamoyloxy)isopropyl acrylate (PhCPA), 2-(hexylcarbamoyl-

Almost 30 years after the introduction of the pulsed laser polymerization−size exclusion chromatography (PLP-SEC) method by Olaj and co-workers,1,2 the IUPAC-recommended method for the determination of the Arrhenius parameters of the propagation rate coefficients has been employed to expand the understanding of the fundamental kinetics of a broad variety of acrylate and methacrylate type as well as other monomers in free radical polymerization. In the past two decades, the number of available temperature-dependent propagation rate coefficients, kp, and Arrhenius parameters has increased considerably with various important monomer systems being benchmarked by the IUPAC subcommittee of Modeling of Polymerization Kinetics and Processes.3−10 An important group of monomers for industrial applications are e.g. nitrogen-containing methacrylates that are widely used in home- and personal-care products, in lubricants, as paper chemicals, or as flocculation agents,11,12 yet aresurprisingly only very little investigated with respect to their underlying © XXXX American Chemical Society

Received: September 6, 2016 Revised: October 27, 2016

A

DOI: 10.1021/acs.macromol.6b01957 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Monomer Landscapea

a

Structures of the herein investigated monomers 2-(N,N-diethylamino)ethyl methacrylate (DEAEMA), 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA), and 3-(N,N-dimethylamino)propyl methacrylate (DMAPMAE) are depicted in the round edged box on the left side. On the right, structures of previously studied nitrogen-containing methacrylates 2-(1-piperidyl)ethyl methacrylate (PipEMA), 2-morpholinoethyl methacrylate (MOMA), and 2-(N-ethylanilino)ethyl methacrylate (NEAEMA) are shown.

Table 1. Collation of Monomer and Polymer Specific Physical Data of the Monomers Investigated in the Present Studya monomer DEAEMA DMAEMA DMAPMAE a

Mw [g mol−1] 185.3 157.2 171.2

ρ0 [g mL−1] 0.942 90 0.954 53 0.942 94

b [g m−1 °C] −4

8.8723 × 10 9.5491 × 10−4 9.0342 × 10−4

dn/dc [mL g−1]

Tg [°C]

K [cm3 g−1]

α

0.089 0.086 0.079

−15 0 −6

21.35 × 10−3 4.98 × 10−3 4.09 × 10−3

0.597 0.729 0.731

K and α as well as dn/dc are reported for 35 °C.

nitrogen and the ester moiety. Thus, carefully selected monomers, namely 2-(N,N-diethylamino)ethyl methacrylate (DEAEMA), 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA), and 3-(N,N-dimethylamino)propyl methacrylate (DMAPMAE)depicted in Scheme 1 alongside the previously studied nitrogen-containing methacrylates NEAEMA, MOMA, and PipEMAare prepared and investigated with focus on their temperature-dependent propagation rate coefficients and Arrhenius parameters as well as the Mark−Houwink−Kuhn− Sakurada (MHKS) parameters, K and α, of the respective polymers. For the determination of the MHKS parameters, multiple broadly distributed polymer samples had to be generated via polymerization with a chain transfer agent and analyzed with a triple detection size exclusion chromatography setup with multiangle laser light scattering (MALLS) combined with refrective index (RI) and viscosimetry (visco) detectors. For the determination of absolute weight-averaged molecular weights, Mw, and intrinsic viscosities [η], exact sample concentrations and refractive index increments, dn/dc collated in Table 1have been employed. Applying the polymer specific MHKS parameters, the molecular weights of the PLP samples measured via the universal calibration method employing narrowly distributed poly(methyl methacrylate) standards28 were recalculated according to eq 1:

oxy)ethyl acrylate (HCEA), 2-(hexylcarbamoyloxy)isopropyl acrylate (HCPA),23 and hydroxypropylcarbamate acrylate (HPCA)24 as well as 2-hydroxyethyl acrylate (HEA),25 2hydroxyethyl methacrylate (HEMA),26 and 2-hydroxypropyl methacrylate (HPMA)22). In 2014, our team24 determined the propagation rate coefficients and Arrhenius parameters for ureidoethyl methacrylate (UMA), the to that date first nitrogen-containing methacrylate investigated via PLP-SEC. Because of its high melting point (>40 °C), UMA was solely investigated in solution in N,N-dimethylacetamide (DMAc) and no bulk kp data are available, leaving the possible solvent effects on the propagation rate coefficient within this system (possibly operational based on its ability to break hydrogen bonds) unexplored. However, a recent study by our group addressed the examination of nitrogen-containing methacrylates in bulk, namely 2-(N-ethylanilino)ethyl methacrylate (NEAEMA), 2-(1-piperidyl)ethyl methacrylate (PipEMA), and 2-morpholinoethyl methacrylate (MOMA) adding a new group of monomers to the pool of existing Arrhenius parameters and propagation rate coefficients.27 Investigation of the above monomers led to the detection of a family type behavior and the data could be described with a joint Arrhenius fit with joint Arrhenius parameters of A = 1.83 × 106 L mol−1 s−1 and EA = 20.14 kJ mol−1. UMA, however, did not fit into this family of nitrogen-containing methacrylates, possibly due to the fact that the UMA data have been determined in DMAc solution and that the influence of the ability of UMA to form intramolecular hydrogen bonding is not yet fully investigated. It appears mandatory to investigate how the structure of the nitrogen-containing ester side chain (for example, variation of branching length or chain length) influences the propagation rate coefficient of the monomer and to identify where the boundaries of the proposed family lie. Therefore, two strategies are implemented in the current study, i.e., the (i) stepwise reduction of the number of carbon atoms situated at the nitrogen and the (ii) extension of the alkyl linker between the

[η] = KM α

(1)

In summary, within the current study, we explore the limits of the recently proposed nitrogen-containing methacrylate family via three new monomers and critically evaluate their possible inclusion into the family on well-reasoned physicochemical grounds.

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EXPERIMENTAL SECTION

Pulsed Laser Polymerization Experiments. The experimental PLP setupas well as the molecular weight determination methods B


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Macromolecules

system is conducted using poly(styrene) standards (PSS Mainz). Determination of absolute molecular weights has been carried out using exact sample concentrations as well as the polymer specific refractive index increments (dn/dc). The weight-averaged molecular weight Mw and the intrinsic viscosity [η] were determined using the MALLS detector response and the viscosimeter signal, respectively. Typical triple detection SEC traces of the macromolecular samples incorporated in the determination of MHKS parameters are displayed in Figures S5−S7. Polymerizations with a Thiol as Chain Transfer Agent.27 For the determination of MHKS parameters, a set of polymer samples was prepared via radical polymerization in the presence of a thiol for each monomer. All polymerization reactions were conducted in bulk with azobis(isobutyronitrile) (AIBN) as initiator and with varying amounts of the controlling transfer agent 1-dodecanthiol. Initiator concentrations varied from 0.004 to 0.007 mol L−1 for DMAEMA, 0.006 to 0.015 mol L−1 for DEAEMA, and 0.007 to 0.009 mol L−1 for DMAPMAE polymerizations. Thiol concentrations varied from 0.006 to 1.00 mol % for DMAEMA, 0.002 to 0.6 mol % for DEAEMA, and 0.03 to 0.14 mol % for DMAPMAE polymerizations. The detailed reaction conditions of all samples incorporated into the MHKS plots are collated in Tables S4−S6. Polymerizations of all monomers were conducted in individually sealed glass vials with rubber septa as well as parafilm and purged with nitrogen for approximately 5 min to remove oxygen from the solution. The reaction vials were heated in a shaker at close to 66 °C and removed after 45−120 min. The polymerization process was ended by adding a solution of hydroquinone in THF. Subsequently, all polymers were isolated from the monomer via dialysis against pure THF with Spectra/Por 6 dialysis membranes, prewetted RCtubing, MWCO 1 kDa. Gravimetry was used to determine the final conversions. The previously described triple detection SEC setup was employed for the analysis of the resulting polymer samples. The obtained weight-averaged molecular weights were in the range of Mw = 23 000−484 500 g mol−1 with dispersities between Đ = 1.1 and 2.0. Materials. Azobis(isobutyronitrile) (AIBN, Aldrich, 98%), 2,2dimethoxy-2-phenylacetophenone (DMPA, Aldrich, 99%), 4-methylhydroquinone (MeHQ, Aldrich, 99%), 1-dodecanthiol (Sigma-Aldrich, ≥98%), and tetrahydrofuran (THF, VWR, HPLC grade) were used as received. 2-(N,N-Diethylamino)ethyl methacrylate (DEAEMA 1500 ppm MeHQ, BASF) and 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA 1000 ppm MeHQ, BASF) were freed from inhibitor by passing through a column of basic aluminum oxide. 3-(N,NDimethylamino)propyl methacrylate (DMAPMAE < 30 ppm MeHQ) was used as received from BASF.

have been previously described and are reiterated here to allow for ready reproducibility.14,15,17 The experimental temperatures ranged from 0 to 90 °C and no temperature change exceeding 0.2 °C was observed during the laser-induced polymerization reactions. During laser irradiation, the temperature was monitored directly at the sample. The pathway of the laser beam was directed to hit the sample from below with a laser energy of 2.0 mJ/pulse. To terminate the polymerization process, hydroquinone (HQ) dissolved in tetrahydrofuran was added directly after the reaction and the samples were filtered and analyzed via SEC measurements. To ensure the consistency of the PLP samples, for all samples incorporated in the determination of propagation rate coefficients and Arrhenius parameter sets, at least two inflection points (Li)in some cases up to 5were observed. Further consistency criteria have been assessed by varying the initiator concentration from 10 to 20 mmol L−1. In addition, only samples with kp,1/kp,2 ratios near unity (between 0.95 and 1.05 if not stated otherwise) were employed to determine the final Arrhenius data sets. kp,1/kp,2 ratios as well as detailed sample conditions of all samples incorporated in the Arrhenius data sets are collated in Tables S1−S3 of the Supporting Information. Size Exclusion Chromatography.27 SEC measurements were carried out on a PL-SEC 50 Plus Integrated System, consisting of an autosampler and a PLgel 5 μm bead-size guard column (50 × 7.5 mm) followed by three PLgel 5 μm Mixed C columns (300 × 7.5 mm). For molecular weight detection, a differential refractive index (RI) detector is employed, using THF as an eluent at 35 °C at a flow rate of 1 mL min−1. For the calibration of the SEC system, linear poly(styrene) standards ranging from 476 to 2.5 × 106 g mol−1 and linear poly(methyl methacrylate) standards ranging from 800 to 1.6 × 106 g mol−1 are utilized. To remove the noise from the RI signal, the obtained molecular weight distributions were smoothed, and the molecular weights of the inflection points were determined employing the first derivatives. SEC calculations were conducted using a universal calibration with poly(methyl methacrylate) Mark−Houwink−Kuhn− Sakurada (MHKS) parameters. The polymer specific MHKS parameters of each investigated monomer have subsequently been employed to recalculate the values determined with PMMA MHKS parameters. Exemplary PLP-SEC traces of each polymer can be found in Figures S2−S4 of the Supporting Information. Density Measurements.27 For the determination of the temperature-dependent densities of the monomers, an Anton Paar DMA 5000 M density meter with a precision of 1 × 10−2 °C and 5 × 10−6 g mL−1 was employed, and the monomer specific data are collated in Table 1. Hydroquinone was added to the monomer solutions to prevent a polymerization process during density measurement. The temperature-dependent density curves of all herein investigated monomers are depicted in Figure S1. Mark−Houwink−Kuhn−Sakurada Parameters.27 Polymerspecific Mark−Houwink−Kuhn−Sakurada parameters are critical for the exact determination of propagation rate coefficients for each monomer. To determine the polymer specific values, broadly distributed polymer samples generated via polymerization with a chain transfer agent have been analyzed via triple detection SEC to determine absolute weight-averaged molecular weights and the corresponding intrinsic viscosities. MHKS parameters determined in the current study are collated in Table 1 alongside other polymer and monomer specific data. Molecular weights and intrinsic viscosities employed for the determination of MHKS parameters are collated in Table S7. Triple Detection Size Exclusion Chromatography.27 For the determination of MHKS parameters, polymer samples are specifically prepared via polymerization with a thiol as chain transfer agent and have been analyzed via a triple detection chromatography setup composed of a modular system (Polymer Standard, PSS Mainz/ Agilent 1200 series) utilizing a multiangle laser light scattering (MALLS) unit (PSS Mainz, SLD7000/BI-M w A, Brookhaven Instruments) combined with an ETA2010 viscosimeter (WGE Dr Bures). For macromolecular analysis, two linear columns (PSS SDVLux-1000 and 105 Å, 5 μm) are incorporated in the system using THF as eluent at 35 °C at a flow rate of 1 mL min−1. Calibration of the

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RESULTS AND DISCUSSION The discussion of the herein presented data commences with the introduction of the MHKS parameters determined for the three investigated monomer systems. Subsequently, the Arrhenius parameters of the propagation rate coefficients in bulk are discussed and critically compared to the family of existing nitrogen-containing methacrylates, i.e., 2-(Nethylanilino)ethyl methacrylate (NEAEMA), 2-(1-piperidyl)ethyl methacrylate (PipEMA), and 2-morpholinoethyl methacrylate (MOMA). Mark−Houwink−Kuhn−Sakurada (MHKS) Parameters. Since the application of different MHKS parameters strongly influences the molecular weight of the analyzed polymer samples, it is crucial to evaluate PLP generated SEC elugrams with the correct polymer specific parameters in order to obtain reliable molecular weights. To date, a broad database of MHKS parameters covering different solvents and temperatures is available.29 For DEAEMA, DMAEMA, and DMAPMAE, however, no MHKS parameters were previously published in the literature; therefore, their careful determination was inevitable. The MHKS parameters determined in C


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Macromolecules

Figure 1. [η] vs Mw plots in THF at 35 °C with linear fits for the determination of MHKS parameters for poly(2-(N,N-diethylamino)ethyl methacrylate) p(DEAEMA), poly(2-(N,N-dimethylamino)ethyl methacrylate) p(DMAEMA), and poly(3-(N,N-dimethylamino)propyl methacrylate) p(DMAPMAE).

Figure 2. Plots of ln(kp/[L mol−1 s−1]) vs T−1 [K−1] with linear fits for the determination of the Arrhenius parameters for 2-(N,Ndiethylamino)ethyl methacrylate (DEAEMA), 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA), and 3-(N,N-dimethylamino)propyl methacrylate (DMAPMAE).

Table 2. Collation of Arrhenius Parameters and Their Associated Error Margins of the 95% Joint Confidential Intervals, Propagation Rate Coefficients at 50 °C, and Temperature Intervals Investigated in the Current Study A [L mol−1 s−1]

±

DEAEMA

2.07 × 10

DMAEMA

2.64 × 106

DMAPMAE

1.22 × 106

−7.89 × 10 3.89 × 106 −7.90 × 105 1.98 × 106 −5.35 × 105 8.02 × 106

monomer

6

EA [kJ mol−1]

±

kp50 °C [L mol−1 s−1]

θ interval [°C]

20.45

−2.02 2.28 −1.31 1.32 −2.74 3.83

1024

0−90

1185

0−90

831

0−90

5

20.71 19.59

the current study are collated in Table 1, and all Mw vs [η] plots show a linear behavior over the investigated molecular weight range as can be seen in Figure 1. For DEAEMA, a higher scattering of the data points is observed; however, multiple remeasurements of the polymer samples confirmed the herein presented MHKS parameters. For DMAPMAE, it was not possible to obtain acceptable SEC traces below a molecular weight of 150 000 g mol−1, yet a strictly linear behavior is observed over the entire investigated molecular weight range. For all three monomers, no polymers with molecular weights exceeding 600 000 g mol−1 were obtained, even when reducing the concentration of the controlling agent to 0.002 mol %. Since the MHKS plots show a linear behavior over an extended molecular weight range, it is appropriate to assume that they can reliably be extrapolated into lower molecular weight regions. According to eq 1, the slope of the MHKS plots corresponds to the α exponent, while the y-intercept correspond to the prefactor K. All Mw and [η] data incorporated in the MHKS plots are collated in Table S7. Arrhenius Parameters. With the correct polymer specific MHKS parameters available, the determination of the absolute

molecular weights of the samples derived via PLP and the calculation of the temperature-dependent propagation rate coefficients can proceed. Pulsed laser polymerizations of all monomers were carried out in bulk within a temperature range of 90 °C to reflect the temperature dependence with a reliable set of data points with a clearly linear fit over an extended temperature range. For all monomers, the observation of at least two inflection points was possible within the SEC traces for each sample included in the final Arrhenius data sets. Exemplary PLP-SEC distributions with their first derivatives are depicted in Figures S2−S4 alongside the detailed sample conditions of all polymer samples incorporated in the Arrhenius plots in Tables S1−S3. As depicted in Figure 2, the propagation rate coefficients, kp, deduced via PLP-SEC can be presented in the form of Arrhenius plots (ln(kp) vs T−1). The determined propagation rate coefficients for each monomer over an extended temperature range lead to Arrhenius parameters of A = 2.07 × 106 L mol−1 s−1, EA = 20.45 kJ mol−1 for DEAEMA, A = 2.64 × 106 L mol−1 s−1, EA = 20.71 kJ mol−1 for DMAEMA, and A = 1.22 × 106 L mol−1 s−1, EA = 19.59 kJ mol−1 for DMAPMAE. The resulting Arrhenius parameters jointly with D


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Macromolecules

Figure 3. Left: combined Arrhenius plots for the herein investigated monomers 2-(N,N-diethylamino)ethyl methacrylate (DEAEMA), 2-(N,Ndimethylamino)ethyl methacrylate (DMAEMA), and 3-(N,N-dimethylamino)propyl methacrylate (DMAPMAE). An overlap of the data is clearly not observed. Right: propagation rate coefficients for the herein investigated monomers at different temperatures: blue dashed line = 100 °C, black solid line = 50 °C, green dot-dashed line = 0 °C, and red dotted line = −50 °C. Monomers are displayed in order as investigated and discussed in the text. The lines are no fits and solely for guiding the eye.

Figure 4. Left: combined Arrhenius plots for 2-(N,N-diethylamino)ethyl methacrylate (DEAEMA), 2-(N-ethylanilino)ethyl methacrylate (NEAEMA), 2-morpholinoethyl methacrylate (MOMA), and 2-(1-piperidyl)ethyl methacrylate (PipEMA). Right: propagation rate coefficients for DEAEMA, NEAEMA, MOMA, and PipEMA at different temperatures: blue dashed line = 100 °C, black solid line = 50 °C, green dot-dashed line = 0 °C, and red dotted line = −50 °C. Monomers are displayed in order as investigated and discussed in the text. The lines are no fits and solely for guiding the eye.

Inspection of the combined Arrhenius plots on the left-hand side of Figure 3 clearly indicates that no overlap of the data of the three herein investigated monomers can be observed. Although several data points of DEAEMA and DMAEMA are close to each other, the data scattering within a single monomer is much lower than the scattering between the different monomers, and thus, Figure 3 suggests three individual Arrhenius plots that do not qualify for a joint fit. Over the entire temperature range, DMAEMA exhibits higher kp values, while DMAPMAE exhibits clearly lower kp values than DEAEMA, lying in between the other two monomers. This notion is clearly underpinned by the right-hand side of Figure 3, depicting the propagation rate coefficients at 100, 50, 0, and −50 °C calculated via the determined Arrhenius parameters for each monomer. Although the data seem to be in proximity for the lowest temperature region, the deviation between the highest and lowest value already exceed 30% at 0 °C, reaching

the error margins of the 95% confidential intervals are collated in Table 2. The Arrhenius parameters were determined using the program CONTOUR V2.0.2 by van Herk30 with an assumed error range of 10% set as standard deviations. The average errors per data point range from 2.9% for DMAEMA to 8.0% for DMAPMAE. The comparably high error ranges for the Arrhenius parameters of DMAPMAE are caused by the higher data scatter in the Arrhenius plot, leading to larger deviations for the slope and y-intercept of the linear fit. Although DMAPMAE exhibits a higher error percentage than DEAEMA and DMAEMA, none of the data exceed the initially assumed error range. To ensure the reliability of the obtained data, several consistency criteria were assessed, and thus the propagation rate coefficients have been tested for independence of frequency, initiator concentration, and laser pulse energy, and solely samples with a kp,1/kp,2 ratio between 0.95 and 1.05 have been incorporated in the final Arrhenius plots if not stated otherwise. E


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Macromolecules

Figure 5. Left: combined Arrhenius plots for 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA), 2-(N-ethylanilino)ethyl methacrylate (NEAEMA), 2-morpholinoethyl methacrylate (MOMA), and 2-(1-piperidyl)ethyl methacrylate (PipEMA). Right: propagation rate coefficients for DMAEMA, NEAEMA, MOMA, and PipEMA at different temperatures: blue dashed line = 100 °C, black solid line = 50 °C, green dot-dashed line = 0 °C, and red dotted line = −50 °C. Monomers are displayed in order as investigated and discussed in the text. The lines are no fits and solely for guiding the eye.

Figure 6. Left: combined Arrhenius plots for 3-(N,N-dimethylamino)propyl methacrylate (DMAEMA), 2-(N-ethylanilino)ethyl methacrylate (NEAEMA), 2-morpholinoethyl methacrylate (MOMA), and 2-(1-piperidyl)ethyl methacrylate (PipEMA). Right: propagation rate coefficients for DMAPMAE, NEAEMA, MOMA, and PipEMA at different temperatures: blue dashed line = 100 °C, black solid line = 50 °C, green dot-dashed line = 0 °C, and red dotted line = −50 °C. Monomers are displayed in order as investigated and discussed in the text. The lines are no fits and solely for guiding the eye.

kJ mol−1 for DEAEMA and A = 1.83 × 106 L mol−1 s−1, EA = 20.14 kJ mol−1 for the joint fit); however, both parameters are just slightly higher than A and EA determined for PipEMA (A = 1.96 × 106 L mol−1 s−1, EA = 20.27 kJ mol−1). Inspection of Figure 4 clearly shows an overlay of the data for DEAEMA with the previously investigated monomers over the entire temperature range, and the propagation rate coefficients for all four monomers are in close agreement for all depicted temperatures. The differences between DEAEMA and PipEMA range from 0.3% for 100 °C to 4.4% for −50 °C, while the differences between DEAEMA and the joint Arrhenius fit proposed in the previous publication27 range from 0.8% for 100 °C to 4.4% for −50 °C, allowing for a possible inclusion of DEAEMA into the family of nitrogen-containing methacrylates. DMAEMA. In the subsequent step, 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) has been investigated, eliminating two additional CH2-groups from the end of the ester side chain. Again, an increase of both Arrhenius

over 50% deviation for kp at 100 °C between DMAEMA and DMAPMAE. Detailed Comparison to the Family of NitrogenContaining Methacrylates. To embed the kinetic data determined in the current study into the context of the family of nitrogen-containing methacrylates, the herein investigated monomers will be successively compared to the previously determined Arrhenius plots and propagation rate coefficients of NEAEMA, PipEMA, and MOMA. Subsequently, all findings will be discussed in view of potential underlying trends and principles. DEAEMA. We commence the detailed examination of the herein investigated kinetic data with the discussion of 2-(N,Ndiethylamino)ethyl methacrylate (DEAEMA), reducing the piperidyl ring at the ester side chain of PipEMA by one CH2group. DEAEMA exhibits a higher pre-exponential factor as well as a higher activation energy as the joint fit for NEAEMA, PipEMA, and MOMA (A = 2.07 × 106 L mol−1 s−1, EA = 20.45 F


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Macromolecules parameters A and EA is detected, leading to final values of A = 2.64 × 106 L mol−1 s−1 and EA = 20.71 kJ mol−1. These elevated Arrhenius parameters are also reflected in the consistently higher propagation rate coefficients. The left-hand side of Figure 5 indicates that several data points of the Arrhenius plot of DMAEMA overlap with the plot for PipEMA and for the lower temperature regions with the plot for MOMA as well. However, the DMAEMA data clearly lie at the top end of the plot, and the right-hand side of Figure 5 shows the elevated propagation rate coefficients for DMAEMA for all depicted temperatures. The differences between DMAEMA and PipEMA range from 6.2% at −50 °C up to 17% at 100 °C, and between DMAEMA and the joint fit for NEAEMA, PipEMA, and MOMA the differences range from 6.2% at −50 °C to 20% at 100 °C. DMAEMA thus appears to be just at the edge of the family type behavior, yet due to the differences in the propagation rate coefficientsespecially at higher temperaturesit seems not to be advisible to implement DMAEMA into the family of nitrogen-containing methacrylates. D M A P M A E . F i n a l l y , w e i n v es t i g a t e d 3 - ( N , N dimethylamino)propyl methacrylate (DMAPMAE), featuring a side chain with an additional CH2-group between the nitrogen atom and the ester moiety. In contrast to DEAEMA and DMAEMA, DMAPMAE exhibits significantly lower Arrhenius parameters, leading to final values of A = 1.22 × 106 L mol−1 s−1 and EA = 19.59 kJ mol−1. Inspection of the lefthand part of Figure 6 shows the Arrhenius plot for DMAPMAE at the lower part of the graph and although several data points scatter into the regime of the three previously investigated monomers, the overall plot of DMAPMAE lies clearly below the family of nitrogen-containing methacrylates. The propagation rate coefficients depicted on the right-hand side of Figure 6 reflects this behavior over the extended temperature range of −50 to 100 °C. Differences between the propagation rate coefficients for DMAPMAE and PipEMA lie between 11.3% at −50 °C and 29% at 100 °C. Compared to the joint fit of NEAEMA, MOMA, and PipEMA, the differences are somewhat lower and range from 11.3% at −50 °C to 25.6% at 100 °C. With a deviation of the propagation rate coefficients for the compared monomers of at least 11%, it is evident that DMAPMAE does clearly not fit into the family of nitrogencontaining methacrylates. For the series with decreasing number of CH2-groups at the nitrogen atom (PipEMA, DEAEMA, DMAEMA) an increase of the frequency factor A and also a slight increase of EA, are observed as can be seen in Figure 7 with A [106 L mol−1 s−1] = 1.96 (PipEMA), 2.07 (DEAEMA), and 2.64 (DMAEMA) and EA [kJ mol−1] = 20.27 (PipEMA), 20.45 (DEAEMA), and 20.71 (DMAEMA). Heuts et al. reported in their study of terminal and penultimate unit effects on the polymerization transition state via ab initio calculations a change to bulkier units leads to an increase in the hindrance of the torsion, leading to a change in the frequency factor.31,32 Going from PipEMA to DEAEMA to DMAEMA, the bulkiness of the ester chain decreases with a decreased number of CH2-groups at the amino group and the frequency factor consequently increases. A further interesting point is the significant difference between propagation rate coefficients and Arrhenius parameters of DMAPMAE and DMAEMA (A [106 L mol−1 s−1] = 2.64 (DMAEMA) to 1.22 (DMAPMAE) and EA [kJ mol−1] = 20.71 (DMAEMA) to 19.59 (DMAPMAE)). Although the monomers merely differ by one CH2-group in the linker between the nitrogen atom and the ester moiety, the Arrhenius plot of

Figure 7. Arrhenius parameters of 1-(2-piperidyl)ethyl methacrylate (PipEMA), 2-(N,N-diethylamino)ethyl methacrylate (DEAEMA), and 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA). Data for PipEMA are taken from ref 27. An increase in both Arrhenius parameters, A and EA, can be observed when going from PipEMA to DMAEMA.

DMAEMA is situated just above the top end of the family, while the Arrhenius plot of DMAPMAE clearly lies below the boundaries that would justify inclusion in the family. For the branched alkyl methacrylates, the exact shape, structure, and position of the branching point of the ester side chain was found to not be decisive for the propagation rate coefficient for monomers with similar steric demands.17 In the case of the nitrogen-containing methacrylates, these findings might not be entirely transferrable. In the previous study of the nitrogencontaining methacrylates, we tendered the hypothesis that a more polar ester side chain interacts stronger with the transition state.27 A stronger interaction can lower the activation energy while at the same time leading to an increased hindrance of the internal rotations that reduces the frequency factor. The nitrogen atom allows for potential intramolecular interactions of the side chain with the ester moiety that could possibly lead to induced polarities at the nitrogen atom and the carbonyl carbon. For DMAEMA (as well as the other N-ethyl methacrylates), the distance between the nitrogen atom and the ester moiety is too short to favor an interaction via ring formation, while for DMAPMAE the propyl linker between the nitrogen atom and the ester moiety would lead to a six-membered ring if the interaction between the nitrogen and carbonyl carbon occurs. This possible interaction would significantly increase the (induced) polarity of the monomer while still allowing for the radical polymerization to take place leading to the observed decrease in activation energy and frequency factor. The differences in the steric demands between DEAEMA and PipEMA are sufficiently distinct to lead to a slight yet significant change in the Arrhenius parameters and propagation rate coefficients, yet not exceeding the domain where the family type behavior is still observable. Altering the steric demands even further when going to DMAEMA, the more pronounced increase in A, EA, and kp leads just to a loss of the family type behavior. Extending the linker between the ester moiety and the nitrogen atom by an additional CH2-group increases the bulkiness and polarity and significantly lowers the Arrhenius parameters as well as the propagation rate coefficient. Figure 8 finally depicts the Arrhenius fits of 2-(N,Ndiethylamino)ethyl methacrylate, 2-(N,N-dimethylamino)ethyl methacrylate, and 3-(N,N-dimethylamino)propyl methacrylate G


Article

Macromolecules

(NEAEMA, MOMA, PipEMA, and DEAEMA) do not just display similar steric demands yet are also in regions of comparable polarities. DMAPMAE does not meet the criteria to be incorporated into the family, since the increased polarity of the monomer due to a possible ring formation followed by induced polarities at the nitrogen atom as well as carbonyl carbon leads to a significant decrease in the activation energy and frequency factor and a decrease in the propagation rate coefficient.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01957. Detailed sample conditions of samples obtained via PLP as well as polymerization with a chain transfer agent are provided for each monomer; exemplary molecular weight distributions recorded via SEC and triple detection SEC alongside Mw and [η] values for the determination of MHKS parameters are collated for each monomer alongside their temperature-dependent density curves (PDF)

Figure 8. Arrhenius fits for the herein investigated monomers 2-(N,Ndiethylamino)ethyl methacrylate (DEAMEA), 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA), and 3-(N,N-dimethylamino)propyl methacrylate (DMAPMAE) as well as the joint fit for NEAEMA, MOMA, and PipEMA as calculated via the corresponding Arrhenius parameters. Data for the joint fit are taken from ref 27.

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calculated with the determined Arrhenius parameters alongside the joint Arrhenius fit for NEAEMA, PipEMA, and MOMA. As discussed in the previous paragraphs and depicted in Figure 4, DEAEMA clearly fits into the family of nitrogen-containing methacrylates, overlapping with the joint fit over the entire temperature range. The Arrhenius plot for DMAEMA lies just slightly above the proposed family but does not exhibit clearly overlapping data points, while DMAPMAE shows significantly lower propagation rate coefficients, disqualifying the monomer from incorporation into the family of nitrogen-containing methacrylates. To conclude the discussion, we propose to include DEAEMA into the previously described family of nitrogen-containing methacrylates and update the joint Arrhenius fit leading to the following joint Arrhenius parameters for the family including NEAEMA, MOMA, PipEMA, and DEAEMA: A = 1.55 × 106 L mol−1 s−1 and EA = 19.68 kJ mol−1.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], christopher. [email protected] (C.B.-K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.B.-K. and K.B.K. are grateful to BASF for supporting the current study and the excellent collaboration. In addition, C.B.K. acknowledges long term support from the Karlsruhe Institute of Technology (KIT) as well as the Helmholtz Association in the context of the STN program and the Ministry for Science and Arts of the State Baden-Württemberg. K.B.K. is grateful to Andrea Misske (BASF) for providing the DMAPMAE monomer investigated in the current study.

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CONCLUSIONS The Arrhenius parameters of the propagation rate coefficients of 2-(N,N-diethylamino)ethyl methacrylate (DEAEMA), 2(N,N-dimethylamino)ethyl methacrylate (DMAEMA), and 3(N,N-dimethylamino)propyl methacrylate (DMAPMAE) in bulk have been determined via PLP-SEC within a temperature range of 90 °C alongside the Mark−Houwink−Kuhn− Sakurada (MHKS) parameters of the corresponding polymers. All data determined in the current study have been critically compared to the previously introduced family of nitrogencontaining methacrylates to identify the boundaries of said family by stepwise reduction of the number of CH2-groups at the nitrogen atom and extending the alkyl linker between the nitrogen atom and the ester moiety. While DMAPMAE does clearly not fit into the family and DMAEMA seems to just exceed the family’s boundaries, DEAEMA excellently fits into the proposed family and an updated joint Arrhenius fit is provided. We suggest that in the case of the nitrogencontaining methacrylates polarity and steric demand are the key factors. The monomers incorporated in the family

ABBREVIATIONS PLP, pulsed laser polymerization; SEC, size exclusion chromatography; MHKS, Mark−Houwink−Kuhn−Sakurada; kp, propagation rate coefficient; Mw, weight-averaged molecular weight; DMAEMA, 2-(N,N-dimethylamino)ethyl methacrylate; DEAEMA, 2-(N,N-diethylamino)ethyl methacrylate; DMAPMAE, 3-(N,N-dimethylamino)propyl methacrylate; NEAEMA, 2-(N-ethylanilino)ethyl methacrylate; PipEMA, 1-(2-piperidyl)ethyl methacrylate; MOMA, 2-morpholinoethyl methacrylate; UMA, ureidoethyl methacrylate.

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Free Radical Propagation Rate Coefficients of N-Containing

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