Monomer Structure and Solvent Effects on Copolymer Composition in

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Kinetics, Catalysis, and Reaction Engineering

Monomer Structure and Solvent Effects on Copolymer Composition in (Meth)acrylate Radical Copolymerization Thomas Rooney, and Robin A. Hutchinson Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00451 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Industrial & Engineering Chemistry Research

Monomer Structure and Solvent Effects on Copolymer Composition in (Meth)acrylate Radical Copolymerization Thomas R. Rooney,* Robin A. Hutchinson* –––––––––

R. A. Hutchinson, T. R. Rooney Department of Chemical Engineering, Queen’s University, Kingston, ON, K7L 3N6, Canada E-mail: [email protected] T. R. Rooney Current address: BASF, 67056, Ludwigshafen, Germany E-mail: [email protected] –––––––––

Abstract Solvent effects on reactivity ratios (ri) and overall composition-averaged copolymer propagation rate coefficients (kp,cop) are generally not observed during methacrylic ester radical copolymerization. However, hydroxyl-bearing comonomers, such as 2-hydroxyethyl methacrylate (HEMA), lead to significant deviations from expectation for both ri and kp,cop, an effect which is highly dependent on solvent choice and rooted in hydrogen bond interactions. The current understanding of the influence of hydrogen bonding on organic solution (meth)acrylic ester radical (co)polymerization kinetics is reviewed by summarizing trends in structure/reactivity for methacrylate homopropagation rate coefficients (kp) and methacrylate macromonomer relative reactivity during copolymerization. In addition, the peculiarities which characterize the apparent enhanced reactivity of hydroxyl-bearing monomers during copolymerization are outlined. Finally, a modeling framework to systematically capture the effects of solvent and hydrogen bonding on copolymer composition, through specific intramolecular hydrogen bond associations between monomer and growing chain, is presented for several methacrylate, acrylate, and styrene copolymerizations. 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Introduction Radical polymerization is a mature synthesis methodology used to manufacture a large variety of polymeric materials.1 The favorable properties of homopolymers are often combined through radical copolymerization in order to make accessible an even greater diversity of materials. To achieve the desired end-use properties, good control over copolymer composition and the (co)polymerization rate are required. According to the Terminal Model, the monomer reactivity ratios, r1 and r2, in Eqns. 1 and 2, respectively, define the instantaneous copolymer composition (F1) given by Eqn. 3, where fi is the molar fraction of monomer i. In addition, the overall composition-averaged copolymer propagation rate coefficient (kp,cop; Eqn. 4) is described by ri as well as the corresponding homopropagation rate coefficients (kp,ii). Depending on the values of ri which characterize a copolymerization system, the composition of copolymer produced at the beginning of a batch may differ significantly from the copolymer produced towards the end. Thus, accurate ri as well as reliable kp values are required in order to design monomer feeding strategies aimed at minimizing compositional drift while maximizing reactor productivity.

=

=

, ,

=

, ,

+ + 2 +

, =

+ 2 + + , ,

1

2

3

4

Due to the increasing diversity of available monomers, there is strong impetus in the polymerization kinetics community to establish family type behaviors for kp and ri to improve modeling and prediction of new and existing systems. Copolymer composition can usually be generalized as follows: methacrylate (xMA)/acrylate (xA) yields methacrylate-rich copolymers,2,3 xMA/xMA copolymerize with equal addition probabilities,4 and xMA/styrene (ST) tends towards an alternating 2 ACS Paragon Plus Environment

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sequence.5 While the Terminal Model adequately captures copolymer composition, it was necessary to introduce penultimate effects,6 through the implicit penultimate unit effect model, to simultaneously represent kp,cop for bulk xMA/xA7 and xMA/ST8 copolymerizations. The corollary is that any model used to capture effects of monomer structure or solvent on copolymer composition must also be tested against kp,cop data. In most cases, solvent choice does not influence the relative consumption of comonomers in homogeneous solution; however, significant deviations from expectation occur for both ri and kp,cop when the differences in monomer and solvent polarities are large,9-11 or when functionalized monomers, such as 2-hydroxyethyl methacrylate (HEMA) or 2-hydroxyethyl acrylate (HEA), capable of hydrogen bonding, are copolymerized in bulk and solution.5,12 Explanations for the various effects of solvent on copolymer composition include: true variations in ri values arising from influences on the transition state (TS) for kp of individual propagation steps,13 complexation models,9 and Bootstrap models.14 The latter assumes that solvent does not alter the inherent reactivity of the growing radical, but affects monomer partitioning and causes variations in local monomer concentrations. Predictive methodologies recently developed to estimate solvent effects on kp for a variety of monomer and solvent pairs include: application of a linear solvation energy relationship which correlates kp to solvent solvatochromic parameters,15,16 and a thermodynamic formulation of TS theory that correlates the non-ideality of a system to an apparent kp.17,18 Although promising, neither of these methods have yet been applied to copolymerization systems, let alone to those containing hydroxylbearing comonomers. The kp measured for bulk HEMA homopropagation is the highest methacrylate kp reported to date,19 an effect which must be related to its hydroxyl functionality since the kp for n-butyl methacrylate (BMA) is augmented in alcohol solvents.20 This remarkable propagation behavior manifests during copolymerization as a significant preferential incorporation of HEMA (relative to n-alkyl methacrylates) in HEMA/ST,5 HEMA/BMA,21 and HEMA/butyl acrylate (BA)21 bulk systems. Moreover, this phenomenon is highly dependent on solvent choice, where HEMA’s preferential incorporation can be further augmented in toluene or xylenes, while in hydrogen bond promoting or disrupting solvents, 3 ACS Paragon Plus Environment


HEMA’s relative reactivity is reduced to that of n-alkyl methacrylates.5 The increased relative incorporation of HEMA is also reflected by elevated kp,cop measurements in bulk compared to hydrogen bond disrupting solvents; with the same trends generally observed for HEA bulk and solution copolymerizations.12,22 It should be emphasized that these effects are largely studied under controlled low conversion conditions. Under the constantly changing ratios of monomer, solvent, and polymer, characteristic of batch and semi-batch industrial processes,23,24 the influence of monomer functionality and solvent on relative reactivity (and thus copolymer composition and rate) may vary significantly with conversion in the system. Therefore, to systematically investigate the effect of various functional group type and location on xMA solution copolymerization kinetics, several short-chain polyester macromonomers, shown in Scheme 1, were produced by ring opening polymerization (ROP).25 These methacrylate type macromonomers are useful for studying structure/reactivity trends as the functional group type is specified by ROP initiator choice, while its distance from the reactive double bond is tuned by the average number of polyester units (n) per macromonomer, defined by the stoichiometric ratio of cyclic monomer to initiator in the ROP step. The macromonomers presented in Scheme 1 were synthesized with common end-group functionalities, such as hydroxyl, carboxyl, alkyl, and tertiary amine, that are separated from the methacrylic ester by polycaprolactone (PCL) or polylactic acid (PLA) spacers with similar average molecular weights. The hydroxyl terminated macromonomers (e.g., HEMA-PCL3 and HEMA-PLA5, where the subscript denotes the average number of PCL or PLA units in the macromonomer, respectively) are produced by the ROP of ε-caprolactone (CL) and lactide (LA), respectively, with HEMA as initiator. Furthermore, the end-group functionality of HEMA-PCL3 was modified with carboxyl and propionate ester to produce HEMA-PCL3-COOH and HEMA-PCL3-PR, respectively. In addition, the alkyl terminated (PLA5EMA) and tertiary amine end-functionalized (PCL3DeMA) macromonomers were also investigated.

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Scheme 1: Structures of short-chain polyester methacrylate macromonomers referred to in this work.

This perspective focuses on the structure/reactivity relationship in methacrylate radical copolymerization kinetics with particular attention given to hydroxyl-bearing monomers and solvent effects related to hydrogen bonding in non-aqueous solutions. Although trends in kp do not directly translate to trends in relative reactivity, the effect of solvent and functionality on homopropagation behaviors are also reviewed to help provide context to the various copolymerization effects. A summary of the trends and peculiarities related to the influence of solvent on copolymer composition for several extensively studied HEMA and HEA copolymerization systems is provided. Then, new understandings from methacrylate macromonomer studies about the influence of ester side chain length, functional group type and distance from the polymerizable group (with emphasis on hydroxyl functionalities) on copolymer composition are highlighted. Finally, a modeling framework to systematically capture the influence of solvent and hydrogen bonding on composition is presented.

Impact of Functionality on Methacrylate Homopropagation Rate Coefficients 5 ACS Paragon Plus Environment


The pulsed laser polymerization combined with size exclusion chromatography (PLP-SEC) technique is recommended by the IUPAC subcommittee on “Modeling of Polymerization Kinetics and Processes” as the most accurate and reliable method for determining kp. Benchmark data sets have been used to estimate Arrhenius parameters for the bulk kp of several alkyl ester methacrylates: methyl methacrylate (MMA),26 ethyl methacrylate (EMA), n-butyl methacrylate (BMA), and dodecyl methacrylate (DMA), with similar activation energies (EA) in the range of 23.4–21.0 kJ·mol-1.27 Furthermore, an increase in the length of the alkyl ester side chain correlates to an increase in bulk kp measured by PLP-SEC; this trend was more recently reported to extend to behenyl methacrylate (average of 19.9 C atoms in ester side chain).28 Buback has attributed this behavior to entropic factors affecting the TS for propagation,29 where longer alkyl side chains can better shield the dipolar interactions (between methacrylate groups) to afford greater mobility to the TS and consequently increase kp.13 Consistent with this interpretation, a similar increase in bulk kp with increasing ester side chain length was reported for polyethylene glycol (PEG) ethyl ether methacrylate (PEGEEMA, 3 PEG units) compared to 2-ethoxyethyl methacrylate (EEMA, 1 PEG unit);30 however, no additional increase in bulk kp was found for polyethylene glycol methyl ether methacrylate (PEGMA, 7-8 PEG units).31 The same trend was also documented for PLAnEMA, where the bulk kp for macromonomers with n=1,5 PLA units in the ester side chain could not be distinguished over the 40–100 °C temperature range.32 The propagation behavior for methacrylates containing heteroatoms or sterically hindered groups in the ester side chain is less well-understood. Although the bulk kp for cyclic ester methacrylates glycidyl methacrylate (GMA), cyclohexyl methacrylate, benzyl methacrylate, and isobornyl methacrylate (iBoMA) can be described by a single Arrhenius relation of EA = 21.9 kJ·mol-1 with pre-exponential (A) of 4.24×106 L·mol-1·s-1,33 the bulk kp for several other methacrylates with steric hindrances further away from the methacrylic group could not be described by the same fit,28 indicating the importance of functional group location (with respect to the polymerizable group) in the ester side chain. The Arrhenius parameters for the bulk kp of methacrylates containing various functional groups in the ester side chain are summarized by Table 1. Despite having lower molecular weight than the linear 6 ACS Paragon Plus Environment

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alkyl methacrylate DMA, acetoacetoxyethyl methacrylate (AAEMA), 2-(N,N dimethylamino)ethyl methacrylate (DMAEMA), and GMA all have kp values that are higher than that of DMA at 50 °C, confirming that the presence of functional groups in the ester side is significant. For tertiary amine functionalized methacrylates, steric and polarity factors prompted the family behavior grouping of 2-(Nethylanilino)ethyl methacrylate (NEAEMA),34 2-morpholinoethyl methacrylate (MOMA),34 2-(1piperidyl)ethyl methacrylate (PipEMA),34 and 2-(N,N-diethylamino)ethyl methacrylate (DEAEMA)35 with joint-fit Arrhenius parameters of A = 1.55×106 L·mol-1·s-1 and EA = 19.7 kJ·mol-1. However, the DMAEMA kp data set exceeded the limits of uncertainty for the family behavior, presumably because of its

comparatively

less

sterically

hindered

N,N

substituted

ethyl

amine,

while

3-(N,N-

dimethylamino)propyl methacrylate (DMAPMAE)35 clearly could not be included because the extra CH2 spacer further distances the tertiary amine (and its influence on kp) from the methacrylic double bond. Finally, it should be noted that the hydroxyl-bearing monomers HEMA and 2-hydroxypropyl methacrylate (HPMA) exhibit markedly elevated kp, attributable to their hydroxyl functionalities.19,36



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Table 1: Summary of Arrhenius parameters for bulk kp of diverse methacrylic esters. kp at 50 °C (L·mol-1·s-1)

EA (kJ·mol-1)

A (106 L·mol-1·s-1)

MMA26

647

22.4

2.7

DMA27

1009

21.0

2.5

1053

24.4

9.3

PLAnEMA32 n=1,5

1045

21.1

2.7

DMAPMAE35

831

19.6

1.2

DMAEMA35

1185

20.7

2.6

GMA33

1219

21.9

4.2

AAEMA37

1399

19.7

2.1

HPMA36

1510

21.7

4.9

HEMA19

2554

21.9

8.9

Methacrylate

O PEGEEMA30

O

O 3

Solvent Effects on the Homopropagation Rate Coefficient Solvent effects on radical homopropagation kinetics can arise from both non-specific as well as specific interactions between monomer and solvent.38 In the case of non-specific solvent interactions, the kp measured by PLP-SEC can manifest as an apparent value (kpapp) that may be different than the bulk value, and which correlates to differences in molar volumes between monomer and solvent. For example, in 8 ACS Paragon Plus Environment

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toluene solution, the kpapp/kp,bulk for MMA (lower molar volume than toluene) is greater than unity, while the kpapp/kp,bulk for iBoMA (higher molar volume than toluene) is less than unity.39 This phenomenon can be interpreted as a competition between solvent and monomer molecules for positioning at the radical site; if the monomer size is less than that of a solvent molecule, then the concentration of monomer at the reaction site will be greater than its analytical concentration such that the kpapp measured by PLP-SEC will be higher than kp,bulk, and vice versa. This interpretation is a special case of the general picture provided by Heuts et al. in which solvents with higher molar volume than monomer increase the hindrance to rotational mobility of the transition state structure.29 Accordingly, the correlation between kp measurements and monomer/solvent molar volumes for linear alkyl acrylates in toluene solution has been explained by Buback as an entropic effect in which an increasing molar volume of monomer relative to that of the solvent results in a more hindered TS for propagation and consequently lower value of kp.13,40 Although the extent to which this explanation governs the kp of stearyl and behenyl acrylates in 1 M toluene or butyl acetate solutions is unclear from experimental studies,40-42 a recent computational work that provides a thermodynamic formulation of transition state theory supports the concept that the magnitude and direction of the solvent effect is a function of the size of the ester group.18 The influence of specific hydrogen bonding (H-bonding) in organic solution n-alkyl ester (meth)acrylic propagation kinetics has been documented for many systems. Currently, this influence is understood as an association between hydroxyl and (meth)acryloyl carbonyl group which reduces the electron density around the double bond making it more reactive towards radical addition, as is reflected by an increase in kp.20 Scheme 2 illustrates how the hydroxyl group may be provided externally by an alcohol solvent, or provided internally by a hydroxyl-bearing monomer in bulk or non-specific interacting solvents (herein termed inert solvent) such as toluene and xylenes. Furthermore, the effect of H-bonding on organic solution kp for (meth)acrylic esters can also be disrupted by solvents such as dimethylformamide (DMF) or tetrahydrofuran (THF).38



Scheme 2: Simplified illustration demonstrating the current understanding of specific (H-bond disruption and promotion) and non-specific (termed inert) H-bonding effects in methacrylic ester solution polymerization.

The influence of H-bonding on kp is known to be concentration dependent,20,43 where 50 vol% nbutanol (BuOH) is sufficient to increase the kp of n-butyl methacrylate (BMA) or n-butyl acrylate (BA) by 30% compared to the respective bulk values. PLP-SEC experiments of 1.5 M BMA in n-butanol solution showed a reduction in EA from 23.0 to 20.4 kJ·mol-1 compared to bulk BMA, indicating that the effect of H-bonding on kp is enthalpic in origin.20 However, the elevated kp of bulk HPMA and HEMA (Table 1), both monomers capable of H-bonding, have EA still within the expected methacrylate family type behavior (the range of 23.4–21.0 kJ·mol-1). The kp of hydroxyl-bearing monomers such as HEMA, HPMA, and ethyl-α-hydroxymethacrylate (EHMA) in various solvents are summarized by Table 2. When a H-bond disrupting solvent is employed, the minimum kp is reached, while in H-bond promoting solvents there is a slight, perhaps negligible, decrease in kp compared to bulk for HEMA and HPMA. In toluene, an inert solvent (towards specific H-bonding interactions), it is not understood why elevated kp were measured compared to bulk for HPMA and EHMA; interestingly, as the weight fraction of toluene is increased from 25 to 75%, the kp of EHMA increases by roughly 60% over the 10–30 °C temperature range.44 More recently, the influence of solvent dipolarity and polarizability were determined to be more influential on the kp of HPMA than its classical H-bonding abilities.15


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Table 2: Influence of solvent on kp for hydroxyl-bearing monomers. Monomer Bulk

kp in various solvents (L·mol-1·s-1) H-Bond H-Bond Disruptor Promoter

Conditions Inert

HEMA19

1201

-

973 (n-butanol)

-

50 vol% mon. at 22 °C

HPMA*36

1211

720 (2.8 M in THF)

1170 (3.0 M in BzOH)

1350 (2.7 M in Toluene)

57 wt% mon. at 40 °C

EHMA44

-

580 (1.8 M in THF)

647 (1.7 M in propan-1-ol)

1635 (1.8 M toluene)

75 wt% mon. at 15 °C

* Contains 20% 1-hydroxypropan-2-yl methacrylate isomer.

Effect of Hydrogen Bonding on Copolymer Composition Hydroxyl-bearing (meth)acrylic esters are frequently copolymerized with a variety of methacrylates, acrylates, or styrene (ST). For such copolymerizations, the expected trends in copolymer composition in terms of (meth)acrylic ester size and functionality are outlined for “common” monomers. Then, the ways in which the hydroxyl functionality and solvent choice contribute to significant deviations in “expected” copolymer composition in bulk and various solution copolymerizations are presented. The Mayo-Lewis curves in Figures 1-3 are generated using best-fit reactivity ratios from extensive data sets, with full details provided in the original references cited. It should be noted that acrylate (co)polymerization systems have the added complexity of mid-chain radical formation through an intramolecular H-atom abstraction reaction involving acrylate units. However, since the amount of monomer consumed by reaction with mid-chain radicals is negligible in these systems,24 the reaction has no influence on the estimated reactivity ratios or the shapes of the copolymer composition plots. It has been found that n-alkyl methacrylate (xMA) copolymerizations with ST tend towards alternating systems, where similar copolymer composition is expected from MMA/ST, BMA/ST, and DMA/ST, independent of the ester side chain length.45 However, functionalized monomers, such as 11 ACS Paragon Plus Environment


GMA, are significantly more reactive towards ST radicals.45,46 In addition, as shown in Figure 1, the relative incorporation of HEMA, like GMA,45 is elevated compared to BMA during bulk ST copolymerization,5 and the relative incorporation of HEA is also increased compared to BA during bulk ST copolymerization.47 H-bonding is thought to be responsible for HEMA’s behavior because during HEMA/ST solution copolymerization in DMF, a H-bond disrupting solvent, the relative reactivity of HEMA is reduced to that of BMA/ST. Similarly, the relative reactivity of BMA during BMA/ST solution copolymerization in BuOH, a H-bond promoting solvent, is increased towards that of HEMA/ST, whereas the reactivity of GMA/ST does not vary with solvent choice.5 Nonetheless, it still remains unclear why HEMA/ST copolymerizations performed in toluene, a non-specific interacting solvent, produce copolymers further enriched by HEMA compared to bulk copolymerization, a feature which is even more pronounced when the toluene content is increased from 25 to 50 vol%. This peculiarity is also evidenced by EHMA homopropagation, where a 60% increase in kp was measured from 25 to 75 wt% toluene solution between 10 and 30 °C.44 A recent computational study calculated the TS geometries for HEMA and chain-end polyHEMA as 7-membered rings formed by H-bonding between hydroxyl proton and methacryloyl carbonyl in bulk and toluene solutions, a treatment that led to qualitative reproduction of the HEMA/ST copolymer composition behaviors in bulk, DMF, and toluene solutions.48


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Figure 1: Mayo Lewis plots for ST copolymerizations with HEMA (left)5 and HEA (right)47 in 50 vol% DMF (BA/ST r values used for the HEA/ST system; solid line), bulk (dashed line), 25 vol% toluene (dash-dotted line), and 50 vol% toluene (dotted line). Curves were reproduced from r values fit to data sets measured at 90 °C and 20-60 °C, respectively.

The reactivity ratios which characterize xMA/xMA copolymerizations are typically near unity meaning that both xMA react with equal addition probability. This is true for simple systems like MMA/DMA,4 but also for xMA bearing diverse functional groups in their ester side chains such as 2-(2bromopropionyloxy)

propyl

methacrylate,49

AAEMA,37

or

1-ethylcyclopentyl

methacrylate.50

Furthermore, a variety of xMA/n-alkyl acrylate (xA) pairs consistently produce methacrylate-rich copolymers.2,3,7,51 For such xMA/xMA and xMA/xA copolymerization systems, it is found that the effect of solvent choice on copolymer composition is small as long as the difference in monomer and solvent polarities is not large.21,52-55 As summarized by Figure 2, the H-bonding capabilities of the HEMA/BMA 13 ACS Paragon Plus Environment


and HEMA/BA systems cause copolymer composition to deviate from their respective expected behaviors. In bulk, the relative reactivity of HEMA is augmented compared to its comonomer, whereas in DMF H-bonding is disrupted such that the relative reactivity of HEMA is reduced to the expected xMA/BMA and xMA/BA copolymerization behaviors, respectively. Additionally, the same composition results are achieved in BuOH because the introduction of additional H-bonding increases the reactivity of BMA and BA towards radical addition, respectively.21 Although the explanations for HEMA/B(M)A composition behaviors in DMF and BuOH are clear, an interpretation of the bulk behavior is not obvious. The elevated bulk kp measured for HEMA and HPMA is related to the H-bonding between hydroxyl and methacrylic carbonyl which increases the monomer’s reactivity towards radical addition.19,36 However, it is also well-known that in alcohol solvents the kp of BMA can be significantly elevated by the same mechanism. Therefore, since the reactivity of both HEMA and BMA (or BA) can be augmented in the presence of hydroxyl moieties, this mechanism is not sufficient to explain why only HEMA is preferentially incorporated in HEMA/BMA and HEMA/BA copolymerization systems. Similar findings were reported for HEMA/DMA copolymerization, with reactivity ratios near unity in DMF, tert-butyl alcohol, and THF solvents, while in inert benzene an abnormally high incorporation rate of HEMA was attributed to an aggregation tendency due to H-bonding.56


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Figure 2: Mayo Lewis plots for HEMA copolymerizations with BMA (left) and BA (right) in 50 vol% DMF (solid line) and bulk (dashed line). Curves were reproduced from r values fit to data sets measured at 90-100 °C and 50 °C, respectively.21

The effect of H-bonding on copolymer composition was extended to HEA/BMA and HEA/BA systems as summarized by Figure 3. Similarly to xMA/xMA copolymerization, the copolymer composition for xA/xA is generally expected to fall along the diagonal in a Mayo-Lewis plot and to be insensitive towards solvent choice, as demonstrated for 2-methoxyethyl acrylate/BA.22 However, for both HEA/BMA and HEA/BA bulk systems there is a significant increase in the relative rate of HEA incorporation compared to expectation for xA/BMA and xA/BA systems, respectively. Consistent with other hydroxyl-bearing monomer copolymerization systems, the relative reactivities of HEA/BMA and HEA/BA in DMF and BuOH are reduced to that of BA/BMA and BA/BA, respectively. Since both BMA and BA can accept H-bonding it is still not clear why HEA is preferentially incorporated; this is 15 ACS Paragon Plus Environment


particularly surprising for the HEA/BMA system where the increased stability of xMA compared to xA radicals should lead to methacrylate-rich copolymers. It is even more surprising that HEA/BMA copolymerization in the non-specific interacting xylenes solvent produces acrylate-rich copolymers over the entire composition range.

Figure 3: Mayo Lewis plots for HEA copolymerizations with BMA (left)12 and BA (right)22 in 50 vol% DMF (solid line), bulk (dashed line), and 50 vol% xylenes (dotted line). Curves were reproduced from r values fit to data sets measured at 90-100 °C and 50 °C, respectively.


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The main peculiarities of the hydroxyl-bearing (meth)acrylic ester copolymerization systems are summarized as follows: 1. For bulk HEA/BMA,12 HEMA/BMA,21 and HEMA/BA,21 the hydroxyl-bearing (meth)acrylates are preferentially incorporated (compared to non-hydroxylated analogs) even though all the comonomers are also capable of accepting H-bonding at their (meth)acryloyl carbonyl (to reduce electron density at the double bond making them more reactive towards radical addition). 2. In the HEMA/ST5 and HEA/BMA12 systems, the preferential incorporation of hydroxyl-bearing monomer exhibited in bulk, is exacerbated in increasingly dilute solutions of non-specific interacting solvents (e.g., xylenes, toluene). 3. xMA/acrylate copolymerizations are known to produce xMA-rich copolymers,3 yet the bulk and solution HEA/BMA12 copolymerizations generate copolymers increasingly enriched by acrylates.

Macromonomers to Probe Copolymerization Kinetics In order to gain a deeper understanding about the role of the ester side chain size as well as functional group type and distance from the reactive double bond, a study about methacrylate type macromonomers (shown in Scheme 1) and ST solution copolymerizations was conducted using the in situ 1H-NMR technique.25 The ability of in situ 1H NMR to capture trends in structure/reactivity, such as the effect of H-bonding on radical copolymerization kinetics, is visualized in Figure 4, where the methacrylate composition drift (fxMA) as a function of molar conversion (x) is normalized by the initial methacrylate composition (fxMA,0=0.2). In the non-specific interacting solvent toluene-d8, the relative consumption of HEMA is significantly faster than that of BMA due to H-bonding. However, in DMSO-d6 solvent which is able to disrupt H-bonding, the relative consumption of HEMA is reduced to that of BMA, consistent with trends seen in the Mayo-Lewis plots of Figure 1. The enhanced relative reactivity of BMA in DMSO-d6 compared to in toluene-d8 was also documented for DMAEMA, GMA, and PLA1EMA copolymerizations with ST under identical conditions and was attributed to the reduced reactivity of ST in polar media.5,25 17 ACS Paragon Plus Environment


Figure 4: Monomer composition drifts normalized by fxMA,0 as a function of conversion for ST copolymerizations with fxMA,0 = 0.2 for BMA (circles) and HEMA (triangles) in 80 wt% toluene-d8 (closed symbols) and 80 wt% DMSO-d6 (open symbols) performed at 80 °C. Reprinted with permission from Macromolecules. Copyright 2017 American Chemical Society.25

To explore the effect of end-group functionality on macromonomer copolymerization kinetics, the relative consumptions of macromonomers in toluene-d8 are compared to their monomeric analogs in Figure 5, with the BMA (dotted line) and HEMA (solid line) profiles provided as visual guides. While the relative consumption of DMAEMA is enhanced compared to BMA because of its tertiary amine functionality, the relative consumption of the macromonomer, PCL3DeMA is even more azeotropic than BMA. This indicates that the impact of the xMA’s tertiary amine functionality on its relative reactivity is reduced as it moves several units beyond the methacryloyl carbonyl; similarly, a reduction in kp was measured in passing from DMAEMA to DMAPMAE (see Table 1).35 In the case of the alkyl terminated 18 ACS Paragon Plus Environment

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macromonomer, PLA5EMA, there is no difference in relative consumption behavior compared to its monomeric analog, PLA1EMA, indicating that the alkyl end-group and total length of the ester side chain are not important considerations for macromonomer copolymerization behavior; rather their common isobutyrate bridge next to the methacryloyl carbonyl must be the determining factor. This conclusion is supported by kp measurements for PLA1EMA and PLA5EMA homopropagation which could not be distinguished over the 40-100 °C temperature range32 as well as n-alkyl methacrylate/ST family behavior45 and a n-alkyl methacrylate/n-alkyl acrylate kp,cop study well-fit by a single pair of r values.7 Finally, the H-bond capable hydroxyl (HEMA-PCL3) and carboxyl (HEMA-PCL3-COOH) functionalized macromonomers

were

compared

to

their

monomeric

analogs,

HEMA

and

mono-2-

(methacryloyloxy)ethyl succinate (HEMA-COOH), respectively. As expected, both monomers exhibit enhanced relative reactivities in toluene-d8 due to their ability to form H-bonds. Although the relative reactivities of the corresponding macromonomers, HEMA-PCL3 and HEMA-PCL3-COOH, are augmented compared to BMA, they do not attain the H-bond dependent enhanced reactivity characteristic of their monomeric analogs.



Page 20 of 38

Figure 5: (Macro)monomer composition drifts normalized by fxMA,0 for ST copolymerizations with fxMA,0 = 0.2 as a function of conversion for monomer (closed circles) and macromonomer (open circles) pairs in 80 wt% toluene-d8 with amine (panel A), hydroxyl (panel B), alkyl (panel C), and carboxyl (panel D) end-group functionalities. Best fit lines for HEMA/ST (solid line) and BMA/ST (dotted line) in 80 wt% toluene-d8 are provided as visual guides. Reprinted with permission from Macromolecules. Copyright 2017 American Chemical Society.25

Placement of the hydroxyl or carboxyl functionality farther into the ester side chain reduces the impact of H-bonding on xMA incorporation compared to HEMA/ST. This is supported by previous copolymer composition measurements for HEMA-PCLn (n=2,3) and MMA bulk copolymerizations with fmacromonomer≤ 0.66 which displayed equal addition probabilities,57 in agreement with the composition behavior

for

HEMA-PLAn/MMA

solution

copolymerizations.58,59

However,

unlike

the

DMAEMA/PCL3DeMA pair, distancing the hydroxyl/carboxyl group farther away from the methacrylic 20 ACS Paragon Plus Environment

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double bond did not completely reduce the macromonomers’ relative reactivities to that of BMA. To test whether the diluted H-bond interaction between hydroxyl and carbonyl is responsible for the intermediary relative reactivity of HEMA-PCL3 and HEMA-PCL3-COOH copolymerization systems, the same experiments were conducted with HEMA-PLA5 (hydroxyl to carbonyl ratio of 6:1) and alkyl end-capped HEMA-PCL3-PR.25 All hydroxyl functionalized macromonomers and derivatives exhibited nearly identical consumption behaviors. This confirmed that the effect of H-bonding in macromonomer/ST systems is so dilute that its influence on relative reactivity is negligible. Instead, it is likely that the ester moiety, separated from the methacrylic double bond by an ethyl bridge, is more influential on macromonomer relative reactivity. In support of this claim, the kp measured for the structurally similar AAEMA (see Table 1) was elevated compared to BMA but not as high as HEMA.37 The diluted influence of H-bonding on macromonomer relative reactivity confirms that the current mechanism shown in Scheme 2, where H-bonding between a hydroxyl and the (meth)acryloyl carbonyl reduces electron density at the double bond making it more reactive towards radical addition, is not sufficient to completely describe the copolymer propagation kinetics of hydroxyl-bearing monomers. Consider the parameter δ, defined by Eqn. 5 as the ratio of H-bond donors to acceptors (i.e., ratio of hydroxyl to carbonyl groups), to qualitatively track H-bond dilution.

=

# ℎ # ℎ

5

On one hand, the elevated relative reactivity exhibited by HEMA (δ=1) during batch xMA/ST copolymerizations in toluene-d8, was significantly reduced for HEMA-PCL3 (δ=1/4) in HEMAPCL3/ST.25 On the other hand, the parameter δ is alternatively reduced by copolymerizing HEMA with alkyl (meth)acrylate comonomers, such as BMA or BA. For bulk copolymerizations with δ

Monomer Structure and Solvent Effects on Copolymer Composition in

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