Article pubs.acs.org/Macromolecules
Quantification of Intramolecular Cyclization in Branched Copolymers by 1H NMR Spectroscopy Julien Rosselgong and Steven P. Armes* Department of Chemistry, Dainton Building, University of Sheffield, Sheffield, South Yorkshire S3 7HF, U.K. S Supporting Information *
ABSTRACT: Statistical copolymerization of a monovinyl with a divinyl monomer leads to macroscopic gels, microgels, or soluble branched copolymers, depending on the precise reaction conditions. This approach is widely used for the manufacture of soft contact lenses and also to prepare various biomedical hydrogels for the separation and purification of proteins, DNA, etc. According to Flory−Stockmayer theory, gelation should occur in such copolymerizations if there is more than 0.50 fully reacted divinyl comonomer per primary chain. However, many experimental studies indicate significant deviations from this classical theory, which are generally believed to be due to wastage of the divinyl comonomer via intramolecular cyclization. Unfortunately, experimental verification of this side reaction has proven elusive for the past seven decades. In the present study, we use a disulfide-based cleavable bifunctional comonomer to undertake the first experimental quantification of the extent of intramolecular cyclization in nonlinear methacrylic copolymers using 1H NMR spectroscopy.
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INTRODUCTION It is well-known that the statistical copolymerization of a monovinyl monomer with a divinyl comonomer can lead to either cross-linked gels,1 microgels,2 or highly branched soluble copolymers3,4 depending on the precise experimental conditions. This approach has been widely used for the manufacture of soft contact lenses,5 for the synthesis of macroporous styrene−divinylbenzene beads for chromatography applications,6 to prepare various biomedical hydrogels for the separation and purification of proteins or DNA,7,8 to act as rheological modifiers,9 and recently for the development of socalled “engineered emulsions”.10−13 Over the past two decades, the emergence of controlled radical polymerization techniques such as nitroxide-mediated polymerization,14 atom transfer radical polymerization,15,16 and reversible addition−fragmentation chain transfer polymerization (RAFT)17 have led to many new studies of well-defined vinyl branched copolymers, including two recent review articles.4,18 Potential applications include peptide-degradable biocompatible fibers,19 thermoresponsive fluorescent copolymers for bacterial detection,20 biodegradable DNA-binding copolymers,21 and model nonlinear glycopolymers.22 Flory developed a so-called “mean field” theory,23,24 later extended by Stockmayer,25 for an idealized formulation whereby a divinyl cross-linker reacts statistically with the monovinyl monomer to link up perfectly monodisperse primary chains. This analysis, which ignores intramolecular cyclization, predicts that a minimum of 0.50 fully reacted divinyl comonomers per primary chain is required to form an insoluble polymer network.4,18 However, many experimental studies indicate significant deviations from this theory, which are generally believed to be due to participation © 2012 American Chemical Society
of the divinyl comonomer in a side reaction known as intramolecular cyclization.26−43 When a monovinyl monomer is copolymerized with a divinyl comonomer, reaction of the first double bond of the latter species creates a pendent vinyl group. If the remaining double bond of this bifunctional comonomer undergoes subsequent reaction, there are two possibilities: it can either react intermolecularly with another growing primary chain to create two branch points or it can react intramolecularly to make a cycle (or loop) within the same primary chain. This second event represents wastage of the divinyl comonomer in the branched copolymer synthesis. Unfortunately, direct experimental quantification of the extent of intramolecular cyclization has proven elusive for the past seven decades, so this explanation for the failure of classical gelation theory has never been verified. In the present study, which involves the 1H NMR analysis of methacrylic branched copolymers prepared by living radical copolymerization, we report the first direct quantification of the relative amounts of intermolecular branching and intramolecular cyclization in any nonlinear polymer system. Recently, we conducted a series of statistical copolymerizations of methyl methacrylate (MMA) with a disulfide-based dimethacrylate (DSDMA) branching comonomer using reversible addition−fragmentation chain transfer (RAFT) polymerization.35,36 Control experiments involving the RAFT homopolymerization of MMA in the absence of any DSDMA branching comonomer indicated chain transfer agent efficienReceived: February 7, 2012 Revised: February 28, 2012 Published: March 13, 2012 2731
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homopolymer chains were cross-linked via quaternization chemistry at various concentrations both above and below c*.41 In the present system, we calculate the c* for PMMA55 homopolymer chains to be ∼11 vol % (or ∼14.6 wt %).35 If the copolymerization of MMA with DSDMA is conducted at 50 wt %, this is well above c* and should hence lead to the relatively efficient production of branched copolymer, whereas the same copolymerization performed at 10 wt % would be expected to suffer from very inefficient branching (and hence contain substantial amounts of intramolecular cycles). These scenarios have been verified for the first time in the present study, in which 1H NMR spectroscopy has been utilized to distinguish between the fully reacted DSDMA units involved in intermolecular branches and intramolecular cycles, respectively.
cies of between 80 and 90% [higher efficiencies (≈ 90%) were obtained for polymerizations conducted at [MMA]0 = 50 wt % compared to [MMA]0 = 10 wt %] for the cumyl dithiobenzoate (CDB), as judged by both 1H NMR spectroscopy and gel permeation chromatography (GPC) analyses.22 Monomer conversions of 96−99% were achieved within 30−96 h at 90 °C, with approximately first-order kinetics with respect to monomer, linear evolution of molecular weight with conversion, and relatively low final polydispersities (1.19 < Mw/Mn < 1.29) being observed.35 Thus, the MMA homopolymerization shows pseudoliving character, which means that the primary chains in the corresponding branched copolymer syntheses should have a relatively narrow molecular weight distribution. Indeed, this was confirmed by GPC analyses of the branched copolymers after selective reductive cleavage of the disulfide bonds in the DSDMA units using tributylphosphine.35 Moreover, monomer depletion studies using HPLC confirmed that the RAFT copolymerization of MMA and DSDMA is almost purely statistical at [MMA]0 = 50 wt %.35 According to our previous work,35,36 and also that of others,18,32,37−42 it was expected that intramolecular cyclization should become much more prevalent at lower monomer concentrations. In order to examine this hypothesis, the initial monomer concentration was systematically varied from 10 to 50 wt % in order to control the relative proportions of intramolecular cyclization and intermolecular branching occurring in these branched copolymer syntheses (see Figure 1).
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RESULTS AND DISCUSSION Figure 2 shows the 1H NMR spectra recorded for five PMMA− DSDMA branched copolymers synthesized at [MMA]0 = 10, 30, and 50 wt % (spectra A and B were recorded for copolymers synthesized at 50 and 30 wt %, respectively, while spectra C−E were recorded for copolymers prepared at 10 wt % in the presence of increasing amounts of DSDMA per primary chain) under conditions that were very close to the experimental gel point, as demonstrated by the high molecular weights reported by GPC (see Table 1). Also shown in the same Figure are the corresponding 1H NMR spectra obtained after selective reductive cleavage of the disulfide bonds in the DSDMA units to afford the linear, near-monodisperse thiolfunctionalized primary chains.35,36 The most compelling evidence for our NMR peak assignments comes from recent work by Hawker’s group, who reported the synthesis of a novel disulfide-based cyclic monomer, followed by its ring-opening polymerization to produce a linear polyester (see Figure S1).45,46 The S−S−CH2 (thiamethylene) protons in the cyclic monomer appear as a triplet at approximately δ 3.0, but this signal is shifted to approximately δ 2.9 in the polyester. These observations mirror almost perfectly what is observed in the 1H NMR spectra of our branched vinyl copolymers (see Figure 2), which contain a range of overlapping signals at or above δ 3.00 due to various intramolecular cycles and a relatively welldefined signal at δ 2.92 due to intermolecular branches. Moreover, our assignments are supported by selective reductive cleavage of the disulfide bonds in the DSDMA units using excess tributylphosphine, since all the intra/intermolecular thiamethylene proton signals are shifted to a new singlet at δ 2.8, which is assigned to the HS−CH2 signals of 2-thioethyl methacrylate (TEMA) residues. Similar subtle differences are also observed for the O−CH2 (oxymethylene) protons of the copolymerized DSDMA units, but these signals are somewhat broader and harder to interpret since these atoms are closer to the methacrylic polymer backbone and hence less mobile. Presumably, the well-known gauche effect, whereby the dihedral angle for the disulfide bond is almost 90°,44 also causes the adjacent methylene protons of the copolymerized DSDMA units to become particularly sensitive to their specific conformation (i.e., whether present as an intramolecular cycle or as a linear intermolecular branch). 1H NMR spectrum A (see Figure 2) was recorded for the branched copolymer obtained at [MMA]0 = 50 wt % (allowing for the CTA efficiency of 90%, the copolymer composition was PMMA53−DSDMA0.95 when PMMA50−DSDMA0.90 was targeted, see entry 1 in Table 1). This system has two signals due to O−CH2 and S−S−CH2 protons at δ 4.22 and δ 2.92 which are assigned to the
Figure 1. Schematic representation of the microstructure of methacrylic branched copolymers prepared using reversible addition−fragmentation chain transfer (RAFT) chemistry for the statistical copolymerization of methyl methacrylate (MMA) with a disulfidebased dimethacrylate (DSDMA) using cumyl dithiobenzoate (CDB) chain transfer agent and 1,1′-azobiscyclohexanecarbonitrile (ACCN) as initiator in toluene at 90 °C. The subsequent reductive cleavage of these branched PMMAn−DSDMAx copolymers using tributylphosphine (Bu3P) afforded the corresponding PMMAn−TEMAx linear primary chains (where TEMA denotes 2-thioethyl methacrylate).
To a zeroth-order approximation, the primary chains are generated first, with most of the branching being confined to the latter stages of the reaction.35 Thus, the branching efficiency is strongly influenced by the solution concentration of the linear primary chains. Below the critical overlap concentration (c*), intramolecular cyclization becomes favored because the individual polymer coils are effectively isolated from each other. On the other hand, at concentrations well above c*, intermolecular branching becomes favored, since the chains can interpenetrate. This situation has been confirmed by our recent study of a model system, whereby near-monodisperse 2732
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Figure 2. 1H NMR spectra recorded for RAFT-synthesized PMMAn−DSDMAx branched copolymers prepared at [MMA]0 = 50, 30, and 10 wt %, both before (left) and after (right) selective cleavage of all disulfide bonds in the DSDMA comonomer residues using excess tributylphosphine. This chemical degradation affords PMMAn−TEMAx linear primary chains (where TEMA denotes 2-thioethyl methacrylate residues).
Table 1. Summary of the Targeted and Actual Compositions (Calculated by 1H NMR) for a Series of Soluble Branched Copolymers Prepared by RAFT at Various [MMA]0 Using Increasing Amounts of DSDMAa [MMA]0 (wt %)
targeted composition
50 30 10 10 10
PMMA50-DSDMA0.90 PMMA50-DSDMA1.50 PMMA50-DSDMA1.00 PMMA50-DSDMA3.00 PMMA50-DSDMA5.00
1
H NMR calculated composition
Mw (kg mol−1)
Mw/Mn
PMMA53-DSDMA0.95 PMMA56-DSDMA1.70 PMMA55-DSDMA1.15 PMMA56-DSDMA3.43 PMMA55-DSDMA5.68
3325 4509 17 70 609
36.1 21.6 1.7 3.7 13.9
inter (%) 61 36 8 8 10
± ± ± ± ±
4 6 3 2 5
intra (%) 39 64 92 92 90
± ± ± ± ±
4 6 3 2 5
a
The percentage degree of intermolecular branching and hence intramolecular cyclization (denoted inter and intra, respectively) was determined by peak deconvolution of the thiamethylene signal at δ 2.92 in the 1H NMR spectra of the original branched copolymers (see also Figure 4). Molecular weight data were obtained by THF GPC using a triple detection system comprising refractive index, viscosity, and light scattering (15° and 90°) detectors. Final comonomer conversions in these branched copolymer syntheses ranged between 96% and 99%.36
larger O−CH3 signal at δ 3.6 confirmed that these two signals are equivalent to ∼1.70 DSDMA units per primary chain. Moreover, the additional complexity (see signals a′ and b′) in the 1H NMR spectrum of this branched copolymer correspond to around 66% of the integrated intensity of signals a and b. This suggests that there are ∼0.50 DSDMA units per primary chain in the form of an intermolecular branch and a further 1.2 DSDMA units per primary chain present as intramolecular cycles. Finally, inspection of 1H NMR spectra C, D, and E recorded for the branched copolymers (see entries 3−5 in Table 1) synthesized at [MMA]0 = 10 wt % confirmed substantial complexity and line broadening for the O−CH2 and S−S−CH2 signals at δ 4.1−4.5 (labeled a, a′) and δ 2.9−3.2 (labeled b, b′). Comparing these two features with that of the O−CH3 signal at δ 3.6 indicated that up to 5.68 DSDMA units can be incorporated per primary chain without causing gelation, as shown in spectrum E.
intermolecular branch form of the DSDMA in the copolymer structure: these structures are labeled a and b, respectively, in Figure 2. These initial assignments were confirmed by comparison with 1H NMR spectra recorded for both the DSDMA comonomer (see spectrum D in Figure 3) and also the soluble fraction extracted from a DSDMA homopolymerization by RAFT that had been quenched at low conversion (see spectrum C in Figure 3). Similarly, 1H NMR spectrum B in Figure 2 was recorded for the branched copolymer prepared at [MMA]0 = 30 wt % (PMMA56−DSDMA1.70, see entry 2 in Table 1). This spectrum also has two signals due to O−CH2 and S−S−CH2 protons at δ 4.22 and δ 2.92, respectively (see signals labeled a and b). However, there is clearly some complexity in these features, as indicated by the additional signals a′ and b′. Integration of signals c and d observed after disulfide cleavage (see spectrum B after Bu3P treatment) and normalizing with respect to the 2733
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deconvolution (see Figure 4), we estimate that ∼61 ± 4% of the DSDMA units react to form intermolecular branches in this case, while the remaining DSDMA units are present as intramolecular cycles (see Table 1). This experimental observation is thus reasonably consistent with Flory−Stockmayer theory, which predicts 50% intermolecular branching and ignores the possibility of any intramolecular cyclization. Experimentally, many research groups have reported that up to approximately one fully reacted divinyl comonomer can be incorporated per primary chain without inducing macroscopic gelation.4,27,35,36,38−41,48,49 This empirical observation seems to be true regardless of whether conventional free radical or living radical polymerization chemistry is utilized, provided that the solution polymerization concentration is well above the critical overlap concentration (c*). Thus, the extent of intramolecular cyclization revealed by 1H NMR spectroscopy in the present work allows the apparent discrepancy between theory and experiment to be reconciled: approximately one fully reacted DSDMA comonomer per primary chain does indeed correspond to the onset of gelation, but only ∼61% of these branching/cross-linking units are actually present in the form of intermolecular branches, with the remaining units forming various types of intramolecular cycles. Peak deconvolution of the complex features based on the known line shapes of signals a and b shown in spectra C, D, and E suggest that more than 90% of the DSDMA units per primary chain are incorporated as intramolecular cycles within the PMMA 55 −DSDMA 5.68
Figure 3. 1H NMR spectra recorded in CDCl3 for (A, B) RAFTsynthesized PMMAn−DSDMAx branched copolymers prepared at [MMA]0 = 50 and 10 wt %, respectively, (C) the soluble fraction extracted from the RAFT homopolymerization of DSDMA, and (D) the DSDMA comonomer alone.
Based on these spectroscopic observations and our previous results reported for this formulation,35,36 our initial hypothesis was that the DSDMA branching comonomer was incorporated exclusively in the form of intermolecular branches under the stated synthesis conditions, with negligible intramolecular cyclization occurring at [MMA]0 = 50 wt % (i.e., spectrum A in Figure 2). However, this did not prove to be the case. In fact, using peak
Figure 4. Partial 1H NMR spectra recorded in CDCl3 at 55 °C show the thiamethylene signals of the copolymerized DSDMA residues for soluble PMMAn−DSDMAx branched copolymers prepared at [MMA]0 = 50, 30, and 10 wt % using RAFT chemistry. The stacked spectra shown on the left correspond to the original partial 1H NMR spectra (black lines) and the corresponding simulated spectra (blue lines; arbitrarily offset vertically above the real spectra) generated from peak-fitting. The stacked simulated 1H NMR spectra shown on the right illustrate the various subpeaks used in each fitting procedure. In each spectrum, the red subpeak is assigned to the intermolecular branched structure, which is significantly reduced relative to the various subpeaks assigned to intramolecular cycles when branching copolymerizations are conducted at lower [MMA]0. 2734
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chemistry as “hyperbranched”.28,33 Based on the results described herein, this term appears to be inappropriate, since essentially all of the “additional” fully reacted DSDMA units in the copolymer chains prepared at lower monomer concentrations are merely present in the form of intramolecular cycles, which make no contribution at all to the degree of branching. Thus, we recommend that the term “hyperbranched” is best avoided when describing such branched vinyl copolymers, unless supported by compelling additional experimental evidence. In retrospect, the choice of MMA as a model monovinyl monomer in our studies was somewhat fortuitous. Almost any other alkyl methacrylate monomer would have had an additional O−CH2 proton signal, which would have likely obscured the relatively weak O−CH2 signal due to the DSDMA comonomer; such overlapping signals would have made our initial 1H NMR peak assignments problematic. Moreover, the unusual sensitivity of the disulfide bond in the DSDMA comonomer to its local environment also played a key role,44 as did the unexpected availability of almost ideal model cyclic compounds in the literature.45,46
branched copolymer synthesized at [MMA]0 = 10 wt %. Thus, only 0.57 fully reacted DSDMA units per primary chain are actually present in the form of intermolecular branches when such branched copolymer syntheses are conducted in dilute solution. This explains the relatively low molecular weight obtained for this branched copolymer (see entry 5 in Table 1) compared to those prepared at 50 or 30 wt % monomer concentration (see entries 1 and 2 in Table 1). In principle, the intramolecular cycles formed by fully reacted DSDMA comonomer can range from a 13-membered cycle (where the pendent vinyl group reacts immediately with the chain end; (i.e., x = 0 in Figure 5) up to relatively large cycles.
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CONCLUSIONS In summary, we have shown for the first time that 1H NMR spectroscopy can be used to distinguish between intramolecular cyclization and intermolecular branching in methacrylic branched copolymers. As expected, intramolecular cyclization becomes much more prevalent for branched copolymers synthesized in dilute solution, with up to 92% of the branching comonomer being consumed in this side reaction. The proportion of intermolecular branching that is observed for copolymer syntheses conducted in more concentrated solution is ∼61 ± 4%, which allows the long-standing apparent discrepancy between theory and experiment to be reconciled. This work is expected to have broad implications for the synthesis of both microgels and gels as well as the growing literature on branched vinyl copolymers. Since intramolecular cyclization can now be explicitly quantified experimentally, our work should also inspire more sophisticated theoretical studies of gelation.
Figure 5. From left to right: (i) fully reacted DSDMA forming an intermolecular branch; (ii) fully reacted DSDMA forming an intramolecular cycle; (iii) a semireacted DSDMA with a pendent vinyl group. Far right: the pendent 2-thioethyl methacrylate (TEMA) residues that are formed after selective cleavage of all the disulfide bonds originally present in the branched copolymer as either intermolecular branches or intramolecular cycles using excess tributylphosphine.
In the latter case, the upper limit ring size corresponds to the mean length of the primary chains (∼55 MMA residues). This rather wide range in ring size explains the complexity of the −O−CH2 and S−S−CH2 NMR signals that are observed in Figure 2. We suspect that our method is relatively insensitive to very large cycles, since these are likely to be spectroscopically indistinguishable from intermolecular branches. Thus, our 1H NMR analysis may only provide a lower limit for the extent of intramolecular cyclization. Although we have focused on RAFT-synthesized branched copolymers in this paper, we have also made similar observations for the equivalent branched copolymers prepared using an ATRP formulation under comparable conditions.50 Moreover, preliminary analysis of our RAFT-synthesized PMMA−DSDMA branched copolymers using quantitative 13C NMR spectroscopy50 indicates similar levels of intramolecular cyclization in each case. Since a delay time of 20 s is required between each scan to ensure full relaxation of the nuclear spins, recording such spectra requires rather long accumulation times (up to 60 h) to achieve sufficiently good signal-to-noise ratios. Nevertheless, the successful demonstration of 13C NMR spectroscopy in this context is important, since in principle this technique offers much better resolution (and hence fewer overlapping signals) than 1H NMR spectroscopy. These additional results will be reported elsewhere in due course. Several research teams have described soluble branched copolymers prepared by such living radical polymerization
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EXPERIMENTAL SECTION
The synthesis of all the branched copolymers and homopolymer reference materials described in this article has been recently reported elsewhere.35 The ACCN initiator was used in conjunction with the CDB chain transfer agent for the RAFT polymerization of MMA at 90 °C since these reaction conditions had been previously shown35 to give high conversions (which are essential for high degrees of branching) and relatively high RAFT agent efficiencies. All 1H NMR spectra were recorded using a 500 MHz spectrometer in CDCl3 at 328 K, and subpeak line shapes were fitted using Topspin 2.1 software (Bruker Biospin). This approach allows deconvolution of the selected area of the spectrum by fitting a Lorentzian function to every subpeak so as to determine their respective areas, their chemical shifts, and widths (in ppm). Our peak deconvolution strategy was as follows. We fitted the relatively well-resolved S−S−CH2 signal at δ 2.92 (see spectrum A in Figure 2) due to intermolecularly reacted DSDMA units first and then used this characteristic Lorentzian peak shape and peak width when fitting the more complex S−S−CH2 signals observed for the various branched copolymers prepared at higher dilution (see spectra B−E in Figure 2). Peak-fitting was conducted so as to reconstruct the original complex spectral envelope using the minimum possible number of subpeaks, with one of these subpeaks always designated as corresponding to the intermolecularly reacted DSDMA units. The envelope shape and the deconvoluted peaks are presented 2735
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in Figure 4; the peaks attributed to the intermolecular branch form of the DSDMA (δ 2.92 for the S−S−CH2 signal) are shown in red. We did not attempt to assign any of the various intramolecular cycles but simply focused on estimating the relative proportions of intermolecular branching and intramolecular cyclization. We estimate that this rather conservative approach introduces an experimental uncertainty of approximately 2−6% in the extent of intermolecular branching presented in Table 1.
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ASSOCIATED CONTENT
S Supporting Information *
1
H NMR spectra (CDCl3) recorded for the cyclic 3methylidene-1,9-dioxa-5,12,13-trithiacyclopentadecane-2,8dione monomer and its copolymer with methyl methacrylate after RAFT ring-opening copolymerization, as reported by Paulusse et al.45,46 This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The Lubrizol Corporation (Hazelwood, UK) is thanked for funding a PhD studentship for J.R. and for permission to publish this research. Mrs. S. Bradshaw, Dr. B. Taylor (U. Sheffield), and Dr. I. Prokes (U. Warwick) are thanked for performing the 1H NMR experiments and for fruitful discussions. Prof. C. J. Hawker and Dr. J. Paulusse are thanked for allowing us to reproduce the 1H NMR spectra of their cyclic disulfide monomer and also the corresponding linear copolymer (see Figure S1 in the Supporting Information). Dr. J. Craven, Dr. A. Hounslow, and Dr. P. Portius (U. Sheffield) are also thanked for fruitful technical discussions.
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NOTE ADDED AFTER ASAP PUBLICATION This article published ASAP on March 13, 2012. Reference 34 has been revised. The correct version posted on March 16, 2012.
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