Article pubs.acs.org/Macromolecules
Influence of Primary Structure on Responsiveness. Oxidative, Thermal, and Thermo-Oxidative Responses in Polysulfides Richard d’Arcy,† Alessandro Siani,† Enrique Lallana,†,‡ and Nicola Tirelli*,†,‡ †
Centre for Injury and Repair, Institute of Inflammation and Repair, University of Manchester, Manchester, M13 9PT, U.K. NorthWest Centre for Advanced Drug Delivery (NoWCADD), Manchester School of Pharmacy, University of Manchester, Manchester, M13 9PT, U.K.
‡
S Supporting Information *
ABSTRACT: Using polymers with a double stimulus responsiveness, we show that primary structure, in addition to chemical composition, can significantly influence the response. This, however, applies to a stimulus of supramolecular nature (temperature) and not to a molecular one (oxidation). In the anionic ring-opening copolymerization of ethylene sulfide (ES) and propylene sulfide (PS), ES polymerizes faster and yields polysulfide chains with a gradient in composition; this primary structure can be varied by modifying the mode of monomer addition. Polysulfides with different primary structures (length of ES sequences) were linked to poly(ethylene glycol) (PEG) in amphiphilic block copolymers, whose (micellar) aggregates in water showed responsiveness to oxidants (solubilization) and gelation upon heating, similar to Pluronics. Both phenomena depended on composition (ES content), but only the second was affected by primary structure (the length of ES sequences), which was attributed to the influence of the latter on supramolecular aggregation. poly(ethylene glycol) (PEG)11 or poly(glycerol methacrylate)12 are introduced as initiators, end-capping agents or produced through sequential polymerization, the resulting amphiphiles are capable of self-assembly, e.g., into vesicular13 or micellar14 structures. Since polysulfide-based colloids solubilize when oxidized (Scheme 1A), responsive actions are possible, e.g., the release of encapsulated actives, which allows them also to be considered among the systems capable to perform inflammation-responsive actions.15,16 In this study hydrogen peroxide (H2O2) was used as an oxidant, since its reactivity with polysulfides is easier to control than that of other compounds: unless very high concentrations are used, H2O2 converts thioethers selectively to sulfoxides, whereas e.g. hypochlorite leads also to sulfones and to polymer chain fragmentation,17,18 and superoxide produces a negligible response, probably due to the scarce solubility of this anion in the hydrophobic polysulfide domains.19 (2) Temperature-sensitive constructs have been produced by combining blocks of polysulfides with blocks of e.g. PNIPAm in amphiphilic copolymeric constructs.20 Here we have used PEG−polysulfide−PEG triblock copolymers (Scheme 1A) that, due to their structural analogy to amphiphilic block polyethers such as Pluronics (PEG− poly(propylene glycol)−PEG triblock polymers), may present a gelation mechanism sensitive to the features of
1. INTRODUCTION “Smart” materials are typically defined by a change in properties in response to an external stimulus (temperature, pH, REDOX potential, magnetic field, etc.); their response may lead to macroscopic (gelation, hardening, solubilization, etc.) or molecular differences (release, spectroscopic changes) that are used in bioimaging, drug delivery and regenerative medicine.1−3 Varying the chemical composition, i.e., the identity of the responsive units, is how most typically these responses are controlled and tuned; for example, the collapse temperature of poly(N-isopropylacrylamide) (PNIPAm) can be altered via copolymerization with other monomers,4,5 or the pH of solubilization of poly(N,N-dialkylamino)ethyl methacrylate) block copolymers can be tuned by the changes in their pKa values induced by different alkyl substituents.6,7 The main point of this study is that, depending on the nature of the response, the spatial organization of the responsive units could be equally important; for example, we have recently shown that the branching slows down the oxidative response.8 Here, we specifically focus on the role of primary structure, i.e., how repeating units distribute along polymer chains, and we have used responses with a molecular (chemical reactivity toward oxidants) or a supramolecular character (temperaturesensitive colloidal self-assembly): (1) As oxidation responsive structures, we have employed poly(alkylene sulfides), hereafter termed polysulfides, whose hydrophobic thioethers can be converted into more polar sulfoxides or sulfones upon oxidation.9 Polysulfides are typically produced via the ring-opening polymerization of episulfides through a living anionic mechanism;10 when hydrophilic structures such as © 2015 American Chemical Society
Received: September 11, 2015 Revised: November 1, 2015 Published: November 10, 2015 8108
DOI: 10.1021/acs.macromol.5b02007 Macromolecules 2015, 48, 8108−8120
Article
Macromolecules
Scheme 1. (A) General Mode of Action of Polysulfide-PEG Block Copolymers (B) Different Modes of Addition of the in Situ Generated Initiator onto Ethylene and Propylene Sulfide and (C) Sketch of How Different Primary Structures May Influence Intermolecular Associationa
a Key: (A) The hydrophobicity of the polysulfide block in PPS−PEG polymers (here described as Pluronic-like PEG−PPS−PEG triblock copolymers) drives the self-assembly in e.g. micellar structures and then possibly in micella gels. The response to oxidants reverses this self-assembly. (B) (S,S'-((ethane-1,2-diylbis(oxy))bis(ethane-2,1-diyl)) diethanethioate) (DOD) was used to produce in situ a dithiol, i.e. a bifunctional initiator, via saponification of its two thioacetate groups; the resulting thiolates act as initiators of the ring-opening episulfide polymerization. Both the thiolates of the initiator and those at the end of growing chains (omitted from the scheme) would react differently with the two monomers employed in this study, ES and PS. Every molecule of ES has two sites of high and equal reactivity, whereas the attack to a PS molecule occurs only at the CH2 and is likely to be hindered by the neighbouring methyl group. (C) The copolymerization of ES (grey) and PS (blue) is likely to create a steep gradient in the primary structure of a polysulfide block: if all monomers are added in a single batch (method [1]), the central part of the block would be predominantly composed by ES, and there ES-mediated aggregation will be favored. If the same amount of monomers is provided in smaller, sequential additions, effectively re-initiating the polymerization each time (method [2]), the gradients and therefore also the possible homosequences would be shortened, also reducing the likelihood of supramolecular homo-ES aggregation. Finally, if ES would be replenished via accurately dosing it throughout the polymerization, it would be possible to further minimize any drift in composition (method [3]).
solvents, whereas poly(ethylene sulfide) (PES) is highly insoluble due to its extremely high crystallinity,25 and even oligomeric ES sequences tend to strongly associate.26 For steric reasons, we expect a different reactivity of ES and PS in their copolymerization (Scheme 1B), and thus gradient polymers to be produced where the chain is enriched in the more reactive ES close to the points of initiation and in the less reactive PS at the termini (Scheme 1C). Several options exist to reduce the extent of composition gradients; for example, monomer
the hydrophobic cores, e.g., their water content and stiffness, as expected for phenomena of micellar crowding.21−23 We have tackled the modulation of polysulfide primary structure through the copolymerization of racemic propylene sulfide (PS) and ethylene sulfide (ES). ES and PS polymerization products have a very different tendency toward aggregation: atactic poly(propylene sulfide) (PPS) is a completely amorphous, low Tg24 polymer soluble in most 8109
DOI: 10.1021/acs.macromol.5b02007 Macromolecules 2015, 48, 8108−8120
Article
Macromolecules
Figure 1. (A) Monomer conversions of PS/ES copolymerizations from various monomer molar feed ratios at different polymerization times. The conversion of ES typically was about 40% after 5 min and approached 60−80% within 15 min of polymerization whereas that of PS was respectively 50% ES content. (C) 1H NMR resonance of the end-capping methyl groups (polymerization time: 2.5 min) depending on the nature of the terminal group, with the peak around 2.17 ppm clearly related to terminal ES units. The peak located at 2.13−2.15 ppm relates to PS units, but it has a fine structure that may reflect a further dependency on the penultimate unit (shoulder/peak at 2.13 ppm possibly related to an ES−PS terminal diad). In the chemical structures the star indicates the location of the polymer chain. (D) Correlation between average ES content in the polymer chain (horizontal axis) and in the terminal groups (vertical axis) in the polymerization kinetics experiments (here showed with Na as a counterion). The line represents y = x and is not a fitting.
evaluated only on other polysulfides obtained via step polymerization).27
mixture can be added sequentially in small portions (hereafter referred to as method 2, opposed to the single-batch method 1). The repeated reinitiation would produce a succession of short blocks with identical ES/PS compositional drift; however, by minimizing the length of these blocks, it would be possible to reduce the average length of the ES sequences, and therefore also the tendency toward ES-based association. A third and more effective option is the dosing of only the fastest reacting monomer, aiming to a quasi-constant monomer ratio in the feed (method 3). Here we have compared methods 1 and 2, in order to establish whether these relatively small changes in the gradient composition may yet have measurable effects on the responsive behavior of polysulfides. Along the way we have also evaluated the reactivity ratios for the PS/ES copolymerization, which to our knowledge is the first case of such calculations applied to the episulfide polymerization (reactivity ratios have been
2. RESULTS AND DISCUSSION 2.1. ES and PS Copolymers: Reactivity Ratios and Primary Structure. In comparison to PS, ES is likely to react faster with a growing, thiolate-ending chain, since it has a higher number (2) of accessible electrophilic sites, and their steric hindrance is lower than that of PS. Moreover, the resulting ESterminating growing chain would react more rapidly than a PSterminating chain due to the lower steric hindrance of a primary vs a secondary thiolate; this would accelerate the polymerization and thus further contribute to a preferential ES consumption. Indeed we have confirmed that ES polymerized faster than PS, with a higher conversion than PS at any time point and specifically in the early stages of the polymerization (Figure 1A). Consequently, the copolymers produced at low monomer conversion were always considerably richer in ES 8110
DOI: 10.1021/acs.macromol.5b02007 Macromolecules 2015, 48, 8108−8120
Article
Macromolecules
Table 1. Summary of the Chemical Composition for PS/ES Statistical Copolymers (150 s Polymerization, Followed by EndCapping with Methyl Iodide) PS/ES feed molar ratio
counterion
PS/ES degree of polymerizationa (molar composition)
PS/ES terminal unit compositionb
average monomer conversion (mol %)c
mass recovery (wt %)d
95/5 90/10 85/15 75/25 65/35 95/5 90/10 85/15 75/25 65/35
Na+ Na+ Na+ Na+ Na+ DBUH+ DBUH+ DBUH+ DBUH+ DBUH+
46.0/4.4 (91/9) 42.3/9.4 (82/18) 51.2/19.2 (73/27) 40.1/29.2 (58/42) 32.4/38.8 (46/54) 61.7/11.9 (84/16) 53.3/20.6 (72/28) 44.5/29.0 (61/39) 35.6/41.0 (46/54) 31.3/53.8 (37/63)
88/12 81/19 74/26 59/41 50/50 85/15 62/38 54/46 46/54 40/60
10.1 10.3 14.1 13.9 14.2 14.7 14.8 14.7 15.3 17.0
95.6 93.9 93.9 96.4 97.0 93.6 100.0 89.0 87.0 90.0
a The degree of polymerization per arm is calculated using the resonance of the PPS methyl peak at 1.34−1.45 ppm, or of the PES peak (−S−CH2− CH2−S−) at 2.74−2.82 ppm, both normalized against the initiator peak at 3.61−3.69 ppm (−S−CH2−CH2−O−CH2−CH2−O−CH2−CH2−S−). b Calculated from the ratio of the resonance peaks at 2.13 and 2.15 ppm, which are respectively associated with ES and PS terminal units. The endcapping was always quantitative, with a yield >99% as determined by the comparison of the integral of the combined (ES and PS) terminal methyl unit peaks and that of the initiator peak. cCalculated through the comparison of the total monomer (ES + PS)/initiator molar ratio in the polymer and in the feed. dMass of polymer recovered divided by the theoretical mass of the polymer as determined using the composition measured by 1H NMR (based on the DPs and end-capping yields calculated) × 100.
Information, Table 1SI and Figures 1SI and 2SI). This analysis provided a coherent picture of a strongly nonideal copolymerization (Table 2), with rES always >2, i.e., the tendency of
than the feed (Figure 1B and Table 1, third column). A higher enrichment was detected when protonated DBU (DBUH+) was used as the counterion of the propagating thiolates instead of sodium (compare the first five rows with the second five rows in Table 1). Because of the virtual absence of termination and chain transfer reactions, this implies that polymer chains produced at high conversions would present a strong gradient in composition; using a bifunctional initiator such as DOD (Scheme 1B), most ES would be localized proximal to the initiator, i.e., in the central portion of the polysulfide chain, while most PS would be found closer to the end-capping groups, i.e., in the two terminal areas. Although several methods can be used to calculate reactivity ratios at high conversions,28 more accurate results can be obtained when the conversions are sufficiently low to allow an approximation of the instantaneous composition with the average polymer composition. We have verified this assumption using polymers produced at moderate, 10−20%, conversion (Table 1, fifth column) and comparing the composition of terminal groups with that of the whole macromolecular chains. When growing polysulfide chains were end-capped with methyl iodide, the 1H NMR resonance of the terminal methyl groups differed depending on whether it had reacted with a primary thiolate, i.e., an ES unit, or a secondary one, i.e., a PS unit (Figure 1C). It is thus possible to estimate the molar ratio between ES- and PS-terminating monomer units at the moment of end-capping, which by definition is the instantaneous polymer composition at that time. The substantial agreement between these data and the average polymer composition (Figure 1D) allowed to conclude, first, that the PS/ES ratio underwent no significant drift using 150 s as the polymerization time, and thus, second, that the reactivity ratios can be reliably calculated on the basis of the average polymer compositions. In terms of reactivity ratios, rPS indicates the relative preference of a PS-terminating macromolecule to react with PS vs ES, and rES the corresponding parameter for a ESterminating growing chain, and we have applied the Finemann−Ross29 (F-R) and Kelen−Tüdös30 (K-T) methods for their calculation (see section 2 of the Supporting
Table 2. Summary of the Monomer Reactivity Ratios Calculated through the Finemann−Ross (F-R) and Kelen− Tüdös (K-T) Models and the Corresponding Number Average Lengths of ES and PS Sequences average sequence lengtha 1:1 PS/ES rPS
rES
F-R K-T
0.58 0.52
3.11 2.84
F-R K-T
0.26 0.24
2.72 2.29
rPSrES
nPS
Na+ Counterion 1.81 2.72 1.46 2.93 DBUH+ Counterion 0.70 3.59 0.54 3.01
3:1 PS/ES
nES
nPS
nES
4.11 3.83
2.04 1.94
2.75 2.55
7.89 8.67
1.43 1.39
1.86 1.67
a
Number-average sequence lengths were calculated for 50:50 and 75:25 PS/ES molar ratios, under the assumption of a much larger degree of polymerization. For the calculations, see the Supporting Information, section 3, and eqs 1 and 2 therein.
ES homopropagation, and rPS always
