Synthesis of Polystyrene-Based Hyperbranched Polymers by Thiol

Sep 30, 2014 - The first part of this report concentrates on studying the effects upon the thiol–yne photopolymerization process caused by the varia...
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Synthesis of Polystyrene-Based Hyperbranched Polymers by Thiol− Yne Chemistry: A Detailed Investigation Raphael Barbey and Sébastien Perrier* Key Centre for Polymers & Colloids, School of Chemistry, Building F11, University of Sydney, Sydney, NSW 2006, Australia S Supporting Information *

ABSTRACT: This contribution reports on the preparation of hyperbranched polymers via thiol−yne photopolymerization of macromolecules bearing a thiol group at one of their chain end and an alkyne moiety at the other. These thiol/yne macromonomers were produced by the reversible addition− fragmentation chain transfer (RAFT) polymerization of styrene mediated by an alkyne-containing chain transfer agent, followed by the aminolysis of the trithiocarbonate groups into thiol moieties. The first part of this report concentrates on studying the effects upon the thiol−yne photopolymerization process caused by the variation of several experimental conditions, such as the size of the thiol/yne macromonomer utilized, the amount of photoinitiator added, or the initial concentration of macromonomer used. Among the various parameters investigated, the latter was demonstrated to be the leading factor that contributes to the final size of the hyperbranched structures. The second part of this article discusses the multidetector size-exclusion chromatography (SEC) analysis of the materials obtained by thiol−yne photopolymerization. This analytical technique allows the molecular weight of the hyperbranched polymers to be determined and the hyperbranched topology to be confirmed, as highlighted by the increased density of the materials compared to that of linear equivalents of the same chemical composition and identical molar masses.

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INTRODUCTION Dendritic polymers, such as dendrimers and hyperbranched polymers, are characterized by an architecture consisting of a globular shape, a highly branched topology, and a high density of functional groups on their periphery.1−3 These specific structural features give dendrimers and hyperbranched polymers markedly different properties compared to linear analogues of equivalent molar masses and make them exciting materials for a wide variety of applications,4−17 including in nanomedicine, (bio)sensing, light-harvesting, or catalysis. The main difference between dendrimers and hyperbranched polymers resides in the degree of structural control present in their respective architecture. Dendrimers are per definition organized as perfectly uniform, highly symmetrical structures. However, such faultless topologies only come at the price of tedious, generational (either divergent or convergent) synthetic approaches, which often limit their commercial production.6,18−22 Hyperbranched polymers are usually described as attractive alternatives to dendrimers. Although being irregularly structured, with their branching points randomly distributed, hyperbranched polymers possess most of the typical dendrimer features, such as a high degree of branching and a high density of peripheral functional groups. Moreover, compared to dendrimers, their preparation is nearly effortless and is usually performed in a one-step process using simple and efficient reactions, such as self-condensing vinyl polymerization of AB* © 2014 American Chemical Society

inimers (where A is a vinyl group and B* is an initiating center) or step-growth polymerization of ABx (macro)monomers (where A and B are moieties that selectively react with each others, and x ≥ 2).4,5,23−32 The use of ABx macromonomers (rather than the commonly utilized small ABx molecules) to produce hyperbranched polymers is particularly interesting as it offers the possibility to influence the spacing between branching points in the hyperbranched structures by finetuning the architecture of the macromonomers.27−32 For example, Hutchings and co-workers used functionalized AB2 polystyrene macromonomers produced by anionic polymerizations to generate hyperbranched polymers, which they refer to as “HyperMacs”, via one-pot polycondensation reactions.27,28 The same group successfully transferred their methodology to other macromonomers.33 The radical-mediated addition of thiols to alkyne moieties (thiol−yne) proceeds via the addition of thiyl radicals to unsaturated carbon−carbon bonds. The reaction starts with the addition of a thiyl radical to an alkyne, thus forming a carboncentered radical vinyl sulfide. Following abstraction of an atom of hydrogen from another thiol (which consequently produces a thiyl radical), the vinyl sulfide species can undergo a second Received: April 4, 2014 Revised: August 29, 2014 Published: September 30, 2014 6697

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Table 1. Quantitative Information for the Preparation of Polystyrene by Reversible Addition−Fragmentation Chain Transfer (RAFT) Polymerization styrene PS10b PS22b PS53b

CTAa

AIBN

[g]

[mmol]

[g]

[mmol]

[g]

[mmol]

[S]:[AIBN]:[CTA]

8.009 10.01 10.01

76.90 96.11 96.11

0.0841 0.0393 0.0085

0.5121 0.2392 0.0518

1.418 0.6762 0.1468

5.130 2.446 0.5310

15.0:0.1:1.0 39.3:0.1:1.0 181.0:0.1:1.0

a CTA: chain transfer agent, prop-2-yn-1-yl 2-(((butylthio)carbonothioyl)thio)propanoate. bNumber-average degree of polymerization approximated based on the respective number-average molar mass (see Table 2).

thiyl radical addition to generate a dithioether compound.34−37 The thiol−yne reaction proved to be a powerful tool for the preparation of hyperbranched polymers, as a branching point is generated upon every addition of two thiols to one alkyne moiety. This approach, pioneered by our group in 2009,38 consists in the photopolymerization of (macro)molecules bearing a thiol group at one end and an alkyne moiety at the other. Such thiol/yne (macro)monomers are equivalent to the AB2 (macro)monomers classically used in the production of hyperbranched polymers via step-growth polymerization routes, with the thiol representing the A unit and each of the π bonds of the alkyne the B units. Hyperbranched polymers have been produced by this means from small thiol/yne molecules,38−40 from thiol/yne-functional polystyrene,38,41 poly(dimethyl acrylamide)-b-polystyrene42 and poly(tert-butyl acrylate)-b-polystyrene42 block copolymers, and poly(styrene-co-3-isopropenylα,α-dimethylbenzyl isocyanate)41 statistical copolymers as well as thiol/yne-functionalized poly(ε-caprolactone)43 and poly(εbenzyloxycarbonyl-L-lysine).44 Moreover, each hyperbranched structure obtained by thiol−yne reaction exhibits numerous peripheral alkyne groups as well as one thiol focal point that can both be utilized as chemical handles for further postpolymerization modification reactions.41,44,45 Therefore, these topologies can be envisioned as a platform with almost endless functionalization possibilities. For these materials to reach their full potential, a clear understanding of the effects of reaction parameters on the synthesis is needed. The main objective of this contribution was to explore the influence of several reaction parameters over the final dendritic structures of hyperbranched polymers produced by the thiol−yne approach. To this end, a series of thiol/yne polystyrene macromonomers with different molar masses were prepared. Reversible addition−fragmentation chain transfer (RAFT)46−48 polymerizations of styrene mediated by an alkyne-containing chain transfer agent, followed by the reduction of the trithiocarbonate moieties located at the ωchain ends into thiols with a large excess of isopropylamine, led to the formation of the macromonomers. The UV irradiation of mixtures containing these thiol/yne building blocks and a small amount of photoinitiator readily produced hyperbranched polymers. The first part of this article concentrates on studying the kinetic of the thiol−yne photopolymerization of each of the thiol/yne polystyrene macromonomers and discusses how the variation of reaction parameters, such as the photoinitiator or thiol/yne macromonomer concentrations, affects the ultimate structure of the synthesized hyperbranched polymers. Among all the reaction parameters studied, the initial concentration of the thiol/yne macromonomer solution was demonstrated to influence the most the final topology of the dendritic structure. Therefore, the second part of this report exclusively focuses on the in-depth analyses by multiple-detector size-exclusion chromatography (SEC) of hyperbranched polystyrene pro-

duced from various concentrations of thiol/yne polystyrene macromonomers.

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EXPERIMENTAL SECTION

Materials. Unless stated otherwise, all chemicals were obtained from commercial suppliers and were used as received. Ultrahigh quality water with a resistance of 18.2 MΩ·cm (at 25 °C) was obtained from a Millipore Milli-Q Plus water purification system fitted with a 0.22 μm filter. Styrene (S) was freed from its inhibitor (4-tertbutylcatechol) by passing the monomer through a column of activated, basic aluminum oxide. 2,2′-Azobis(2-methylpropionitrile) (AIBN) was recrystallized from methanol. The alkyne-containing chain transfer agent, prop-2-yn-1-yl 2-(((butylthio)carbonothioyl)thio)propanoate, was synthesized from 2-(((butylthio)carbonothioyl)thio)propanoic acid (received from DuluxGroup Australia) and 2-propyn-1-ol following a published protocol.41 Analytical Methods. NMR spectra were recorded at 300 K either on a Bruker AVANCE UltraShield 300 (300 MHz) or on a Bruker AVANCE III Ascend 500 (500 MHz) spectrometer with a delay time (d1) set to 10 s. Chemical shift values (δ) are reported in ppm. The residual proton signal of the NMR solvent was used as internal standard (CDCl3: δH = 7.26 ppm). Size-exclusion chromatography (SEC) analyses were performed on a Polymer Laboratories PL-GPC 50 instrument equipped with a PL-AS RT autosampler, a PL-RI differential refractive index (DRI) detector, a PL-BV 400 viscometer, and a Precision Detectors PD2020 light scattering detector. The system was fitted with a PolarGel-M guard column (50 × 7.5 mm) and two PolarGel-M analytical columns (300 × 7.5 mm). N,NDimethylformamide (DMF) containing 0.04 g L−1 hydroquinone and 0.1 wt % lithium bromide was used as the mobile phase, eluting at 0.7 mL min−1 at 50 °C. Analyte samples were dissolved in DMF containing 0.04 g L−1 hydroquinone and 1.15 wt % water (flow rate marker). Prior to injection, the samples were filtered through PTFE membranes (0.45 μm pore size). The molar masses of the polymers were determined using Cirrus GPC software by either a conventional calibration obtained from polystyrene standards ranging from 6.82 × 102 to 1.67 × 106 g mol−1 or by combining the multiangle light scattering (MALS) detector response with that of the concentrationsensitive detector (DRI). For this purpose, the system was calibrated with a narrow polystyrene standard of 215 kg mol−1, which has a refractive index increment dn/dc of 0.197 in DMF at 50 °C.49 The dn/ dc value for hyperbranched polystyrenes (0.1883) was determined experimentally from the slope of the plot representing the area under the DRI detector response as a function of four samples of known concentrations, as detailed in Figure S1. The parameters of the Kuhn− Mark−Houwink−Sakurada relation, K and α, for linear polystyrene in DMF at 50 °C were obtained from the slope and intercept of a doublelogarithmic plot of the intrinsic viscosity as a function of the molecular weight (Figure S2). The UV source used for the thiol−yne photopolymerizations was a Spectroline ENF-280C/FE lamp operating at 365 nm. Preparation of Polystyrene (PS) via Reversible Addition− Fragmentation Chain Transfer (RAFT) Polymerization. Polystyrene samples were produced in bulk according to the following general procedure. Inhibitor-free styrene (S) was added to 20 mL vials containing a stirrer bar, AIBN, and the chain transfer agent (CTA) prop-2-yn-1-yl 2-(((butylthio)carbonothioyl)thio)propanoate to give 6698

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Scheme 1. Preparation of Thiol/Yne Polystyrene Macromonomersa

a

AIBN: 2,2′-azobis(2-methylpropionitrile); THF: tetrahydrofuran; RT: room temperature.

initial molar ratios [S]:[AIBN]:[CTA] as tabulated in Table 1. The vials were sealed with a septum and placed in an ice bath, and the yellow solutions were bubbled with nitrogen for 15 min. The polymerizations were carried out at 65 °C for 20 h, after which the polymerization mixtures were cooled down to room temperature and aerated. Aliquots were taken to calculate the conversion by 1H NMR and gravimetric analysis, and the remainder of the polymer mixtures was dissolved in dichloromethane and precipitated into 500 mL of icecold methanol to yield yellow powders. After being dried overnight in a vacuum oven at 50 °C, the polymers were analyzed by size-exclusion chromatography and 1H NMR spectroscopy. Preparation of Thiol/Yne Polystyrene Macromonomers (PSSH) via Aminolysis. A vial containing a 0.05 M solution of polystyrene in THF was capped with a septum and placed in an ice bath, and the mixture was bubbled with nitrogen for 15 min. Isopropylamine was deoxygenated separately by bubbling nitrogen through the solution for 15 min. To the polymer mixture was added ∼40 equiv (relative to the number of moles of the Z-group) of isopropylamine via syringe. The solution was allowed to stir for 20 h before the aminolyzed PS was precipitated in ice-cold methanol. The polymer was filtrated and washed twice with cold methanol to remove any residual isopropylamine or aminolysis byproducts (i.e., butyl isopropylcarbamodithioate, 1,3-diisopropylthiourea, or butanethiol). After drying overnight in a vacuum oven at 50 °C, the PS-SH samples were recovered as white powders. The polymers were analyzed by sizeexclusion chromatography and 1H NMR spectroscopy. General Photopolymerization Procedure. Aminolyzed polystyrene (PS-SH) was added to a 1.5 mL glass vial containing the photoinitiator 2,2-dimethoxy-2-phenylacetophenone (DMPA). To this vial, a solution of DMF/toluene (4/1, v/v) was added in order to obtain the targeted concentration. The flask was wrapped in aluminum foil and capped with a septum. Once a homogeneous solution was obtained, the aluminum foil was removed and the reaction mixture was placed under UV irradiation at a distance of 5 cm to the source at room temperature. After a predefined amount of time, the photopolymerization was stopped. A small quantity of the solution was used without purification for size-exclusion chromatography analysis, while the remainder of the solution was diluted with dichloromethane and precipitated into ice-cold methanol. The hyperbranched polystyrene (hbPS) samples were recovered by centrifugation to yield white to offwhite powders and were characterized by size-exclusion chromatography.

Table 2. Conversion, Number-Average Molar Masses, and Dispersity Values before and after Aminolysis

PS10c PS22c PS53c

conva [%]

Mn,beforeb [g mol−1]

Đbeforeb

Mn,afterb [g mol−1]

Đafterb

91.2 69.9 36.9

1300 2510 5640

1.13 1.14 1.12

1330 2520 5720

1.13 1.14 1.15

After 20 h of polymerization at 65 °C as determined by 1H NMR. Number-average molar mass and dispersity obtained from conventional calibration with polystyrene standards. cNumber-average degree of polymerization approximated based on the respective numberaverage molar mass. a b

by reducing the trithiocarbonate moieties with a large excess (∼40 equiv relative to the number of polymer chains) of isopropylamine in THF for 20 h. The thiol/yne macromonomers thus produced showed molar masses and dispersity values nearly identical to that of the precursor polystyrene (Table 2). In addition, the comparison of the size-exclusion chromatograms before and after aminolysis clearly indicated that the formation of disulfide bonds between polymeric chains did not occur to a noticeable extent (Figure S3). The successful aminolysis of the polymers was ascertained by 1H NMR (Figures S4−S6) with the disappearance of the signal at 3.3 ppm due to the two protons adjacent to the trithiocarbonate group and the appearance of a new, broad signal at 3.5 ppm (corresponding to the first proton of the polymer backbone next to the thiol group). Hyperbranched Polystyrene by Thiol−Yne Photopolymerization. Hyperbranched polystyrenes were synthesized from thiol/yne polystyrene macromonomers via thiol− yne reaction following the general procedure depicted in Scheme 2. The thiol−yne reaction proceeds with the radical-mediated addition of thiols to alkyne moieties via a two-step mechanism. First, a thiol is added to an alkyne moiety to produce a vinyl sulfide. This intermediate can then react with another thiol to yield a dithioether compound. A branching point is therefore generated upon completion of each thiol−yne reaction. Moreover, the use of bifunctional macromonomers during the process implies that the branched structure obtained after each cycle becomes a new, larger thiol/yne macromonomer, which can participate in a subsequent thiol−yne reaction. Ideally, each generation of branched polymers contains 2n+1 − 1 of the original, linear thiol/yne macromonomers and possesses one single thiol focal group and 2n alkyne moieties (where n is the “generation”). However, it can be anticipated that this process is subject to some side reactions. Bowman and co-workers showed that with stoichiometrically balanced mixtures of thiols and alkynes (i.e., two thiols per alkyne) the monohydrothiolated compounds (vinyl sulfides) were consumed almost immediately upon formation.34 This infers that mostly only unreacted (i.e., alkynes) and fully reacted (i.e., dithioethers) species are present during the

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RESULTS AND DISCUSSION Preparation of Thiol/Yne Polystyrene Macromonomers. The thiol/yne polystyrene macromonomers used in this study for the preparation of hyperbranched polymers by thiol− yne photopolymerizations were produced following a two-step strategy (Scheme 1). The first step involved the reversible addition−fragmentation chain transfer (RAFT) polymerization of styrene in bulk at 65 °C, using 2,2′-azobis(2-methylpropionitrile) (AIBN) as initiator and prop-2-yn-1-yl 2-(((butylthio)carbonothioyl)thio)propanoate as chain transfer agent. The use of this alkyne-containing chain transfer agent to mediate the polymerization allowed the synthesis of alkyne-functionalized polystyrene (PS) samples with well-defined molar masses and narrow dispersity values (Table 2). The thiol groups were generated at the ω-chain end of the polymers in a second step, 6699

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The hyperbranched polymers examined in this study were generated at room temperature via the UV irradiation at a wavelength of 365 nm of solutions of thiol/yne polystyrene macromonomers and a photoinitiator (2,2-dimethoxy-2-phenylacetophenone, DMPA) in a N,N-dimethylformamide (DMF)/toluene (4/1, v/v) solvent mixture. In a first set of experiments, the effect of the size of the thiol/ yne macromonomer on the thiol−yne process, as well as on the final dendritic structure, was studied. To this end, the thiol−yne photopolymerization was conducted on 0.15 M mixtures of thiol/yne polystyrene macromonomers of different numberaverage molar masses (i.e., 1300, 2500, and 5700 g mol−1) in the presence of ∼0.5 equiv (relative to the number of polymeric chains) of DMPA. Figure 1 shows the size-exclusion chromatograms, which are reported as the normalized differential refractive index (DRI) detector response as a function of the retention time, that result from the irradiation of the three different macromonomer solutions for various amounts of time. The rapid decrease upon UV irradiation of the signals attributed to the original thiol/yne macromonomers (black curves), as well as the broadening and the shift to smaller retention times of the DRI detector response traces with increasing reaction times, clearly indicate the successful production of high molar mass material. The evolution in the size-exclusion chromatograms is nearly identical for all three thiol/yne macromonomer solutions, with the continuous growth of the dendritic structures in the first 4 h. After that, no further changes in the DRI detector response were observed. Interestingly though, the results obtained upon irradiation of the smallest thiol/yne macromonomer (Figure 1A) slightly deviate from this general trend, with the appearance of a new set of hyperbranched structures at low retention time after 2 h of irradiation. Similar observations were made by Hutchings.28 The emergence of these large structures (at a retention time of 18.2 min) arises from the reaction of smaller hyperbranched structures (with retention times located between 19 and 21 min) together and confirms the step-growth nature of the thiol−yne process. This phenomenon also suggests that the diffusion of reactive species within the solution is slightly improved in this case compared to the other two reactions. Indeed, the diffusion of larger reactive species is expected to be slowed down through the more viscous solutions, which as a consequence limits the growth of the hyperbranched structures. This diffusion limitation was further evidenced with experiments involving an even larger (approximately 15 000 g mol−1) thiol/yne polystyrene macromonomer (Figure S7). In this instance, the UV irradiation of concentrated solutions of the macromonomer did not produce any appreciable amount of hbPS structures. It is worth noting that an increased amount of intramolecular cyclization due to the longer chain length could also be responsible for such behavior, but one would have then expected a shoulder at higher retention times. The size-exclusion chromatograms in Figure 1 also suggest the presence of a residual amount of the initial thiol/yne macromonomers even when the thiol−yne reaction is carried out for up to 8 h. The depletion of the amount of radicals generated by the irradiation of the photoinitiator as the reaction proceeds was envisioned as a potential cause for this observation. Therefore, a new series of experiments were performed to better understand the role played by the photoinitiator concentration on the thiol−yne photopolymerization. Figure 2 represents the size-exclusion chromatograms

Scheme 2. Simplified Mechanism for the Preparation of Hyperbranched Polymers from Thiol/Yne Macromonomers via the Thiol−Yne Processa

a

The red spheres represent either linear or branched structures, as the reaction not only involves the starting, linear thiol/yne macromonomers but also the branched thiol/yne macromonomers obtained as products from previous iterations of the thiol−yne process.

reaction. Conversely, with stoichiometrically unbalanced mixtures exhibiting an excess of alkyne groups (i.e., [alkyne] > 2[SH]), the authors observed a substantial accumulation of vinyl sulfides.34 This observation is of particular interest for the present study. Indeed, thiol−yne reactions using thiol/yne macromonomers are inherently thiol-limited, with a ratio between thiols and alkynes at the beginning of the reaction equal to 1 (i.e., [alkyne] = [SH]). The excess of alkyne groups is even further reinforced (i.e., [alkyne] > [SH]) over the course of the reaction as the depletion of thiol groups is faster than the consumption of alkynes during the thiol−yne process, which suggests that, unlike previously thought,38 the amount of vinyl sulfides introduced within the hyperbranched structures is not negligible. While the dihydrothiolated species are representative of branching units, the monohydrothiolation products can be described as the linear units. Therefore, the anticipated accumulation of such semireacted products over the course of the thiol−yne photopolymerization would result in decreasing the degree of branching of the hyperbranched structure. Nevertheless, although this is not the subject of the present study, it is interesting to mention that the presence of vinyl sulfide within the hyperbranched structure could also be turned to an advantage, as these functional groups could be used as chemical handles for further postpolymerization modification reactions (such as radical-mediated thiol−ene reactions or Michael additions). Another side reaction of the thiol−yne process that can be envisioned while using thiol/yne macromonomers resides in the intramolecular cyclization that could occur via addition of the thiol focal point to a peripheral alkyne or internal vinyl sulfide of the same molecule during the reaction. This would further decrease the amount of thiols available for subsequent additions and therefore strengthen the thiol/yne stoichiometric disparity, thus definitely hampering the thiol−yne process. Finally, it is worth noting that, although only the βmonosubstituted and the α,β-disubstituted adducts are represented in Scheme 2, the thiol−yne process could also give rise to α-monosubstituted, as well as α,α- and β,βdisubstituted products, depending on the local electronic environment and/or steric hindrance.37 6700

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Figure 2. Size-exclusion chromatograms (normalized DRI detector response versus retention time) resulting from the UV irradiation of solutions containing a thiol/yne polystyrene macromonomer (2500 g mol−1) and different amounts of photoinitiator (ranging from 0 to ∼3 equiv relative to the number of polymeric chains). Reaction conditions: 0.15 M solutions, DMF/toluene (4/1, v/v), 2 h of UV irradiation.

occur and thus confirms that an external radical source is required to induce the process. The results displayed in Figure 2 also indicate that, above a certain concentration threshold (between 0.1 and 0.25 equiv), the amount of photoinitiator has only a limited influence on the hyperbranched structure produced within the time frame of the reaction. The size and topology of the hyperbranched structures obtained after 2 h of reaction are independent of the amount of photoinitiator used during the thiol−yne photopolymerization. Also, the sizeexclusion chromatograms clearly show that the signal attributed to the linear thiol/yne macromonomer does not totally vanish, independently of the photoinitiator concentration. The same observation was made from experiments that involved the addition of a second dose of ∼0.5 equiv of DMPA after the first hour of reaction (Figure S8). Together with the diffusion limitations and cyclization side reactions discussed previously, the structure of the thiol/yne macromonomer itself could explain the remaining material observed despite relatively long irradiation times (up to 8 h, Figure 1B) or the use of large quantities of photoinitiator (up to ∼3 equiv, Figure 2). Indeed, it can be assumed that not every single thiol/yne macromonomer possess both the alkyne and thiol functional end-groups that are needed for an effective thiol−yne reaction. This imperfect end-group retention is inherent to the RAFT process and can become particularly important for slow-propagating monomers (such as styrene), which require long polymerization times. In the present case, after 20 h of polymerization at 65 °C, we can assume from statistical analysis that approximately 86.5% of the chains contain both the thiol and alkyne functionalities, 6.5% only the thiol, 6.5% only the alkyne, and 0.5% of the polymer chains do not contain any of the functionalities. The absence of one or both of the functionalities would undoubtedly plague the thiol− yne reaction. In addition to the size of the thiol/yne macromonomer, the reaction time, and the amount of photoinitiator discussed in the previous sections (vide supra), the effect on the thiol−yne process of varying the initial concentration of the thiol/yne macromonomer has also been explored. To this end, the thiol−

Figure 1. Size-exclusion chromatograms (normalized DRI detector response versus retention time) as a function of irradiation time for the thiol−yne photopolymerization of thiol/yne polystyrene macromonomers (PS-SH) with an Mn of 1300 (A), 2500 (B), and 5700 g mol−1 (C). Reaction conditions: 0.15 M solutions, DMF/toluene (4/ 1, v/v), ∼0.5 equiv of DMPA.

obtained UV irradiation of 0.15 M solutions of PS22-SH containing various amounts of DMPA. The data in Figure 2 show that, in the absence of photoinitiator, the thiol/yne macromonomer remains stable upon irradiation, with the exception of a slight broadening of the peak, which is attributed to the formation of a small amount of disulfide bridges between polymer chains. This indicates that the UV irradiation of the macromonomer did not directly generate (enough) thiyl radicals for the thiol−yne reaction to 6701

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analysis and that the chromatograms obtained were fully representative of the injected samples. A relatively straightforward method to prove that the hyperbranched structures remained intact during the separation is to reduce the shear forces that are applied onto the samples by decreasing the flow rate at which the size-exclusion analyses are performed. The analysis by size-exclusion chromatography of the largest hyperbranched polymer produced in this study has therefore been performed at a reduced flow rate of 0.3 mL min−1 and has been compared to the data obtained at the usual flow rate of 0.7 mL min−1. The normalized responses of the DRI detector are reported for both flow rates as a function of elution volume (Figure S9). Plotting the detector signal as a function of elution volume, rather than retention time, is necessary for the direct comparison of size exclusion chromatograms generated at different flow rates. The nearly perfect superimposition of the traces undeniably confirms that the hyperbranched polymers reported in this article did not suffer from any shear degradation under the standard size-exclusion analyses flow rate of 0.7 mL min−1. Multiple-Detector Size-Exclusion Chromatography of Hyperbranched Polystyrene. The analysis of hyperbranched polymers by multidetector size-exclusion chromatography (SEC) offers opportunities to gather information that cannot be obtained when only a concentration-sensitive detector (such as differential refractive index detectors) is used.24,51 Because dendritic structures possess hydrodynamic volumes that are smaller than their linear counterparts of the same molecular weight,24 the molar masses of hyperbranched polystyrene cannot be accurately determined from a conventional calibration curve constructed from linear polystyrene standards, as this would induce a significant underestimation of the molar mass values. This drawback can be overcome by the use of multidetector SEC. Indeed, SEC systems with both concentration-sensitive and multiangle light scattering (MALS) detectors allow the absolute molar masses to be determined without even the need of a calibration curve.51 Figure 4 represents the response of several detectors (differential refractive index, multiangle light scattering, and viscometer) as a function of the retention time for the hyperbranched polystyrene obtained after 24 h irradiation of a 0.3 M solution of the 2500 g mol−1 thiol/yne macromonomer in DMF/toluene (4/1, v/v). The size-exclusion chromatograms for the hyperbranched polymers generated upon irradiation of solutions of different macromonomer concentrations are presented in the Supporting Information (Figures S10−S12). The data collected by the light scattering detector at both 15° and 90° angles, in combination with the response of the concentration-sensitive (DRI) instrument, allow the calculation of the molecular weight of the hyperbranched polymers produced by thiol−yne photopolymerization for each “slice” of retention time. Given the discrepancy in sensitivity between the concentration-sensitive refractometer and the molecularweight-sensitive light scattering detector (which is more sensitive for high molecular weights, but less sensitive for low molecular weights, than the refractometer), molar masses determined for both the highest and lowest retention times systematically deviate due to poor signal-to-noise ratios.52,53 To avoid introducing systematic errors for these specific retention times, and thus to obtain reliable results, the molar masses were extrapolated from the section of the size-exclusion chromatograms that is not plagued by this lack of sensitivity, allowing for

yne photopolymerization was carried out on solutions of different concentration (ranging from 0.05 to 0.3 M) of the 2500 g mol−1 thiol/yne macromonomer and was followed by size-exclusion chromatography (Figure 3).

Figure 3. Size-exclusion chromatograms (normalized DRI detector response versus retention time) of hyperbranched polystyrene produced by thiol−yne photopolymerization of 0.05, 0.1, 0.15, and 0.3 M solutions of thiol/yne macromonomer (2500 g mol−1). Reaction conditions: DMF/toluene (4/1, v/v), ∼0.5 equiv of DMPA, 24 h of irradiation.

As highlighted in Figure 3, the initial concentration of the macromonomer has a clear impact on the size of the dendritic structure, as increasing the concentration directly translate into the production of larger hyperbranched polymers. Interestingly, despite a reaction time that is longer than required (24 h) and in the presence of a sufficient amount of photoinitiator (∼0.5 equiv), irradiation of a 0.05 M solution of thiol/yne macromonomer produced only relatively small hyperbranched structures. In contrast, the thiol−yne photopolymerization of the 0.3 M solution yielded structures bigger than the ones that were obtained previously while varying reaction parameters for 0.15 M solutions of thiol/yne macromonomer. Hutchings and co-workers reported qualitatively similar results (i.e., the higher the concentration of the macromonomer, the larger the final hyperbranched structure), but in their study they also observed the formation of compounds that have a smaller hydrodynamic volume compared to the original macromonomer, especially when decreasing the macromonomer concentration.27 They attributed this effect to the production of cyclized products via intramolecular coupling.27 There is no evidence for such phenomenon to occur for our macromonomers, although intramolecular cyclization events most probably happen within the produced hyperbranched polymers. Hence, among the different parameters studied in this report, the initial concentration of the thiol/yne macromonomer represents the factor that has the most significant influence on the thiol−yne reaction and final product. It can therefore be anticipated that choosing the concentration wisely would allow the control of the final hbPS size and thus the related properties. This will be discussed into more details in the following section. Finally, as shear forces obtained in size-exclusion columns can be relatively destructive for high molecular weight polymers,50 it appeared crucial to verify that the hyperbranched polymers produced in this study were not degraded during the 6702

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weight is indicative of the topology of the hyperbranched polymers produced (Figure 5). Figure 5A illustrates the evolution of the intrinsic viscosity as recorded by the viscometer as a function of the molecular weight for hbPS produced by thiol−yne photopolymerization of 0.05−0.3 M solutions of a thiol/yne polystyrene macromonomer (2500 g mol−1). The values of intrinsic viscosity for linear polystyrene were obtained from the Kuhn−Mark− Houwink−Sakurada (KMHS) relation (eq 1) [η] = KM α

where [η] is the intrinsic viscosity, M is the molecular weight, and K and α are the KMHS parameters. The values of K and α were determined experimentally (Figure S2) for linear polystyrene in DMF at 50 °C as 32.05 × 10−5 and 0.604, respectively. As predicted, for identical molecular weights, the intrinsic viscosities measured for the hyperbranched polystyrenes were lower than those calculated for the linear polymer. Also, the difference in intrinsic viscosity between the two architectures increased with increasing the size of the structures, indicating that the larger the branched system, the more compact the structure is. The study of the value of the KMHS parameter α further confirmed this observation. Indeed, the value of α is indicative of the expansion of a polymer in a specific solvent (for a defined temperature).54 Typical values of α range from 0 to 2. A value of 0 is for a fully aggregated, compact sphere; a value of 0.6−0.8 is typical for flexible polymer chains in a good solvent; and a value of 2 is for rigid-rod structures.54 Therefore, the decrease of the value of α from 0.604 (linear PS) to approximately 0.25 (high molecular weight hbPS) observed in Figure 5A is a clear indication that the structure undergoes a contraction and becomes more compact as the molecular size increases, which is expected for hyperbranched systems. Furthermore, the viscosity contraction (or shrinking) factor g′, which is defined by the ratio between the intrinsic viscosity of the branched system compared to that of its linear analogue at identical molecular weight,51 decreased with increasing the molecular weight, as illustrated in Figure 5B. Small values of g′ are indicative of highly branched structures, as they are directly related to the extent of contraction observed for the branched system compared to the linear structure. The minimal intrinsic viscosities measured for the hbPS produced in this study represent only 20−50% (depending on the thiol/yne macromonomer concentration utilized for the thiol−yne photopolymerization) of the value obtained for the linear system. Finally, assuming that the Fox−Flory relation55 corrected by Ptitsyn and Éizner56 for nontheta conditions (eq 2)51 is valid for the branched system described within this report, the radii of gyration of the hbPS structures can be roughly estimate from the intrinsic viscosity data (Figure 6)

Figure 4. Size-exclusion chromatograms for the hyperbranched polystyrene produced by photopolymerization of a 0.3 M solution of thiol/yne macromonomers (2500 g mol−1). The evolution of the normalized responses of the DRI, viscometer, and light scattering (at 15° and 90° angles) detectors as a function of retention time is represented on the right axis, while the logarithmic progression of the calculated and extrapolated molecular weights is reported on the left axis. Reaction conditions: DMF/toluene (4/1, v/v), ∼0.5 equiv of DMPA, 24 h of irradiation.

the determination of the number-average and weight-average molar masses as well as the dispersity values (Table 3). As expected for such systems, the weight-average molar mass increased much more rapidly than the number-average molar mass,24 which resulted in relatively high dispersity values. Furthermore, the data reported in Table 3 confirmed the visual observation made in Figure 3 that the molar masses of the hyperbranched structures directly correlate with the concentration of thiol/yne macromonomer utilized for the photopolymerization. Table 3. Number-Average Molar Masses, Weight-Average Molar Masses, and Dispersity Values for Hyperbranched Polystyrenes Produced by Thiol−Yne Photopolymerizations of Various Initial Concentrations of Thiol/Yne Macromonomers (2500 g mol−1) As Determined by Multiangle Light Scattering (MALS) Size-Exclusion Chromatographya [thiol/yne macromonomer]0 [mol L−1]

Mn,MALS [g mol−1]

Mw,MALS [g mol−1]

Đ

0.05 0.10 0.15 0.30

9300 14100 16000 18400

24700 53400 87300 214500

2.66 3.79 5.46 11.66

(1)

a

Reaction conditions: DMF/toluene (4/1, v/v), 24 h of UV irradiation, ∼0.5 equiv of DMPA (relative to the number of polymeric chains).

Rg =

The use of a multidetector SEC system that is equipped with an online viscometer offers an extended range of characterization possibilities, as it gives direct access to the intrinsic viscosity values of the hyperbranched structures analyzed. As pointed out earlier, at equivalent molecular weights, hyperbranched polymers possess a smaller hydrodynamic volume than their linear counterparts, which directly translates into a lower intrinsic viscosity for the former. As a result, measurement of the intrinsic viscosity as a function of the molecular

1 6

3

1023[η]M ϕ* F

(2)

with ⎛ (2α − 1)2 ⎞ 2α − 1 ϕF* = ϕF⎜1 − 2.63 + 2.86 ⎟ 9 3 ⎝ ⎠

where Rg is the radius of gyration in nm, [η] the intrinsic viscosity in dL g−1, M the molecular weight in g mol−1, φF* the 6703

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Figure 5. Intrinsic viscosity (A) and viscosity contraction factor g′ (B) as a function of molecular weight for linear polystyrene as well as for hyperbranched polystyrenes obtained by thiol−yne photopolymerization of solutions of thiol/yne polystyrene macromonomers (2500 g mol−1) with concentrations ranging from 0.05 to 0.3 M. Reaction conditions: DMF/toluene (4/1, v/v), ∼0.5 equiv of DMPA, 24 h of irradiation.

thiol−yne photopolymerizations was followed by size-exclusion chromatography (SEC) for three thiol/yne macromonomers of different molar masses. The diffusion of the thiol/yne reactive species within the reaction solutions was shown to limit the overall size of the hyperbranched polymers produced by thiol− yne photopolymerization. These limitations were confirmed by studying the thiol−yne photopolymerization of even larger thiol/yne macromonomers. While the amount of photoinitiator present in the reaction solution only demonstrated little effect on the thiol−yne reaction, the initial concentration of thiol/yne macromonomer was shown to have a significant influence upon the control of the final size of the hyperbranched structures produced by thiol−yne photopolymerization. Finally, multidetector SEC analyses allowed the absolute molecular weights of the hyperbranched materials to be determined and the hyperbranched structure to be confirmed. As expected, at equivalent molecular weights, the hyperbranched structures prepared by thiol−yne photopolymerization were reported more compact as compared to a linear analogue of the same chemical composition.

Figure 6. Radius of gyration as a function of molecular weight for hyperbranched polystyrene produced by irradiation of 0.05−0.3 M solutions of thiol/yne macromonomer (2500 g mol−1). Reaction conditions: DMF/toluene (4/1, v/v), ∼0.5 equiv of DMPA, 24 h of irradiation.

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corrected Flory constant in mol−1, φF the Flory constant (2.86 × 1023 mol−1), and α the exponential factor of the KMHS equation. As expected, the radius of gyration increases with increasing the molar mass of the hyperbranched structure. The data in Figure 6 also nicely show that the size of the hyperbranched structures can be easily tuned by choosing the initial thiol/yne macromonomer concentration for the thiol−yne reaction.

ASSOCIATED CONTENT

S Supporting Information *

Determination of dn/dc for linear polystyrene, Kuhn−Mark− Houwink−Sakurada plot for linear polystyrene, 1H NMR spectroscopy spectra, size-exclusion chromatograms. This material is available free of charge via the Internet at http:// pubs.acs.org.

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CONCLUSIONS The work described in this article discussed the production of polystyrene-based hyperbranched materials via a strategy consisting in the UV irradiation of polystyrene macromonomers containing alkyne and thiol functional groups at their α- and ω-chain ends, respectively. The preparation of such thiol/yne macromonomers was proven relatively straightforward and involved the reversible addition−fragmentation chain transfer (RAFT) polymerization of styrene mediated by an alkyne-functional chain transfer agent, followed by the reduction with a large excess of isopropylamine of the trithiocarbonate moieties into thiol groups. The kinetic of the

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax +44 2476 524 112; Tel +44 2476 528 085 (S.P.). Present Address

S.P.: Department of Chemistry, University of Warwick, Coventry, CV4 7AL, United Kingdom; Faculty of Pharmacy and Pharmaceutical Sciences, Monash University, 381 Royal Parade, Parkville, VIC 3052, Australia. Notes

The authors declare no competing financial interest. 6704

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ACKNOWLEDGMENTS The authors acknowledge the Swiss National Science Foundation (PBELP2-135833) for the provision of a fellowship (R.B.) and the Australian Research Council Future Fellowship program for funding (S.P.). Patrice Castignolles is sincerely thanked for fruitful discussions about multidetector sizeexclusion chromatography. Algi Serelis from DuluxGroup is thanked for the provision of the 2-(((butylthio)carbonothioyl) thio)propanoic acid chain transfer agent, used as a precursor for the synthesis of prop-2-yn-1-yl 2-(((butylthio)carbonothioyl) thio)propanoate.

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ABBREVIATIONS SEC, size-exclusion chromatography; DMF, N,N-dimethylformamide; THF, tetrahydrofuran; DMPA, 2,2-dimethoxy-2phenylacetophenone; CTA, chain transfer agent; Mn, numberaverage molar mass; Đ, dispersity (= Mw/Mn); PS, polystyrene; PS-SH, thiol/yne functional polystyrene macromonomer; hbPS, hyperbranched polystyrene; KMHS, Kuhn−Mark− Houwink−Sakurada.

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