Copolymers with Controlled Molecular Weight Distributions and

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Copolymers with Controlled Molecular Weight Distributions and Compositional Gradients through Flow Polymerization Nathaniel Corrigan,†,‡ Rodrigo Manahan,† Ze Thong Lew,† Jonathan Yeow,†,‡ Jiangtao Xu,*,†,‡ and Cyrille Boyer*,†,‡ †

Centre for Advanced Macromolecular Design (CAMD), School of Chemical Engineering, and ‡Australian Centre for NanoMedicine, School of Chemical Engineering, UNSW Australia, Sydney, NSW 2052, Australia S Supporting Information *

ABSTRACT: We report a novel semicontinuous method for producing polymer mixtures with tailored molecular weight distributions (MWDs) and chemical compositions. In contrast to recent methods disclosed for the production of tailored MWDs, the current approach allows the MWD to be tailored at any point in a multistep process. Optimization of a photoflow polymerization process has been performed, where polymer fractions with variable compositions can be produced and independently processed downstream. This independent processing allows complex and tedious operations to be significantly simplified and the polymer structure to be manipulated by varying the production conditions. To illustrate the versatility of our approach, we prepared low dispersity block copolymer mixtures with tailored composition gradients through PET-RAFT polymerization, using a facile one-pass flow technique.

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INTRODUCTION The evolution of controlled polymerization techniques, particularly controlled/living radical polymerization (CLRP), over the past two decades has facilitated the production of an incredibly diverse range of materials, with varied physical and chemical properties.1−6 In particular, reversible addition− fragmentation chain transfer (RAFT) polymerization and atom transfer radical polymerization (ATRP) techniques have garnered significant attention due to the facile polymerization of many different monomer families with diverse functionality, under a wide range of reaction conditions.7−10 Additionally, the use of light as an external stimulus to activate and control chain growth in RAFT and ATRP has increased the scope of these systems.11−25 Most notably, the ability for these techniques to precisely modulate chain growth has also allowed the production of macromolecules with controlled primary structures and chemical compositions. Moreover, the control exhibited in CLRP has led to the creation of polymers with diverse architectures, including block, gradient, and graft copolymers,26−30 multiarm star,31 and hyperbranched polymers32−34 as well as network structures,35−37 among many others.38−40 The development of these interesting architectures has led to applications in various fields due to their unique properties.41−44 Since controlled polymerization techniques allow for exquisite control over chain lengths, a great deal of emphasis has been placed on synthesizing polymer materials with narrow molecular weight distributions. Comparatively, there has been far less interest in tuning polymer molecular weight distributions (MWDs), with only a few groups presenting such protocols.45 Notably, Fors and co-workers developed procedures for tailoring © XXXX American Chemical Society

MWD shape through anionic and nitroxide mediated polymerization and investigated the mechanical, thermal, and rheological properties of the resulting polymers.46−48 MWD control has also been described using flow processing techniques; Zhu and coworkers have utilized a recycle loop to create multimodal MWDs using thermal RAFT,49 while our group has altered flow, chemical, and light parameters throughout a photoinduced RAFT process to create tailored MWDs.50 Although some other protocols for altering MWDs have been disclosed, these techniques either alter the dispersity rather than the molecular weight distribution or have limited versatility in controlling the shape of the distribution.51−55 Interestingly, while there are limited strategies for controlling MWDs, the effects of disperse blocks on self-assembly behavior have been studied by several groups.56 Hillmyer and coworkers,57−63 Matsushita and co-workers,64−66 Mahanthappa and co-workers,67−69 and others70−72 have shown that the dispersity of block copolymers plays a critical role in determining the morphologies, domain spacing, and other properties of selfassembled structures. Fors and co-workers also recently developed a method to tune the domain spacing in thin films by consideration of the moments of polymer MWDs.73 Other studies have shown that MWDs also play a critical role in determining rheological and other properties of polymer materials.46,48 Since the properties of polymer materials are heavily dependent on their MWDs, the ability to control MWDs Received: March 30, 2018 Revised: May 18, 2018

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DOI: 10.1021/acs.macromol.8b00673 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Statistical and controlled chain extensions of tailored homopolymer MWDs. Early work performed by multiple authors. Previous work performed by Fors and co-workers.46,75

Figure 2. Proposed mechanism and photocatalyst, RAFT agents, and monomers used in this study.91 ZnTPP: 5,10,15,20-tetraphenyl-21H,23Hporphine zinc; DTPA: 2-(((dodecylthio)carbonylthio)thio)propanoic acid; BTPA: 2-(n-butyltrithiocarbonate)propionic acid; DMAm: N,N′dimethylacrylamide; NAm: N-acryloylmorpholine; BzA: benzyl acrylate; 3Σ: triplet oxygen; 1Δ: singlet oxygen; DMSO: dimethyl sulfoxide; DMSO2: dimethyl sulfone.

is critical for future materials production. For example, the preparation of polymers with disperse blocks in the aforementioned self-assembly studies was usually accomplished by the tedious preparation and mixing of several homopolymer fractions. Although this method is effective for laboratory scale production, synthetic techniques for producing polymer mixtures with varied compositions and controlled MWDs that are rapid, facile, and scalable are needed. Moreover, previous one-pot (batch)46,47 or one-pass (flow)50,74 methods for tuning MWDs have a limitation in that the MWD is only tunable for a single homopolymer block; chain extension of the tailored MWDs produces copolymer mixtures with a precisely tailored block and a statistical chain extension (Figure 1). Recent efforts for the preparation of such diblock copolymers with a single

tunable MWD block still present advantages over traditional low dispersity diblock copolymers, in that the polymer properties can be more precisely tuned.75 Control over the distribution of chains for all blocks, rather than just a single block, is more desirable, however, as a higher level of control over the final polymer topology can be realized. Previously, our group examined a photoinduced flow polymerization system (using photoinduced electron/energy transfer−RAFT (PET-RAFT) polymerization) for controlling MWDs; flow polymerization provides some inherent benefits over batch polymerization including increased heat transfer, more uniform light distributions, and facile upscaling through reactor parallelization.76−80 Moreover, sequential processing by linking flow reactors provides the opportunity to streamline B


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Macromolecules Table 1. Polymerization in Photoflow Reactors with Varied Geometries and Plug Sizesa no.

tube i.d.b (mm)

tube vol (μL)

flow rate (μL/min)

plug sizec (μL)

Reynolds numberd

sample timee

Αf (%)

Mpg (kg/mol)

Đg

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1.59 1.59 1.59 3.18 3.18 3.18 3.18 3.18 3.18 3.18 3.18 3.18 3.18 3.18 3.18

1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 52000 52000 52000

10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 433.3 433.3 433.3

2400 2400 2400 2400 2400 2400 200 200 200 200 200 200 2400 2400 2400

4.1 4.1 4.1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1929.5 1929.5 1929.5

start middle end start middle end 1-start 1-end 2-start 2-end 3-start 3-end start middle end

90.1 90.3 91.8 86.6 90.8 91.2 96.4 96.7 96.2 96.7 96.7 93.2 93.7 94.1 94.4

21.3 21.6 23.5 19.6 21.9 26.5 20.6 20.7 20.4 20.3 20.3 21.2 20.3 20.2 20.3

1.07 1.08 1.12 1.06 1.13 1.23 1.11 1.11 1.10 1.11 1.10 1.12 1.12 1.12 1.12

a

Experimental conditions: a two-pump system was used to pump a homogenized reaction mixture through the photoflow reactor at ambient temperature. The catalyst concentration was 50 ppm relative to monomer, and the monomer solids content was 20 vol % in DMSO for all experiments. Irradiation wavelength was 530 nm, and intensity was 3.8 W/m2. Irradiation time was 90 min for all experiments. bi.d. = inside diameter. cPlug size refers to the volume of each distinct section of reactant injected into the flow reactor. dReynolds number calculated from initial reaction mixture; see ref 50 for calculation details. eSample time relative to the plug volume, i.e., a sample time of “middle” for a 2400 μL plug represents a sample taken after 1200 μL polymer elution. fMonomer conversions determined by 1H NMR. gMolecular weight and dispersity determined by GPC analysis (DMAc used as eluent) using PMMA standard for calibration.

complicated processes;81 however, this has seldom been explored for polymerization systems.82−85 Although complexity can be easily incorporated in such flow processes, the effects of fluid dynamics within each system must be understood in order to generate the necessary products and minimize side-product formation. Herein, we present the results of a systematic optimization of a semicontinuous, sequential photoflow processing technique. The optimized system was subsequently employed to produce tailored MWDs as well as homo- and copolymer mixtures with controlled compositions and tailored MWDs.

mixing behavior and reactant residence times. In order to gain greater control over the system, the effects of the fluid dynamics in these systems must be understood. As such, the influence of tube diameter, tube volume, flow rate, and reactant volume on the polymerization process was investigated. Table 1 shows the monomer conversions, peak molecular weights (Mp), and dispersities of poly(dimethylacrylamide) (PDMAm) produced in four photoflow reactors with different geometries. The target molecular weights (20.0 kg/mol) and reaction times (2 h) were constant throughout the reactor geometry investigation, and a two-pump system was used (Supporting Information, Figure S2). The chemical concentrations were also maintained for the geometry alteration experiments as follows: N,N′-dimethylacrylamide (DMAm) = 1.94 M, DMSO = 11.37 M, ZnTPP = 97 μM, 2-(((dodecylthio)carbonylthio)thio)propanoic acid (DTPA) = 12 mM. “Plugs” are herein defined as a section of liquid within the flow reactor, which is separated from other sections of liquid by a controlled volume of gas, such as nitrogen or air. The “plug size” is thus representative of the volume of the liquid reactant injected into the flow reactor; this value was altered during the optimization study. The first reactor geometry (Table 1, nos. 1−3) was chosen to mimic the conditions used in our previous work; 1.59 mm inside tubing diameter (i.d.), 1200 μL tube volume, and 2400 μL reactant volume.50 Under these conditions the Reynolds number (Re) at the start of the polymerization was determined to be ∼4.1, which represents laminar (Re ≲ 2000) fluid profiles.92 Samples were taken directly from the reactor exit at the start, middle, and end of the product elution to show the instantaneous polymer composition at different time points in the reaction. The samples taken at the start and middle of the polymerization presented narrow dispersities and reasonably high monomer conversions; however, the final sample (corresponding to the final drops of the polymerization mixture) displayed a slightly higher dispersity, higher molecular weight, and more tailing at high molecular weight compared to the earlier samples. This can be attributed to the longer retention times experienced by this

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RESULTS AND DISCUSSION Optimization of the Photoflow Process. To perform polymerizations in the photoflow reactor, PET-RAFT polymerization was employed. In contrast to conventional RAFT, where a continuous external source of radicals is used to perform RAFT polymerization, PET-RAFT polymerization can be achieved using a photocatalyst which directly activates the RAFT agent.23 A variety of photocatalysts with specific photophysical properties have been employed to activate PET-RAFT polymerization under a range of wavelengths, including organophotocatalysts, metal-based photocatalysts, natural photocatalysts, etc.86−90 Throughout this work, 5,10,15,20-tetraphenyl-21H,23H-porphine zinc (ZnTPP) was selected as photocatalyst due to its excellent oxygen tolerance, allowing fast polymerization in the presence of air; different RAFT agents and monomers were used, while dimethyl sulfoxide (DMSO) was used as solvent and singlet oxygen quencher for all experiments (Figure 2).91 All polymerization was carried out at room temperature, under green light (λ = 530 nm, intensity = 3.8 W/m2 max). During our previous investigation into controlling molecular weight distributions through photoinduced polymerization in a flow reactor, some effects of the fluid profiles on the final polymer products were observed.50 Notably, the low flow rates utilized for polymerization resulted in laminar flow profiles, which adversely affected the polymerization by producing inconsistencies in C


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were very similar. Clearly, the use of small plugs of fluid increased the homogeneity of the polymer products and reduced the long molecular weight tails and broad dispersities that were evident in previous systems. Importantly, the use of plugs rather than fully continuous streams reduces the time needed for flow systems to reach steady state and, in turn, increases product yield and purity.96,98,99 Moreover, the production of polymers via this plug flow methodology circumvents the need to use very thin tubes that allow for increased light penetration, as the increased mixing in each plug ensures even light irradiation over the course of the reaction.93−95 Following the consistent molecular weights obtained through polymerization of discrete plugs, reactant mixtures with varied targeted molecular weights were sequentially injected into the photoflow reactor to further investigate any mixing between each plug.96 During these reactions a four-pump setup was used to alter the molecular weight of each polymer fraction and separate the reactant plugs with small sections of nitrogen (Figure S3). Although the PET-RAFT polymerization technique that was used as a model for these systems is tolerant to molecular oxygen, nitrogen was used instead to increase the reproducibilty and decrease any adverse effects that may occur due to the presence of large amounts of oxygen.91,100−103 Initially, four discrete reaction solutions were injected into the reactor, with targeted molecular weights of 15.0, 20.0, 26.7, and 35.6 kg/mol. As expected, the nitrogen separated the fractions throughout the polymerization, which produced four separate polymer fractions with molecular weights in line with theoretical predictions (Table 2). The accompanying dispersities were narrow, and monomer conversions were above 90% (Figure S7 and Table S2).

sample within the reactor due to the laminar flow profile. The fluid at the tube edges has a lower velocity compared to the fluid in the middle of the tube; as a result, the final sample consists of reaction mixture which has experienced a longer mean residence time.50 As the light intensity is also higher at the tube edge, chains with disproportionately high molecular weight are formed here and eluted from the flow reactor later in the process. The second reactor geometry (Table 1, nos. 4−6) had a wider tubing diameter (3.18 mm) compared to the first reactor, while the reactor and reactant volumes were maintained. For this reactor geometry, molecular weights, monomer conversion, and dispersity all increased throughout the reaction. The higher dispersity and overall lower product consistency compared with the first reactor geometry can be attributed to the very low Reynolds number (∼1) and decreased light penetration within the thicker tubing. The combination of these effects produced a larger discrepancy between retention times at the start and end of the reaction as well as some differences in reaction rate depending on the reactants position within the tube, i.e., closer to or further from the light source. Although the dispersity of the final sample was only 1.23, the inconsistency compared to the first samples and increasing breadth of the distribution is undesirable for specialized applications where precise control over polymer molecular weight is constantly required (Figure S5b). The third reactor geometry was identical to the second, though the volume of reactant injected into the reactor (plug size) was decreased to 1/6th of the total reactor volume (200 μL). It was envisioned that by reducing the reactant plug size the polymerization mixture would behave in a pseudo-plug-flow manner with increased mixing, even though the Reynolds number would remain very low, i.e., well within the laminar region.93−95 Three reactant plugs with equal volumes, separated by 100 μL sections of air, were injected into the third reactor, and samples were taken at the start and end of each plug. The molecular weights of all the plugs were more consistent than the polymers produced in previous reactors and close to theoretical values (Table 1, nos. 7−12). Moreover, the monomer conversion and dispersity were reasonably consistent for all three plugs, and the MWDs for all samples were almost indistinguishable, as shown by the GPC traces (Figure S5c). Although this reactor had the same geometries and Reynolds numbers as the second reactor, the smaller plug size appears to have changed the fluid behavior and induced greater mixing within the plug.93−96 As such, the inconsistencies that were seen in the previous reactors are not present under these conditions due to the homogenization of residence times for all polymerization mixtures (Figure S5a−c). Similarly, the first reactor geometry was trialed with three 200 μL plugs, with the results showing a more consistent polymerization process (Figure S6). While the introduction of smaller plugs of reactant produced consistent polymer products, the flow rates used in these processes were quite low, which limits the production rate for a single stream. To overcome this limitation, the reactor volume (tube length) was increased while the diameter was fixed at 3.18 mm, allowing a higher production rate, i.e., higher flow rate, for the same targeted mean residence time.81 Moreover, the increased flow rate led to a higher Reynolds number that defined the fluid profile closer to transitional flow (Table 1, nos. 13−15) and led to further improvements in production consistency due to increased homogeneity within the plug.94,97 The GPC traces for a 2400 μL plug injected into a 52 000 μL, 3.18 mm i.d. reactor were almost identical (Figure S5d), and monomer conversions

Table 2. Production of PDMAm with Varied Molecular Weights in a One-Pass Flow Process fractiona

target MW (kg/mol)

αb (%)

Mpc (kg/mol)

Mn,theob (kg/mol)

Mn,expc (kg/mol)

Đc

1 2 3 4

15.0 20.0 26.7 35.6

92.5 94.8 94.8 95.2

14.8 19.6 26.0 34.6

14.2 19.2 25.5 34.1

13.9 18.6 24.6 32.3

1.07 1.08 1.08 1.10

a

Experimental conditions: a four-pump system was used to pump reaction mixture through the photoflow reactor at ambient temperature. Irradiation wavelength was 530 nm, and intensity was 3.8 W/m2. The catalyst concentration was 25 ppm relative to monomer, and the monomer solids content was 20 vol % in DMSO for all experiments. Irradiation time was 180 min for all experiments. bTheoretical MWs were calculated from the following formula: Mn,theo = DPn × MWDMAm × α/100 + MWBTPA, where DPn, MWDMAm, MWBTPA, and α correspond to targeted degree of polymerization, DMAm molar mass, BTPA molar mass, and conversion determined by 1H NMR, respectively. cMolecular weight and dispersity determined by GPC analysis (DMAc used as eluent) using PMMA standard for calibration.

To highlight the ability for the plug flow processing technique to generate polymer fractions with distinct molecular weights, a control experiment was performed without the addition of gaseous sections to separate the initial reaction mixtures. Four adjacent 600 μL polymer fractions with molecular weights increasing logarithmically from 10.0 to 35.6 kg/mol were injected into a photoflow reactor; five samples were taken at equally spaced intervals to represent the instantaneous polymer composition as the product eluted from the reactor. A sixth sample representing the cumulative polymer composition over the entire reaction was also taken. Unsurprisingly, all six samples D


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Figure 3. Tailored MWDs produced in a single pass flow system. (a) Tailored MWD 1, composed of four distinct molecular weight polymer fractions. (b) Tailored MWD 2, composed of six distinct molecular weight polymer fractions. (c) Polymer characteristics for tailored MWDs 1 and 2 and the individual fractions of tailored MWD 1. aExperimental conditions: a four-pump system was used to pump reaction mixture through the photoflow reactor at ambient temperature. Irradiation wavelength was 530 nm, and intensity was 3.8 W/m2. The catalyst concentration was 25 ppm relative to monomer, and the monomer solids content was 20 vol % in DMSO for all experiments. Irradiation time was 180 min. bMonomer conversions determined by 1H NMR. cMolecular weight and dispersity determined by GPC analysis (DMAc used as eluent) using PMMA standard for calibration. d Skewness and kurtosis calculated from Rudin.50,106 eRatio of full width at one-quarter maximum to full width at three-quarter maximum.

was used to achieve the targeted MWD (Figure 3a, black line). Clearly, the tailored MWD presents a nearly equal response over the targeted range. Furthermore, the tailored distribution displayed a larger dispersity than the individual fractions; the tailored distribution displayed a nominal increase in skewness and a similar kurtosis to the individual fractions (Figure 3c, nos. 1−5). Interestingly, while the tailored MWD appears to be broad in the GPC trace, the dispersity was only 1.21. As such, it must be noted that dispersity is not necessarily a good indicator for characterizing these tailored MWDs, as highlighted by Rane and Choi and by Harrison.104,105 Values for skewness (a value of 0 indicates a perfectly symmetrical distribution, larger values indicate the distribution is skewed toward higher molecular weights) and kurtosis (a value of 3 is typical for standard normal distributions, larger values indicate distributions with heavier tailing) have also been included to aid in characterizing these distributions.106 It must also be noted, however, that these moments (skewness and kurtosis) do not give a full description of distributions such as those shown in Figure 3.107 As such, we have included another value to characterize such distributions that represents the ratio of the full peak width at one-quarter of the maximum response (FWQM) to the full peak width at threequarters of the maximum response (FWTQM) (Table S10). For distributions with an equal targeted response over a defined molecular weight range, the ratio of FWQM to FWTQM approaches unity as the MWD becomes more “rectangular” in shape. The tailored MWDs show a significantly decreased FWQM:FWTQM ratio compared to the individual polymer fractions, as expected. A second tailored MWD with an equal response over a larger range (15.0−63.2 kg/mol) was also produced using the plug flow processing technique (Figure 3b, black line). To increase the width of the distribution, additional polymer fractions with higher molecular weights were synthesized such that all the fractions would be evenly spaced on the logarithmic x-axis.

presented identical MWDs, indicating complete mixing of the adjacent fractions throughout the reaction (Figure S8). As such, the inclusion of separatory sections of gas ensures that multimolecular weight polymer products can be formed sequentially in a semicontinuous, one-pass flow process. Molecular Weight Distribution Control with New Protocol. Following the optimization of the flow polymerization, more complex processing was investigated with the new plug flow regime. An added advantage of using plugs of polymer separated by gas is the ability to process polymer fractions separately and easily add complexity that would otherwise be extremely difficult or tedious with other processing techniques. For instance, our previous publication focused on controlling molecular weight distributions (MWDs) by alteration of flow rates, chemical concentrations, or the light source during the polymerization.50 This process relied on the continuous production of polymer fractions with distinct molecular weights; however, the continuous injection of the reactant stream led to unavoidable, unwanted mixing of the fractions throughout the polymerization. By adding separatory sections of nitrogen during the injection of the reactants, each polymer fraction can be completely separated and mixing issues between the fractions can be eliminated. As a result, more well-defined and complex polymer products can be easily synthesized. To demonstrate this, a PDMAm tailored MWD with an equal GPC refractive index (RI) intensity response over the range of 15.0−35.6 kg/mol was targeted. The molecular weights of the polymer fractions were chosen such that they were evenly spaced on a logarithmic x-axis, and the volumes were chosen such that the response profile in the GPC would produce a relatively even response over the targeted range (Table S3). The complete procedure and details on the four-pump system used to produce these tailored MWDs can be found in the Methods section of the Supporting Information. The sequential, semicontinuous production of four distinct polymer fractions with targeted molecular weights of 15.0, 20.0, 26.7, and 35.6 kg/mol, and subsequent collection, E


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while pump 4 injected gaseous nitrogen to separate the reactant plugs. After polymerization of the first blocks in reactor 1, pump 5 injected varied volumes of solvated monomer solution into each of the macro-RAFT agent solutions. Following a mixing section designed to ensure homogenization, each fraction was passed through a second reactor where chain extension took place. Collection of all fractions produced diblock copolymer mixtures with tailored A and B blocks. For the initial controlled chain extension experiment, four polymer fractions with molecular weights of 15.0, 20.0, 26.7, and 35.6 kg/mol were targeted for the first blocks. The tailored MWD produced using these blocks was assumed to be the same as shown in Figure 3a; however, there was slight deviation between the same targeted MWDs due to differences in the individual polymer fractions (Figure S9). The pumps used for the injection of reactants into the flow setup introduced some discrepancy in the reaction volumes due to their sensitivity to syringe diameters and injection rates. It is envisioned that the use of a more precise reaction setup would eliminate these issues by producing more consistent individual polymer fractions. The volumes of solvated monomer solutions injected for the first chain extensions were targeted such that the final molecular weights of all four fractions was 60.0 kg/mol, leading to a tailored MWD that was monomodal and narrowly distributed once the fractions had been collected (Table S6, 1a−8a). Figure 5a, blue line, shows the GPC trace representing the collection of all four poly(DMAm-b-DMAm) chain extension products produced in the controlled chain extension experiment. The chain extensions of the initial fractions led to polymers with relatively constant molecular weights, as demonstrated by the monomodal and narrow MWD of the copolymer mixture. The individual fractions all displayed similar molecular weights close to the theoretical values, though there was some shouldering at low molecular weights for the chain extended fractions; this shouldering was ascribed to the relatively large amounts of oxygen that diffused into the reaction mixture during addition of the solvated monomer solution, which quenched some propagating species (Figure S10). Regardless, the initially broad MWD was tailored through chain extension to produce a narrower distribution. To further demonstrate the ability to alter the MWD through this process, an initially narrow MWD was broadened through chain extension. Figure 5b, black line, shows a mixture of four separate homopolymer fractions, each with targeted molecular weights of 15.0 kg/mol. Subsequently, each fraction was chain extended by addition of varying amounts of the solvated monomer solution, designed to produce a broad distribution over the range of 20−39 kg/mol when collected (Figure 5b, blue line). Again, the individual fractions were close to theoretical predictions, and the distribution was broadened, as demonstrated by the nearly equal GPC response profile over the targeted range (Figure S11). The chain extension for both the broad to narrow, and narrow to broad distributions presented expected changes in the dispersity, skewness, and kurtosis (Figure 5c); the skewness and kurtosis increased when the distribution was broadened, while the skewness decreased, and kurtosis remained similar for the narrowed distribution. Moreover, the ratio of FWTQM to FWQM ratio was lower for the broad targeted distributions (Table S10). Low Dispersity Copolymers with Tunable MWD Blocks. Following the production and alteration of tailored PDMAm MWDs, different monomers were used for chain extension to demonstrate the versatility of our approach; the use of different

Because the overall MWD represents the superimposition of the individual polymer plugs, the volume fractions of each plug also required alteration to form the tailored MWD (Table S4). The tailored MWD displayed an approximately equal response over the targeted range and showed a slight increase to both skewness and kurtosis compared to the individual fractions (Table S5). Not surprisingly, the dispersity increased to 1.44, while the skewness and kurtosis also increased (Figure 3c, no. 6). Maintaining MWD Control after Chain Extensions. As the distribution of chains in a polymer sample influences the physical, chemical, and other properties of the resulting materials, synthetic techniques for tailoring MWDs, especially for controlled polymerization systems, are quite useful.108−112 However, the production methods used for synthesis in previous systems severely limit the ability for downstream processing to be performed, such as successive and controlled chain extensions of the tailored MWDs.46−48,50,74,113 For instance, Fors and coworkers produced a series of poly(styrene-block-isoprene) (PS-bPI) copolymers with varied dispersities and MWD “shapes” for the PS block.46 Because control over the PS MWD was achieved through metered addition of initiator in a one-pot batch process, the subsequent addition of isoprene resulted in a statistical chain extension of the tailored PS MWD. Assuming even chain growth after addition of isoprene, the chain extended copolymer mixture consists of a low dispersity PI block and a tailored PS block. (Figure 1, middle). Although such copolymers have demonstrated variable properties, protocols for controlling the initial MWD and subsequent chain extension are more valuable as they provide additional options for precise control over polymer structures.75 Similarly, our previous flow system was not amenable to controlled chain extensions due to the eventual mixing of the targeted fractions throughout the flow process (vide supra). However, by keeping the polymer fractions separated through this semicontinuous plug flow process, downstream processing is easily achieved. Particularly, chain extensions can be selectively undertaken on each polymer fraction via the addition of a second monomer solution to the macro-RAFT agent solutions leaving the first reactor to create gradient diblock copolymers with tailored compositions and degree of polymerization for each block. As such, controlled chain extensions can be performed, and the tailored MWD can be effectively adjusted in a one-pass process to produce a new type of diblock copolymer composed of blocks with tailored compositions and MWDs (Figure 1, bottom). Figure 4 shows the setup used for producing diblock copolymers with precisely tailored block lengths; pumps 1, 2, and 3 controlled the chemical composition of the reactant solutions,

Figure 4. Flow setup for the controlled chain extension of tailored MWDs. Pump 1 injected monomer 1 and catalyst (in solvent), pump 2 injected a RAFT agent solution, pump 3 injected pure solvent, pump 4 injected nitrogen, and pump 5 injected monomer 2 and solvent. F


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Figure 5. Controlling MWDs through chain extensions. (a) Narrowing an initially broad distribution. (b) Broadening an initially narrow distribution. (c) Molecular weights, dispersity, skewness, and kurtosis for MWDs produced through successive chain extension experiments. aExperimental conditions: a five-pump system was used to pump reaction mixture through the photoflow reactor at ambient temperature. Irradiation wavelength was 530 nm, and intensity was 3.8 W/m2. The catalyst concentration was 25 ppm relative to monomer, and the monomer solids content was 20 vol % in DMSO for all experiments. Irradiation time was 180 min. bMolecular weight and dispersity determined by GPC analysis (DMAc used as eluent) using PMMA standard for calibration. cSkewness and kurtosis calculated from Rudin.106

Figure 6. Controlled chain extension of tailored MWDs with different monomers: (a) MWD before and after chain extenson with NAm; (b) MWD before and after chain extension with BzA.

Table 3. Theoretical and Experimental Molar Ratios from NMR for Individual Copolymers Produced in Controlled Chain Extension Experimentsa no.

block 1/ block 2b

block 1 target MW (kg/mol)

block 2 target MW (kg/mol)

αblock 2c (%)

mol frac block 1c

mol frac block 2c

theor molar ratioc

expt molar ratioc

1 2 3 4 5 6 7 8

DMAm/NAm DMAm/NAm DMAm/NAm DMAm/NAm DMAm/BzA DMAm/BzA DMAm/BzA DMAm/BzA

10.0 20.0 30.0 40.0 15.0 20.0 26.7 35.6

40.0 30.0 20.0 10.0 45.0 40.0 33.3 24.4

89.0 92.1 93.1 92.0 83.4 82.5 80.9 77.4

0.286 0.507 0.696 0.861 0.395 0.498 0.619 0.755

0.714 0.493 0.304 0.139 0.605 0.502 0.381 0.245

0.40 1.03 2.29 6.19 0.65 0.99 1.62 3.08

0.46 1.18 2.69 6.11 0.58 0.88 1.38 2.72

a Experimental conditions: a five-pump system was used to pump a homogenized reaction mixture through the photoflow reactor at ambient temperature. Irradiation wavelength was 530 nm, and intensity was 3.8 W/m2. The catalyst concentration was 25 ppm relative to monomer, and the monomer solids content was 20 vol % in DMSO for all experiments. Irradiation time was 180 min. bMonomers chosen for the initial polymer fractions and chain extensions, respectively. cMonomer conversions and mole fractions determined by 1H NMR; complete conversions of the first block were assumed, and molar ratio represents the molar ratio of the first block to the second block as determined by NMR. See Figures S12−S15 and methods for full details.

tunable for this system, a MWD with a linear, rather than logarithmic, increase in molecular weight was targeted for chain extension with N-acryloylmorpholine (NAm). The initial molecular weights for the first section increased linearly from 10.0 to 40.0 kg/mol, while the subsequent chain extension of each fraction was targeted to produce 50.0 kg/mol poly(DMAm-

monomers allows production of low dispersity diblock copolymer mixtures with tailorable MWDs for both blocks. Copolymer mixtures with varied block lengths and chemical compositions have been shown to significantly influence selfassembly behavior and may present some other interesting properties (vide supra). Given that the molecular weights are G


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Macromolecules b-NAm) copolymers (Figure S12). As such, the copolymer mixture was targeted to have a linear composition gradient of PDMAm-PNAm, not unlike the distribution shown in Figure 1, bottom. Figure 6a shows the initial PDMAm homopolymer (black line) and the copolymer mixture (blue line). The collection of all fractions produced a narrow MWD which was composed of copolymers with varied compositions. NMR analyses showed that the mole fraction of PDMAm decreased, while the mole fraction of PNAm increased, and the experimental molar ratio of the two blocks closely followed the theoretical values (Table 3, nos. 1−4, Figures S13 and S14) The NAm extension also displayed decreased dispersity and skewness, compared to the initial PDMAm MWD, and a slightly increased kurtosis (Table S8). To further demonstrate the ability to form polymer mixtures with tailored composition gradients and MWDs through the plug flow polymerization process, a copolymer mixture composed of a hydrophilic and hydrophobic block was produced. For this experiment a broad PDMAm distribution was targeted for the first block; after formation of the initial PDMAm MWD, benzyl acrylate (BzA) was used to chain extend the homopolymer fractions to produce 60.0 kg/mol copolymer fractions (Figure 6b). Similar to the previous chain extensions with DMAm and NAm, the chain extensions with BzA led to narrower molecular weight distributions for the copolymer mixture compared to the original tailored MWD. Moreover, the composition of each copolymer fraction was close to predictions, with NMR analysis showing the expected decrease in PBzA composition for each successive copolymer fraction; the molar ratio of the PDMAm block to the PBzA block increased with each successive fraction and was close to the predicted values (Table 3, nos. 5−8, Figures S15 and S16). The PBzA extension produced molecular weights slightly lower than predicted values, though this deviation was likely the result of hydrodynamic volume differences between the PBzA blocks and the PMMA standards used for GPC calibration. The combination of the separate fractions still produced a monomodal GPC trace, however, and NMR analysis showed the expected changes in the block copolymer compositions. The PBzA blocks showed a slightly reduced conversion (∼80%) after irradiation for 180 min (Table S9).

collection. The ability to alter the number of these polymer fractions and their molecular weights allows tailored MWDs to be tuned as required. Moreover, the separation of individual polymer fractions allowed chain extensions to be performed separately, which effectively altered the tailored MWD through chain extension in one pass. The ability to sequentially process separate polymer fractions in a semicontinuous, one-pass manner allows complexity to be easily built up and provides the opportunity to produce interesting polymer structures that would normally be impossible or extremely tedious through other processing techniques. Notably, the use of different monomers for the controlled chain extension of separate polymer fractions produced a new class of copolymer mixture with tailored composition gradients. The formation of such polymer mixtures could present some interesting materials properties and is currently being investigated.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00673. Experimental details; Figures S1−S16 and Tables S1−S10 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.X.). *E-mail: [email protected] (C.B.). ORCID

Jiangtao Xu: 0000-0002-9020-7018 Cyrille Boyer: 0000-0002-4564-4702 Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS An investigation into the effects of flow rates, reactant plug sizes, and reactor geometries on polymer products produced in flow has shown that small plugs of fluid injected into a photoflow reactor increase product consistency. Decreasing the reactant plug volume and using gaseous blocks to separate the plugs led to fluid profiles which more closely exhibited plug flow behavior, even though the very low Reynolds numbers implied laminar flow profiles. The increased mixing in the small plugs during polymerization increased the homogeneity of the reaction mixtures and polymer residence times and reduced negative effects including deviation from targeted molecular weights and long tailing at high molecular weights. As such, the use of small plugs rather than a continuous reactant stream provides the possibility to conduct photopolymerization in tubes that have thicker inside diameters and correspondingly increase production rates. The use of this plug-flow polymerization process increased the ability for sequential processing to be achieved; the semicontinuous polymerization of separate reactant fractions allowed for facile production of polymer mixtures with tailored MWDs via concurrent polymerization of the fractions and subsequent H


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