Article Cite This: Macromolecules XXXX, XXX, XXX-XXX
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Controlling Molecular Weight Distributions through Photoinduced Flow Polymerization Nathaniel Corrigan,†,‡ Abdulrahman Almasri,† Werner Taillades,† 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: Molecular weight distribution plays an important role by giving specific properties to polymeric materials. Despite the recent progress in controlled/“living” radical polymerization, polymer chemists do not have the ability to tune and manipulate the shape of molecular weight distributions (MWDs) (except for a few recent examples). In this article, new synthetic procedures have been developed for controlling MWDs using photoinduced electron/energy transfer−reversible addition-fragmentation chain transfer (PET-RAFT) polymerization, in conjunction with flow processes. By adjusting the pump flow rates, the chemical concentrations and residence times of the reactant streams were tailored throughout the polymerization, leading to control over the MWD. Changes to the intensity and wavelength of the light source also induced changes in the polymerization, allowing alteration of the MWD. The protocols described here should be amenable to a range of polymerization techniques and reaction setups.
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INTRODUCTION It is well-known that the distribution of molecular weights (MWs) in a polymer sample determines the properties of that sample, rather than any average molecular weight or the polydispersity index (Đ).1−6 Specifically, the mechanical properties of a polymer sample, including viscosity, elasticity, and plasticity among many others, are all directly dependent on the polymer molecular weight distribution (MWD); as such, the MWD significantly affects the processability and end use of polymeric materials.7−10 For example, polyethylene produced with bi- and multimodal MWDs have superior resistance to stress crack growth, increased toughness, and tensile strength, while maintaining good processability compared with polyethylene possessing a monomodal MWD.11−14 Moreover, chemical, thermal, and other physical properties of polymeric materials are also dependent on the MWD, which can drastically affect the performance of these materials in their respective applications. Hillmyer and co-workers, and others, have shown that the MWD of block copolymer mixtures affects the interaction between respective chains and significantly influences nearly every aspect of self-assembly behavior, resulting in changes to the morphology, size distributions, etc.15−28 By extension, the MWDs of polymer nanostructures affect their final properties, including everything from interfacial interactions to drug loading and release profiles. Clearly, MWDs need to be carefully considered in order to obtain the high level of control required in modern polymeric materials. Although the MWD of polymer mixtures plays such a crucial role in materials properties, most methods proposed to modify © XXXX American Chemical Society
the MWD either broaden the distribution without controlling it or involve the tedious preparation of individual samples and subsequent mixing.29−35 There have been a disproportionately small number of methods disclosed in the literature for more precise and facile control over the MWDs of polymers. Notable studies on controlling MWDs include early attempts by Meira and co-workers to control the shape of MWDs through periodic alteration of the monomer and initiator flow rates in a tubular reactor.36−38 Although the experimental results showed limited success, the theoretical studies provided excellent insights into the requirements for precise MWD control. Other studies include a report by Zhu and co-workers in which solutions of butyl acrylate were polymerized through RAFT polymerization in a thermal tube reactor with a recycle loop.39 By altering the recycle ratio, a residence time distribution was developed, in turn producing multimodal MWDs. Fors and co-workers recently developed an elegant method for controlling the molecular weight distribution in nitroxide-mediated polymerization (NMP) and anionic polymerization systems.10,40,41 By temporally regulating the addition of initiator, precise control over the shape of the MWD was achieved. Importantly, different initiator addition profiles altered the symmetry of the MWDs for poly(styreneblock-isoprene) copolymers, with the symmetry of the Received: September 1, 2017 Revised: September 29, 2017
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DOI: 10.1021/acs.macromol.7b01890 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. MWDs for PDMAm formed through the alteration of light intensity and wavelength. (a) PDMAm formed at varying green light (530 nm) intensities. (b) PDMAm formed at varied irradiation wavelengths at the same intensity (1.4 W/m2). (c) Theoretical and experimental Mp’s, dispersities, and conversions for PDMAm produced under varying light intensities and wavelengths. aExperimental conditions: a one-pump system was used to pump a homogenized reaction mixture through a 600 μL photoreactor at ambient temperature. The catalyst concentration was 50 ppm relative to monomer in all cases. bResidence time. cTheoretical Mp’s were calculated from the following formula: Mp,theo = X × MWDMAm × α/100 + MWDTPA, where X, MWDMAm, MWDTPA, and α correspond to targeted degree of polymerization, DMAm molar mass, DTPA molar mass, and conversion determined by 1H NMR, respectively. dGPC performed in DMAc with PMMA standards. eMonomer conversion based on 1H NMR performed in CDCl3.
Building on this continuous flow system, we envisioned that alteration of the chemical concentrations or reaction rate during the polymerization would lead to the formation of different molecular weight polymers at different stages of the polymerization; combination of these polymers would then form customized MWDs in a one pass system. Since the chemical concentrations and reaction rate can be effortlessly modified through modification of flow rates and light parameters respectively, this technique provides facile access to a range of MWDs and should be applicable to a variety of polymerization systems.
distribution being critical in determining the stiffness of the samples. As is typical with new polymerization processes today, an important motivation is to develop “greener” technologies that are affordable and safe. Systems that utilize visible light as the energy source to initiate and control polymerization are inherently safer, cheaper, and more environmentally benign than traditional, thermally initiated systems and have been the emphasis of a great deal of research in polymer science over the past decade.42 The development of the photoinduced electron/ energy transfer−reversible addition-fragmentation chain transfer (PET-RAFT) polymerization technique has provided polymer chemists with an effective toolbox to enact the controlled radical polymerization of a wide range of monomer families in various solvents, without the need for prior deoxygenation.43−53 As PET-RAFT polymerization exploits visible light to control polymerization at ambient temperatures, it offers a “green” method for polymer production. Moreover, a characteristic of RAFT and thus PET-RAFT polymerization systems is the narrow dispersities that accompany polymerization.54 Given that the accuracy of a controlled MWD is limited by the intrinsic breadth of distributions for a given polymerization type, ionic polymerization should be able to impart the highest level of control in a tailored MWD. However, given the limited monomer scope and stringent reaction conditions required in ionic polymerization, we sought to develop a more robust process for controlling MWDs through the use of PET-RAFT polymerization. Recently, our group disclosed the PET-RAFT polymerization of acrylamides in a continuous flow reactor, with the polymers produced having Đ as low as 1.05.55 Continuous flow polymerization offers a range of benefits compared to batch polymerization, including uniform heat and mass transfer, favorable light penetration, and facile upscaling and sequential processing through polymerization in series and parallel.56−69
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RESULTS AND DISCUSSION Controlling MWs in Continuous Flow. As per our previous work, a PET-RAFT type system was used to enact all continuous flow polymerization in this work.55 Details on the flow reactor setups can be found in the Supporting Information. The model monomer used was N,N′-dimethylacrylamide (DMAm), the RAFT agent selected was 2-(((dodecylthio)carbonothioyl)thio)propanoic acid (DTPA), and the catalyst used was 5,10,15,20-tetraphenyl-21H,23H-porphine zinc (ZnTPP). All systems used dimethyl sulfoxide (DMSO) as solvent. By using a PET-RAFT system in continuous flow, polymer MWs can be tuned by adjusting three main parameters: chemical concentrations, reactant flow rates, and the light source. Specifically, changing the intensity and wavelength of the light source in these polymerization systems, while fixing the residence time and chemical concentrations, can lead to the production of polymers with varied MWs. Under these conditions, the changes in MWs are due to differences in monomer conversion for each of the final polymer samples; adjusting the light intensity or the irradiation wavelength changes the rate of reaction and leads to polymer samples that have differing monomer conversion for a fixed residence time. These effects have been demonstrated by our group previously B
DOI: 10.1021/acs.macromol.7b01890 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. MWD and conversion profiles of PDMAm polymers formed through alteration of RAFT agent concentrations. (a) MWDs for PDMAm polymers produced at varied RAFT agent concentrations; (b) peak molecular weights and monomer conversion of PDMAm produced at varied RAFT agent concentrations; (c) effects of chemical concentrations and residence times on narrowly distributed PDMAm polymers produced through continuous flow PET-RAFT polymerization. aConditions: a one-pump system was used to pump a homogenized reaction mixture through a 600 μL photoreactor at ambient temperature. The catalyst concentration was 50 ppm relative to monomer in all cases. bResidence time. cReactor volume was 6 mL. dTheoretical Mp’s were calculated from the following formula: Mp,theo = X × MWDMAm × α/100 + MWDTPA, where X, MWDMAm, MWDTPA, and α correspond to targeted degree of polymerization, DMAm molar mass, DTPA molar mass, and conversion determined by 1H NMR, respectively. eGPC performed in DMAc with PMMA standards. f Conversion based on 1H NMR performed in CDCl3.
in batch PET-RAFT polymerization.48,55 Figure 1a shows the MWDs for a series of polymers produced in a one-pump system, under different green light intensities (530 nm; 0.3, 1.0, and 3.8 W/m2) at a constant residence time (90 min) and for a fixed concentration of RAFT (4.9 mM), monomer (1.9 M), and catalyst (0.1 mM). The monomer conversions were 39, 63, and 95% for PDMAm irradiated under 0.3, 1.0, and 3.8 W/m2 green light, respectively, with corresponding Mp’s of 15.5, 30.0, and 42.5 kg/mol (Figure 1c, entries 1−3). Similarly, changing the irradiation wavelength, while maintaining a constant residence time and RAFT, monomer, and catalyst concentrations, also changes the reaction rate and leads to polymer samples that have varied monomer conversions. By switching the wavelength from red (630 nm), to green (530 nm), to blue (460 nm), while fixing the light intensity at 1.4 W/m2, PDMAm samples were produced with different final monomer conversions, resulting in polymers with differing degrees of polymerization, and different MWs (Figure 1b). Under blue, green, and red light irradiation, PDMAm was produced with monomer conversions of 57, 79, and 71% and Mp’s of 28.6, 38.4, and 33.6 kg/mol, respectively (Figure 1c, entries 4−6). Changing the concentration of RAFT agent while fixing the residence time (reactant flow rate) and the wavelength and intensity of the light source will also lead to the production of polymers with different MWs. This effect is shown in Figure 2a,b for a series of polymers formed in a one-pump system with a fixed residence time of 90 min, under green light (530 nm, 3.8 W/m2), but with varied RAFT agent concentrations. In these experiments the monomer and catalyst concentrations were fixed at 2.9 M and 0.15 mM, respectively. For an approximately equal monomer conversion the three polymers have different degrees of polymerization, and as such, different MWs that
increase on decreasing RAFT agent concentration.70 The experimental molecular weights were close to theoretical predictions, indicating that manipulation of the RAFT agent concentration yielded PDMAm polymers with differing MWs (Figure 2c, entries 1−3). Alternatively, polymer MWs can be tuned by altering the residence time while fixing the chemical concentrations and light parameters; limiting the residence time of reactants inside the photoreactor reduces the monomer conversion of polymer samples exiting the reactor, producing polymers with lower degrees of polymerization and lower MWs. To demonstrate this effect, six PDMAm polymer samples with equal concentrations of RAFT (5.0 mM), monomer (1.5 M), and catalyst (0.07 mM) were passed through the photoreactor using a one-pump system with different flow rates, where they were irradiated with green light (530 nm, 1.8 W/m2). 20, 40, 60, 80, 100, and 120 min residence times were targeted; the fastest flow rate (30 μL/min) led to a residence time of 20 min, which produced PDMAm with a Mp of 9.9 kg/mol at 26% monomer conversion. The slowest flow rate was 5 μL/min (corresponding to a residence time of 120 min); this flow rate produced PDMAm with a monomer conversion of 84% and a Mp of 27.9 kg/mol. The intermediate residence times produced PDMAm samples with conversions and Mp’s that increased logarithmically with residence time, as was to be expected (Figure 3). As shown in Figures 1−3, polymer MWs can be tuned by adjusting either the chemical concentrations of reactants, the residence time of reactants inside the photoreactor, or the wavelength and intensity of the light source. While using these methods provide a facile route for the production of narrowly distributed single MW polymers, we envisioned that an alteration of these parameters during the flow polymerization will result in fully tailorable MWDs. The most commonly used C
DOI: 10.1021/acs.macromol.7b01890 Macromolecules XXXX, XXX, XXX−XXX
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in order to produce a MWD with a tailored response (W log(M)) over a predefined range of molecular weights. In order to generate the tailored MWDs, a one-pump system with a predefined pumping program was used to continuously alter the residence time of reactant passing through the photoreactor. The flow rates, residence times, and volume fractions of PDMAm polymer fractions used in the continuous residence time distribution trials are shown in Table 1. The initially fast Table 1. Flow Rates, Residence Times, and Volume Fractions for Fully Automated MWD Control through Changing Residence Timesa Figure 3. Mp’s and monomer conversion for PDMAm produced at varying photoreactor residence times.
trial no.
step no.
target RT (min)b
flow rate (μL/min)
volume (μL)
step time (min)
volume fraction
target Mp (kg/ mol)c
1 1 1 1 2 2 2 2
1 2 3 4 1 2 3 4
20 40 60 120 20 40 60 120
30 15 10 5 30 15 10 5
1383 563 164 300 2800 600 200 300
46.1 37.5 16.4 60.0 93.3 40.0 20.0 60.0
0.574 0.233 0.068 0.125 0.718 0.154 0.051 0.077
10.3 18.8 22.3 25.0 10.3 18.8 22.3 25.0
method for preparing tailored MWDs is to prepare several single MW polymers and manually blend them to produce a targeted MWD composed of the individual polymer fractions. This is demonstrated in Figure 4 for a series of polymers
a
Experimental conditions: a one-pump system was used to pump a homogenized reaction mixture through a 600 μL photoreactor at ambient temperature. Irradiation wavelength was 530 nm, and intensity was 3.8 W/m2. The catalyst concentration was 50 ppm relative to monomer, and the monomer solids content was 15 vol % in all cases. bResidence time. cTheoretical Mp’s were calculated from the following formula: Mp,theo = X × MWDMAm × α/100 + MWDTPA, where X, MWDMAm, MWDTPA, and α correspond to targeted degree of polymerization, DMAm molar mass, DTPA molar mass, and conversion determined by 1H NMR, respectively.
Figure 4. MWDs for monomodal PDMAm polymer fractions and their blended mixture. 1: Mp = 9.9 kg/mol; 2: Mp = 17.2 kg/mol; 3: Mp = 22.9 kg/mol; 4: Mp = 27.9 kg/mol.
flow rate in step 1 led to the production of polymers with low conversions, and thus low Mp’s, with the subsequent decrease in flow rate increasing the Mp of each fraction thereafter. Notably, the volume fraction of the high flow rate (low target Mp) steps in both trials is required to be far larger than the other volume fractions due to differences in conversions between the low and high Mp fractions. The steps with low target Mp’s have lower conversions and, as such, lower mass concentrations of polymer. Therefore, a much larger volume fraction of low-Mp PDMAm is required as the response of the GPC is based on the mass concentration of polymer at each elution time (vide supra). Four PDMAm fractions with Mp’s of 10.3, 18.8, 22.3, and 24.5 kg/mol were targeted in both the continuous residence time distribution trials. The flow rates and volume fractions for the first trial (Table 1, trial no. 1) were generated in order to produce a MWD with a response that linearly increased from the low Mp to the high Mp. As shown in Figure 5b, the response was approximately linear from the targeted low Mp (10.3 kg/ mol) to the high target Mp (24.5 kg/mol), although the onset of the low target Mp occurred at a higher molecular weight than expected. The flow rates and volume fractions of the second targeted MWD (Table 1, trial no. 2) were developed in order to produce a MWD with an approximately equal response over the range of the low (10.3 kg/mol) to high (24.5 kg/mol) PDMAm Mp’s. By increasing the volume fraction of the low molecular weight PDMAm fractions, while keeping the other fractions nearly equal, the response at low Mp’s increased to create a MWD with a nearly equal response over the targeted
produced through alteration of the reactant residence time. Four PDMAm polymers with different Mp’s were prepared and blended in a ratio that was targeted to produce a MWD with a relatively equal response (W log(M)) over the range of the lowest to the highest Mp’s, i.e., 9.9−27.9 kg/mol. As shown in Figure 4, mixing of the four polymer fractions in the calculated volume ratio resulted in a MWD that showed a nearly equal W log(M) over the range 9.9−27.9 kg/mol, as expected. The polymer mixture exhibited increases in skewness, kurtosis, and dispersity compared to the individual polymer fractions (Supporting Information, Table S11).71 For the GPC used in this study, the response is proportional to the mass concentration of polymer for each elution time. Therefore, in order to generate a polymer mixture with an equal response over a defined range, the mass of polymer eluted at each elution time in that range must be equal. More specific details on the ratios required for blending these individual polymer fractions can be found in Table S9. Control of MWDs through Residence Time Distributions in Flow. While the production of several individual polymer fractions with differing MWs is tedious in batch polymerization, flow polymerization provides the ability to fasttrack the process through automated polymer formation. Through consecutive formation of polymer fractions in flow, and collection of the separate fractions in a single pot, we envisioned that controlled MWDs were attainable in a one-pass manner. Following the predictable results of the individually mixed polymer fractions, a fully automated system was trialled D
DOI: 10.1021/acs.macromol.7b01890 Macromolecules XXXX, XXX, XXX−XXX
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Figure 5. Controlled MWDs produced through residence time distributions: (a) MWD for single Mp target; (b) MWD for linearly increasing response over the range of 10.3−24.5 kg/mol; (c) MWD for equal response over the range of 10.3−24.5 kg/mol; (d) cumulative volume fraction for monomodal Mp; (e) cumulative volume fraction for linearly increasing response over the range of 10.3−24.5 kg/mol; (f) cumulative volume fraction for equal response over the range of 10.3−24.5 kg/mol.
Figure 6. Tailored MWDs produced through alteration of the light source: (a) MWD produced through changes to the light intensity; (b) MWD produced through changes to the irradiation wavelength.
(0.3 W/m2) to high (3.8 W/m2) throughout the polymerization, to produce two separate polymer fractions that formed a bimodal distribution when collected. Again, a one-pump system was employed for the polymerization as both the reactant composition and reactant flow rates were fixed (Table S3). Figure 6a shows the results of the light intensity alteration trial. The tailored MWD shows the expected bimodal distribution composed of two polymer fractions that correspond well to individual polymers with Mp’s of 15.5 and 42.5 kg/mol for low- and high-intensity green light, respectively. As the rate of reaction can be manipulated by switching the wavelength of the light source, rather than the intensity, tailored MWDs can be produced by switching the light source from one irradiation wavelength to another throughout a flow polymerization. Initial studies showed that the polymerization proceeded slightly slower under blue light compared to red light at the same intensity (Figure 1). In order to create a MWD that was clearly bimodal, the intensity of blue light was increased so that a higher Mp PDMAm fraction would be produced in the tailored MWD trial. Figure 6b shows PDMAm fractions individually produced under blue (460 nm, 4.2 W/m2) and red (630 nm, 1.4 W/m2) light irradiation as well as the tailored MWD generated by switching the light source from red to blue throughout the reaction. Again, an expected bimodal MWD was generated that indicates the production of PDMAm
range (Figure 5c). Increasing the volume fraction of the low molecular weight polymer fraction led to a decrease in the Mn, and an increase in the dispersity, as expected (Table S11). Figure 5d−f shows the cumulative volume fraction profiles for the continuous residence time distribution trials. Figure 5d is a straight line indicating a single Mp was targeted, while Figure 5e,f have increasing volume fractions of lower Mp PDMAm polymer fractions. Clearly, increasing the targeted volume fraction of the low-Mp PDMAm increases the response of that fraction in the overall MWD. In this way, it is possible to impart control on the MWD. Control of MWDs through Light Alteration in Flow. With the success controlling the MWD through continuous alteration of the residence time, control over the MWD was attempted through alteration of other parameters. One advantage of using photopolymerization systems to control MWDs through flow processes is the ability to simply change the properties of the light source to tune the MWD. Since the rates of reaction in PET-RAFT polymerization systems are reliant upon the wavelength and intensity of light absorbed by the photocatalyst, alteration of these parameters generates polymers with different molecular weights for the same polymerization time (Figure 1). To demonstrate the ability to form tailored MWDs in flow by alteration of the light source, the intensity of green (530 nm) light was switched from low E
DOI: 10.1021/acs.macromol.7b01890 Macromolecules XXXX, XXX, XXX−XXX
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Figure 7. Theoretical vs experimental results for controlled GPC shapes: (a) MWD with an equal response over the range 10.6−34.4 kg/mol; (b) MWD with an equal response over the range 17.0−90.1 kg/mol.
Figure 8. PDMAm sample with a targeted equal number of chains from 8.5 to 28.7 kg/mol: (a) GPC trace; (b) normalized number of chains distribution.
equal response over the molecular weight range of 15−30 kg/ mol. Figure 7b shows the theoretically predicted and experimentally determined MWD with four targeted Mp’s of 17.0, 29.7, 52.1, and 90.1 kg/mol at 85% monomer conversion. The four targeted polymer fractions in the experimental curve appear to have Mp’s in line with theoretical predictions, though there is some discrepancy between the experimental and theoretical response. The response of the middle two MW fractions appear to be approximately in line with the theoretical response, while the first (17 kg/mol) fraction is underexpressed and the last (90.1 kg/mol) fraction is slightly overexpressed. Moreover, the high MW fraction exhibits a very long tail at high molecular weights. Controlling the Number of Chains Distribution in Flow. Another advantage of being able to control polymer MWDs is the ability to directly control the number of chains in a polymer sample. Noting that the GPC response is based on the concentration of polymer at a specific elution volume, the number of chains in a polymer sample can be calculated through treatment of the GPC data with the equations of Clay and Gilbert (eq 1).2,72
with varied molecular weights in line with the individually produced polymers. Details on the flow rates and injection volumes for the wavelength alteration experiment can be found in Table S4. Control of MWDs through Alteration of Chemical Concentrations in Flow. The previous sections provided a way to control the MWDs of PDMAm polymers through alteration of the pumping flow rate (reactant residence time) as well as the light source. As shown in Figure 2, the chemical concentrations can also be altered to produce polymers with different MWs at a constant light intensity and residence time. To exploit this phenomenon for the control of polymer MWDs, a three-pump system was used to alter the concentration of RAFT agent while fixing the concentrations of monomer and catalyst. It is possible to constantly alter the MWs of polymers in continuous flow polymerization with the use of a two-pump system, though in this case the concentration of monomer changes throughout the reaction. As the reaction rate in radical chain polymerization is dependent on the concentration of monomer, it was important to fix the monomer concentration in these experiments. To demonstrate the production of tailored MWDs by changing the chemical concentrations in flow polymerization, two relatively distinct MWDs with an equal response (W log(M)) over a predetermined range were targeted. Figure 7a shows the theoretical and experimental MWDs for a distribution with an approximately equal response over the range 10.6−34.4 kg/mol. The target Mp’s of the individual PDMAm fractions were 10.6, 15.7, 23.3, and 34.4 kg/mol at 85% monomer conversion. Theoretical curves were generated based on the superimposition of four monomodal MWDs with Mp’s identical to the experimentally targeted values. More information on the development of the theoretical curves can be found in the Supporting Information. The experimental MWD was narrower than expected but still produced a nearly
P(̂ M ) = (arbitrary constant) ×
G(Vel) M2
(1)
where P̂(M) is the cumulative number MWD, G(Vel) is the polymer concentration at a certain elution volume, and M is the molecular weight. Based on this equation, a number of chains distribution can be developed by dividing the response (W log(M)) at each molecular weight by the square of its molecular weight. Therefore, in order to obtain a polymer sample with equal numbers of chains over a given molecular weight range, the response must quadratically increase from the low to high molecular weights in that range. To demonstrate the ability to autonomously control the number of chains distribution, four F
DOI: 10.1021/acs.macromol.7b01890 Macromolecules XXXX, XXX, XXX−XXX
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Figure 9. Molecular weight profiles for MWDs produced in a 6 mL reactor. (a) Targeted and experimentally determined Mp’s at different elution volumes. (b) MWDs for samples taken at different elution volumes. No. 1: elution volume of 0 mL; No. 2: elution volume of 0.93 mL; No. 3: elution volume of 1.87 mL; No. 4: elution volume of 2.8 mL.
Figure 10. Mixing behavior inside flow reactor tubing.
PDMAm polymer fractions with varying target Mp’s and volume fractions were prepared in a three-pump reaction setup. The target Mp’s for the four PDMAm polymer fractions were 8.5, 12.8, 19.1, and 28.7 kg/mol for the number of chains distribution, with volume fractions equal to 0.051, 0.114, 0.257, and 0.578 for the low to high targeted Mp’s, respectively. More detailed information on the calculation of volume fractions required can be found in the Supporting Information. Figure 8a shows the GPC trace for the autonomously produced number of chains distribution. The GPC trace sharply rises at low retention times until the GPC peaked; thereafter, the curve is approximately quadratic in shape, as expected by the targeted Mp’s and volume fractions. Notably, on the low retention time side of the GPC Mp the curve did not decay as sharply as expected, which introduced some high molecular weight tailing in the normalized number of chains distribution (Figure 8b). Although there was some tailing, the normalized number of chains distribution did show an approximately equal number of chains for molecular weights between 5 and 25 kg/mol, which was close to the theoretically predicted distribution (equal number of chains from 8.5 to 27.8 kg/mol). Effect of Reactor Flow Profiles. As shown above, utilizing photoinduced flow polymerization provides several different approaches for the alteration and control of MWDs. It must be noted that the flow profiles in our photoreactor introduced some mixing behavior to our flow polymerization systems. The mixing behavior is a direct result of the flow reactor geometry, which plays a critical role in determining system fluid dynamics, and, as such, thermal phenomena, residence time distributions, and chemical concentration gradients during the polymerization process.73,74 These effects must be understood in order to identify the limitations of the system. Mixing Behavior. Calculation of the Reynolds number for the above systems shows that all fluids were in the laminar regime for the entire duration of their polymerizations (Re
