The well-established synthetic technique of free radical polymerizations is widely used throughout a variety of different polymer industries, from the R&D laboratory to the full-scale plant production of many different polymeric types. However, the polymerization mechanism offers limited control over the final product making the tailoring of material properties difficult. However, the combination of several monomer components in a polymerization along with the careful addition of materials in stages throughout the course of the reaction provides a degree of control. Using these variables helps to tailor the final product of these reactions to suit end-use application, but characterizing these materials proves a large amount of work.
Described here is a reproducibility study of a series of free radical feed copolymerizations, in or near real time, using a PL polymerization monitoring and control (PL-PMC) system that utilizes rapid online gel permeation chromatography (GPC) techniques. This system shortens the time required to fully investigate the polymerization process as key information is obtained throughout all the stages of the reaction in one experiment, providing molecular weight distribution information across the full reaction profile. This greatly increases the efficiency of the investigation and leads to the possibility of altering the reaction conditions in real time to ensure that the reaction remains under control.
Case Study: Reproducible Online Monitoring of Molecular Weight Distributions and Monomer Conversion in a Free Radical Polymerization Reaction using PL-PMC InstrumentationA PL-PMC instrument was used with chromatographic monitoring in a GPC configuration to make molecular weight and conversion measurements of the polymer synthesized throughout a solution based free radical feed copolymerization reaction of butyl acrylate and acrylic acid. The reaction was conducted under a continuous nitrogen blanket and was investigated using the following reaction conditions with the reaction being repeated three times to investigate batch reproducibility:
Initial Monomer: Acrylic acid and butyl acrylate
Monomer Feed: Acrylic acid and butyl acrylate
Initiator: Azo-bis-isobutyronitrile (AIBN)
Solvent: Ethyl acetate / acetone
Reaction Temperature: 78 °C
Each of the reactions were continuously monitored using Rapid-GPC under the following conditions:
Column: PL Rapide-M (100 x 10 mm) (PL1013-2500)
Solvent: THF
Flow Rate: 3.0 mL/min
Detector: Differential Refractive Index
Temperature: Ambient
Injection Volume: 100 μL
Collection Time: 2.5 minutes
Calibration: Polymethylmethacrylate Narrow Standards
Results and Discussion
Each polymerization was allowed to proceed for a period of roughly 5 hours, with GPC data collected automatically by the PL-PMC system every 2.5 minutes. Figure 1 shows the full raw data GPC chromatograms obtained across the full reaction time. All data from each polymerization was processed automatically to yield a series of molecular weight averages and polydispersity data for each polymerization, with these calculated molecular weight distributions shown in figure 2.
Figure 1. Full raw data GPC chromatograms obtained during the entire course of the free radical feed copolymerization.
Figure 2. Full molecular weight distributions calculated during the full course of the free radical feed copolymerization. The distributions calculated in real time as the chromatograms are obtained provides a key insight into the polymer formed.
Conclusions
As shown from the data obtained, the PL-PMC instrumentation fitted with chromatographic monitoring offered a convenient and automated route to the full characterization of monomer conversion and molecular weight distribution during a complex multi-step free radical feed copolymerization reaction in real time. The system has allowed a rapid analysis of the reproducibility of a series of polymerizations providing fully automated GPC data every 2.5 minutes across a 5 hour run time.
Figure 3 shows the molecular weight average and polydispersity information with respect to time from both reactions, while figure 4 shows the conversion information with respect to time from both polymerizations. The steady and reproducible increase in monomer conversion throughout the reaction is clearly seen, as well as the expected trends for both molecular weight and polydispersity indices indicative of this type of polymerization mechanism.
Figure 3. Repeated calculated molecular weight averages and polydispersity index v reaction time across the full reaction profile showing the progress of the polymerization in near real time.
Figure 4. Repeated conversion v reaction time across the full reaction profile highlighting the evolution of polymer and the consumption of monomer throughout the polymerization.