Cyclic Oligomer Byproducts Separated by High-Resolution Advanced Polymer Chromatography (APC)

Authors: Christopher Kasprzak^a, Katherine Heifferon^a, Josh Wolfgang^a, Ryan Mondschein^a, Carrie Schindler^b, Kyle Williams^b, Timothy Long^a

^aDepartment of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA USA
^bMalvern Panalytical, Houston, TX USA

Introduction

Step-growth polymerization involves the reaction of two complementary functional groups creating repetitive linkages to form long linear chains of repeat units. As this occurs randomly, there is competition between linear polymerization and cyclic oligomer byproducts which comprise dimer, trimer, or higher order structures [1]. These cyclic byproducts can either be beneficial or detrimental depending on the desired final material properties. Due to the lack of end-groups in these cyclic species, characterizing the amount and type of cyclic oligomers formed during the polymerization is challenging. A technique to quickly analyze these byproducts is greatly desired. This is incredibly important for various research areas, such as developing new polymeric precursors for 3D printing techniques. While methods of extracting and concentrating the cyclic content exist, these are quite time consuming. There is a need for a faster analysis to characterize the absolute molecular weight distribution with high-resolution separation of cyclic oligomers. 

Conventional gel permeation/size exclusion chromatography (GPC/SEC) has been paramount to understanding the molecular weight distribution of polymeric systems for decades. However, the drawback of single detector concentration analysis derives from the need for calibration standards matching the sample type and molecular weight range. The introduction of advanced detection, combining viscometer and light scattering detectors with concentration detectors, has provided a method of obtaining a wealth of information from a single injection. Absolute molecular weight, intrinsic viscosity, and hydrodynamic size are among the many parameters obtained by advanced detection GPC/SEC with no dependence on calibration standards. The accuracy of the result critically depends on, among other things, the chromatography or the separation of molecules in solution. One particularly challenging area is the separation of low molecular weight oligomers for absolute molecular weight determination. While the separation may be achieved using several GPC/SEC columns in series, the average run time per sample can be 45-60 minutes in length. In addition, the detection aspect is quite challenging since small molecules generally give poor light scattering responses. This type of analysis typically requires a high concentration of sample injected onto the column to combat the inherent low amounts of scattering and an artificially broadened sample peak. 

The Waters ACQUITY Advanced Polymer Chromatography^TM (APC) system addresses this separation issue as it utilizes ultra-performance liquid chromatography (UPLC) technology to employ low volume columns with smaller particle sizes. This enhances peak resolution and lowers sample volume requirements by decreasing band-broadening, which in turn yields taller and sharper peaks. The APC pump is uniquely designed to safely operate at higher pressures required to flow solvents through the columns at normal GPC/SEC flow rates resulting in benefits, such as shorter run times per sample of 5-10 minutes and lower mobile phase consumption.

To complement this low-volume technique, Malvern Panalytical’s UPLC compatible OMNISEC REVEAL houses refractive index (RI), UV-Vis photodiode array, light scattering, and viscometer detectors in a compact design that minimizes inter-detector tubing volume to maintain maximum resolution. Simultaneous data collection from all detectors on a single injection is accomplished without significantly increasing band-broadening. When combined with the superb separation realized by the APC system and columns, the benefits of advanced detection can be achieved in 1/3 of the time versus standard GPC/SEC columns. 

In this Application Note, we discuss the utility of OMNISEC REVEAL in combination with the resolution and speed improvements of APC to measure cyclic byproducts of a step-growth polymerization. The goal is to resolve individual low molecular weight species from the main polymer peak. The subsequent analysis will be compared with other analytical methods such as NMR or MALDI-TOF to provide confirmation and insight into the length of polymer chains forming cyclics. 

Methods

A polysulfone sample was prepared as a 5.0 mg/mL solution in chloroform and a 50 μL aliquot was injected into the Waters APC equipped with a 45x45x45 Å 150 mm XT column set. The eluent chosen was chloroform at 40 °C with a flow rate of 0.5 mL/min. The dn/dc value determined and used for molecular calculations was 0.194 mL/g. 

Results & Discussion

The polysulfone sample discussed herein was prepared through a solution polycondensation reaction. This process is known to produce small amounts of low molecular weight cyclic byproducts during the polymerization. This has been confirmed in analogous systems through a costly and time-consuming dialysis process (to concentrate the cyclic species) and subsequent analysis with mass spectrometry. Characterization of these low molecular weight byproducts by GPC/SEC has the potential to offer insight into the polymerization reaction in a quicker and more cost-effective manner.

Data collected from the analysis of the polysulfone sample by the combined APC-REVEAL instrument is shown below in Figure 1, with an emphasis on the low molecular weight region. The achieved separation allowed for multiple low molecular weight peaks to be completely resolved from the main polymer peak. This resolution, combined with the sensitivity of the RI and light scattering detectors, allowed the OMNISEC software to analyze and calculate data for each individual species present. The calculation limits for each peak are depicted below in Figure 1; the results from these analyses are presented in Table 1 and Table 2.

Figure 1. The peak labeling of the chromatogram shown in Figure 1 which correlates to the peaks discussed in Table 1.

Table 1. A summary of the molecular weight information obtained through analysis of the chromatogram of the polysulfone sample, represented as the average of three injections. 

Table 2. A summary of the percent RSD from the three injections of polysulfone sample.

Each of the defined peaks was calculated to have a narrow dispersity, as shown in Table 1. While the data suggests the presence of low-molecular weight species produced in this step-growth polymerization, further analysis by mass spectrometry would ultimately confirm the presence of cyclic species. Analysis of the sample using the APC-REVEAL pairing provides a faster, high-throughput analysis method for screening polymers for low molecular weight byproducts during or after polymerization. 

Conclusion

This work demonstrated the powerful combination of UPLC chromatography with advanced detection systems. The Waters ACQUITY APC™ system allowed for superb separation of low molecular weight species, as peaks with molecular weight differences as low as 120 g/mol were observed to be almost entirely resolved from each other. The Malvern Panalytical OMNISEC REVEAL demonstrated quantitative analysis of each peak in the chromatogram. The data gathered from the advanced detection system revealed that each peak was narrow in dispersity. Furthermore, this data was gathered from analyzing only 250 µg of material with a runtime of 20 minutes per injection. When used in tandem, the APC and OMNISEC REVEAL units provided unprecedented resolving power and detection for low molecular weight species such as cyclic oligomer byproducts. 

References

[1] Allcock, Harry. Contemporary Polymer Chemistry. 2003.