Liquid Chromatography At The Critical Condition Research Articles

Effects of macromonomer chain length, solvent and concentration on the cyclization kinetics during AB-type step-growth polymerization

For the first time, this work has explored the cyclization kinetics of multi-mers during step-growth polymerization by using alkyne-PS-azide (l-PS-N3) as model macromonomer, where PS represents polystyrene. First, three AB-type PS macromonomers with different chain lengths have been prepared by atom transfer radical polymerization (ATRP), namely, l-PS77-N3, l-PS240-N3 and l-PS380-N3. Second, the polymerization and cyclization kinetics has been monitored in dimethylformamide (DMF), tetrahydrofuran (THF) and cyclopentane (CyP). Third, by size exclusion chromatography (SEC) and liquid chromatography at the critical condition (LCCC) methods, we have quantified how the macromonomer chain length, solvent type and reaction concentration affect the intra-chain cyclization process. The analysis results have demonstrated that, for macromonomer l-PS77-N3 with shorter chain length, the reaction rate of macromonomer cyclization, multi-mer cyclization and inter-chain coupling, is significantly faster than that for l-PS240-N3 and l-PS380-N3 with longer chain lengths. The total percentage of final cyclic product (wtcyclic) slightly decreases with the increase of macromonomer chain length, namely, 33%, 25% and 23% for l-PS77-N3, l-PS240-N3 and l-PS380-N3, respectively. At the same time, we found that, solvent with lower solvent viscosity and relatively poor solvent quality is more conducive to the cyclization reaction of PS chains. In addition, we also found that the inter-chain and intra-chain reaction rate are both faster than that at other reaction concentrations when the reaction concentration is near the critical overlapping concentration.

Investigation of the Multimer Cyclization Effect during Click Step-Growth Polymerization of AB-Type Macromonomers

In this work, by combining size exclusion chromatography (SEC) and liquid chromatography at the critical condition (LCCC) techniques, we have studied the multimer cyclization effect during click step-growth polymerization of AB-type macromonomers. First, by atom transfer radical polymerization (ATRP), we have synthesized a series of alkynyl and azide functional AB-type polystyrene macromonomers with different molar masses, i.e., PSn. Second, via the click step-growth polymerization of a PSn macromonomer in dimethylformamide (DMF) at varied reaction concentrations (Cs), we have prepared polyaddition products [(PSn)m] with large polydispersity indices, where m represents the average degree of polymerization of the macromonomer. Further, narrowly distributed fractions have been obtained by SEC fractionation. Third, we have quantitatively analyzed how the macromonomer molar mass, reaction concentration, and multimer molar mass influence the percentage of a cyclic topoisomer (wtcyclic) for (PSn)m samples. In particular, we have established the relation between wtcyclic and C for unfractionated (PSn)m, i.e., wtcyclic = 0.86–0.32 log C, and the relation between wtcyclic and m for fractionated (PSn)m prepared at a constant C = 100 g/L, i.e., log wtcyclic = −0.46m + 0.16. For the first time, our result has unambiguously pointed out that neither a cyclic topoisomer nor a linear topoisomer can be ignored when considering the structure–property quantitative relationship for polyaddition products.

Preparation of low molecular weight cyclic polystyrenes with high purity via liquid chromatography at the critical condition

Cyclic polymers synthesized by ring-closure method from linear precursors contain some of linear contaminates. In this work, the origin of linear contaminates in cyclic polystyrenes (c-PS) is demonstrated by the coupling of liquid chromatography at the critical condition (LCCC) with matrix-assisted laser desorption/ionization time-of-flight mass spectra. The linear contaminates are revealed to be the “dead” chains during ATRP by chain termination, the unreacted linear polystyrene (l-PS) precursors, and the dimers by the imperfect ring-closure reaction. The c-PS are purified by LCCC fractionation, and the results show the LCCC fractionation at the critical adsorption point (CAP) of c-PS is more efficient than that at the CAP of linears for low molecular weight (<10,000) PS. A two-step LCCC method is presented for the preparation of c-PS with high purity (>99.6%) via the tandem-coupled LCCC fractionation at the CAP of l-PS and at the CAP of c-PS.

Interactions of complex polymers with nanoporous substrate.

With the advance of polymer synthesis, polymers that possess unique architectures such as stars or cyclic chains, and unique chemical composition distributions such as block copolymers or statistical copolymers have become frequently encountered. Characterization of these complex polymer systems drives the development of interactive chromatography where the adsorption of polymers on the porous substrate in chromatography columns is finely tuned. Liquid Chromatography at the Critical Condition (LCCC) in particular makes use of the existence of the Critical Adsorption Point (CAP) of polymers on solid surfaces and has been successfully applied to characterization of complex polymer systems. Interpretation and understanding of chromatography behaviour of complex polymers in interactive chromatography motivates theoretical/computational studies on the CAP of polymers and partitioning of these complex polymers near the CAP. This review article covers the theoretical questions encountered in chromatographic studies of complex polymers.

A Monte Carlo study on LCCC characterization of graft copolymers at the critical condition of side chains

Liquid chromatography at the critical condition (LCCC) has been used to characterize graft copolymers based on the assumption that one of the copolymer blocks becomes chromatographically “invisible” at the critical condition of the corresponding homopolymer. We investigate the validity of this assumption with lattice Monte Carlo simulations of A-g-B graft copolymers modeled as either random walk (RW) or self-avoiding walk (SAW) chains composed of multiple invisible B blocks grafted to an A backbone that is either in size-exclusion chromatography (SEC) or liquid adsorption chromatography (LAC) mode. The simulations show that, in agreement with recent experimental results, the B blocks have a small, but noticeable, impact on the overall elution of the graft copolymer. This influence exists even when the chains are modeled as RWs, indicating that its fundamental origin is due to chain connectivity, not excluded volume interactions. In general, both models show that grafting B blocks to an A backbone in SEC mode tends to increase copolymer retention, while grafting B blocks to an A backbone in LAC mode tends to decrease copolymer retention. When the copolymer is modeled as a SAW, the excluded volume interaction increases the entropic penalty associated with the addition of a B block and, therefore, the addition of B blocks in SAW chains is more likely to result in a decrease in the retention time of the copolymer when the A backbone is in LAC mode.

Can the individual block in block copolymer be made chromatographically “invisible” at the critical condition of its corresponding homopolymer?

Liquid chromatography at the critical condition (LCCC) method has been proposed as an attractive method to characterize individual blocks in block copolymers because it can make one block chromatographically “invisible” at the critical condition of its corresponding homopolymer. However, observable dependence of retention time on the “invisible” block length was reported in LCCC experimental studies of diblock copolymer [Macromolecules 2001;34:2353–2358; Anal Chem 2001;73:3884–3889.]. In this study, we re-examined the validity of the LCCC method for AB, BAB and ABA block copolymers by the lattice Monte Carlo simulation method with using random walk (RW) and self-avoiding walk chain (SAW) models. In the current study, the A block is set in the size exclusion mode and is chromatographically “visible”. The interaction between the B type monomer and the column surface are varied to identify the critical condition of B block in the block copolymer. Our simulation results establish that individual block, i.e. B block in the current work, from the block copolymer has its own critical condition for the first time. However, it was also found that the critical condition of B block might be different from the critical condition of B homopolymer. The critical condition of B block in the AB diblock is exactly equal to the critical condition of B homopolymer only when the chain is modeled by RW model. However, deviations of the critical condition of B block away from that of the B homopolymer are observed for AB copolymer using SAW model, and also for BAB and ABA copolymers whether the chain is modeled by RW or SAW models. Moreover, under the critical condition of B homopolymer, no dependence of the partition coefficient on the “invisible” B block length was observed for the AB and BAB copolymers when the copolymer chains were modeled by RW model. Distinct dependence of the partition coefficient of these two types' copolymers on the B block length was found when the chain was modeled by SAW model. For the ABA triblock copolymer, slight dependence of the partition coefficient on B block length was observed even for the RW model while this dependence became much stronger when the chain was modeled by SAW model. Moreover, it was also found that the partition coefficient of ABA copolymer is much smaller than those of AB and BAB copolymers under the critical condition of B homopolymer because of the chain architecture effect. The current study confirms that the block in the block copolymer is hard to be made completely chromatographically “invisible” because the intrinsic nature of the excluded volume interaction existed in real polymer system.

Partitioning of star branched polymers into pores at three chromatography conditions

The partitioning of star branched polymers into a slit pore at three different chromatography conditions, namely, size exclusion chromatography (SEC), liquid chromatography at the critical condition (LCCC), and liquid adsorption chromatography (LAC) have been investigated with lattice Monte Carlo simulations. Two different chain models are used: random walks (RW) that have no excluded volume interaction and self-avoiding walks (SAW) that have excluded volume interaction. The simulation data obtained for the two chain models are compared to illustrate the effect of excluded volume interactions on the partitioning of star branched polymers. The two most outstanding effects observed due to the introduction of excluded volume interactions are: (i) stars with a high number of arms can be excluded from the pore at condition corresponding to the LCCC of the linear polymers; (ii) the partition coefficient of stars in LAC mode is not dependent only on the total number of monomers on the chain. These effects illustrated by the current study should be taken into account when interpreting experimental chromatography data for branched polymers.

Retention Behavior of Star-Shaped Polystyrene near the Chromatographic Critical Condition

The retention behavior of star-shaped polystyrene (PS) at the liquid chromatographic critical condition of linear PS was investigated. The star-shaped PS samples were prepared by anionic polymerization of styrene and subsequent linking of the polystyryl anions with divinylbenzene. The linking reaction yields a series of star-shaped PS with different number of branches of equal length. Three star-shaped PS samples with different arm molecular weight (MW) were prepared. To investigate the MW (hence the branch number) dependence of the LCCC (liquid chromatography at the critical condition) retention, the two-dimensional liquid chromatography method was used—first separating the polymers with respect to the molecular weight and subsequently separating the effluent by LCCC. Two different pore size columns were used for the LCCC separation to investigate the pore size dependence. The LCCC retention shows a very complex behavior. The retention time of the star-shaped PS shows a strong variation with MW (branch number) at the coelution condition of linear PS, and the deviation from the coelution behavior is more serious at a smaller pore sized column. The peculiar LCCC retention behavior was successfully delineated by a lattice Monte Carlo method taking into account the excluded volume and weak attractive interaction of the chain ends.

HPLC Characterization of Cyclization Reaction Product Obtained by End-to-End Ring Closure Reaction of a Telechelic Polystyrene

Cyclization reaction product synthesized by the end-to-end ring closure reaction of a telechelic polystyrene with molecular weight of 38K in extremely dilute conditions was carefully characterized by using two kinds of HPLC techniques, that is, liquid chromatography at the critical condition (LCCC) and size exclusion chromatography (SEC). First, the cyclization reaction product was coarsely separated into linear species and cyclic ones by LCCC; second, each fraction was further separated by high-resolution SEC. It was found from the HPLC analyses that the cyclization reaction product contains both linear and cyclic condensation products. Furthermore, dimeric, trimeric, and more multimeric cyclic molecules with reasonable abundance were identified as well as the monomeric cyclic molecule with high yield, as much as 50%, in the cyclic products.

Retention Behaviors of Block Copolymers in Liquid Chromatography at the Critical Condition

The partitioning of a diblock copolymer (AB) and a triblock copolymer (ABA and BAB) into a pore, at a condition similar to that employed in the experimental liquid chromatography at the critical condition (LCCC), was investigated by lattice Monte Carlo simulations with self-avoiding-walk chains. The B block is set at the predetermined critical condition where the partition coefficient of a homopolymer B has a least dependence on its chain length, and hence becomes chromatographically “invisible”. The A block is set at the size-exclusion mode and is chromatographically “visible”. The partition coefficients of these copolymers were compared with that of a homopolymer A with the same (total) length of the visible A block(s). The partition coefficient of a diblock, KAB, was found to be larger than KA, especially when the A block was short and the B block was long. The difference tends to vanish with an increase in the A block length or a decrease in the B block length. A smaller pore also tends to decrease th...

Preparation and Characterization of Cyclic Polystyrenes

Macrocyclic polystyrenes with various molecular weight were prepared by the reaction of linear polystyrene having two end vinyl groups and potassium naphthalenide as a coupling agent. Intermolecular side reactions produced higher molecular weight polycondensates and undesirable chain termination reactions also produced linear precursors in addition to the designed molecules. In order to isolate cyclic polymers from the ring closure reaction mixtures, preparative size exclusion chromatography (SEC) method was employed. The purity of SEC-fractionated cyclic polystyrenes was rigorously examined by liquid chromatography at the critical condition (LCCC) and interaction chromatography (IC). After SEC-fractionation, we obtained large amount of highly pure cyclic polymers, though high molecular weight cyclic polymers contain very small amount of linear precursor (<5%). The purity and isotope effect on reversed-phase liquid chromatography (RPLC) were also rigorously investigated by preparing a deuterated cyclic polystyrene.

Theory of chromatography of linear and cyclic polymers with functional groups

We discuss the chromatographic behavior of linear polymers and rings having specific (functional) adsorption-active groups. Functionalized eight-shaped, daisy-like and theta-shaped macromolecules are considered as well. By using a model of an ideal chain with point-type defects in a slit-like pore we derive equations for the distribution coefficient covering all modes of chromatography of functionalized polymers of any molar mass in both narrow and wide pores. Additionally simple approximate formulae are obtained for a number of important modes of chromatography; chromatograms are simulated for model mixtures of polydisperse non-functional and functional polymers. By using the theory we analyze separation of polymers by molar mass, functionality and topology. Although there is a principal possibility to use adsorption chromatography or size-exclusion chromatography (SEC), we conclude that the liquid chromatography at the critical condition (LCCC) is especially efficient for separation of polydisperse polymers by both functionality and topology. The theory predicts that functionalized linear and cyclic polymers can be separated from each others by LCCC even better than non-functionalized ones. The LCCC behavior of some other types of polymers such as comb-like and semicyclic ones is discussed as well.

Effect of Block Copolymer Chain Architecture on Chromatographic Retention

The chain architecture dependence of the retention behavior of block copolymers in the temperature gradient interaction chromatography (TGIC) and liquid chromatography at the critical condition (LCCC) was investigated. For the purpose, polystyrene (PS)/polybutadiene (PB) diblock (SB), SBS triblock, and BSB triblock copolymers were prepared by sequential anionic polymerization and further fractionated by reversed-phase TGIC to obtain a set of the block copolymers with high purity, narrow distribution, and matched block length. In the TGIC separation with C18 bonded silica stationary phase and a mixture of CH2Cl2/CH3CN mobile phase, retention of the three block copolymers with matched molecular weight and composition shows a significant architecture effect: SBS elutes significantly earlier while BSB or SB elute later at similar retention volume. It indicates that the polymer−stationary phase interaction is less effective for the PB block located at the middle of the chain than the blocks located at the chain end. In LCCC separation at the critical condition for PB block, SBS is eluted early while SB and BSB were eluted later at the same retention time. Therefore, triblock copolymer with an invisible middle block behaves differently from those having invisible end block(s). This behavior is consistent with the theoretical prediction by Guttman et al. [Macromolecules 1996, 29, 5723].

Development of water-based liquid chromatography at the critical condition.

Liquid chromatography at the critical condition (LCCC) is a chromatographic technique that allows for the isolation of one area of the polymer matrix so that other areas of the polymer may be probed with size-exclusion or adsorptive chromatographic modes. This technique has been successfully applied to the analysis of functionality distributions in functionalized oligomers and to polymer distributions within copolymers. Herein, the critical conditions of two polar polymers, poly(acrylic acid) and polystyrene sulfonate, are determined. These conditions were identified by varying buffer concentration, organic modifier within the mobile phase, or both. At the critical condition of poly(acrylic acid), the retention characteristics of a copolymer of acrylic acid and vinyl pyrrolidinone were determined. This extension to water-based mobile phase conditions will substantially broaden the possible applications of LCCC.

Fundamental studies of liquid chromatography at the critical condition using enhanced-fluidity liquids.

The improvement in the analysis of telechelic polymer matrixes continues to be a pursuit for many scientists of varying disciplines. This quest for a new technique has led to the continued development of liquid chromatography at the critical condition (LCCC) or liquid chromatography at the critical adsorption point (LC-CAP). LCCC allows for the isolation of one area of the polymer matrix so that other areas of the polymer can be probed with size-exclusion or adsorptive chromatographic modes. Although this technique has been successfully applied to the analysis of telechelic polymers, the practice of LCCC can be difficult. These difficulties include finding and maintaining a solvent system appropriate for the practice of LCCC as well as deterioration of peak shape once the system is operating at the LCCC mode. Because of the specificity of the mobile phase required for the practice of LCCC, the work is routinely practiced by premixing solvents. Previous work with enhanced-fluidity liquid mobile phases demonstrated that these mobile phases removed many of the aforementioned challenges associated with working at the LCCC mode. These mobile phases utilize both pressure and temperature variation in order to maintain the specific solvent strength necessary for the LCCC work. This work studies the coupling and optimization of enhanced-fluidity, EF, liquid mobile phases for LCCC. Several EF-LCCC systems, differing in mobile phase composition, temperature, and pressure, were routinely established, resulting in the effective practice of critical chromatography. The practice of LCCC with on-line mobile phase preparation is demonstrated using commercially available instrumentation. Finally, EF-LCCC is used to analyze triblock and diblock copolymers.

Characterization of poly(L-lactide)-block-poly(ethylene oxide)-block-poly(L-lactide) triblock copolymer by liquid chromatography at the critical condition and by MALDI-TOF mass spectrometry.

Poly(L-lactide)-block-poly(ethylene oxide)-block-poly(L-lactide) triblock copolymers (PLLA-b-PEO-b-PLLA) were fractionated in terms of the number of LLA units by liquid chromatography at the critical condition (LCCC) of PEO block. The fractionated samples were identified using MALDI-TOF mass spectrometry. The dependence of the LCCC retention of the diblock and triblock copolymers on the degree of polymerization of PLLA block(s) follows Martin's rule very well. Unlike the case of PEO-b-PLLA diblock copolymer reported earlier (Lee, H.; et al. Macromolecules 1999, 32, 4143), however, a splitting of the elution peaks containing the same number of LLA units was found. The peak splitting was ascribed to the different length distributions of PLLA blocks at the two ends of the PEO block. From the relative intensities of the peaks, the split peaks were assigned to different isomeric structures of the PLLA blocks. From these results we conclude that the interaction of the triblock copolymers with the stationary phase is affected by the distribution of the interacting blocks at the two ends of the center PEO block, in addition to the total number of LLA units in the triblock copolymer.

Polymer characterization using packed capillary size exclusion and critical adsorption chromatography combined with maldi-tof mass spectrometry

The complete characterization of polymers requires the evaluation of both the molecular weight and chemical composition distributions. Size exclusion chromatography and liquid chromatography at the critical condition (LCCC) are two methods often used to obtain these distributions. Recently, matrix-assisted laser desorption ionization (MALDI)–time of flight (TOF) mass spectrometry has been used for polymer characterization. The application of packed capillary SEC and LCCC is illustrated here for the characterization of poly(propylene glycol) based materials. Especially in the case of critical chromatography, packed capillary chromatography separations were achieved that were not possible using analytical-scale critical chromatography. In addition, the off-line combination of packed-capillary SEC and LCCC with MALDI-TOF is described. ©1999 John Wiley & Sons, Inc. J Micro Sep 11: 53–61, 1999

Polymer characterization using packed capillary size exclusion and critical adsorption chromatography combined with maldi‐tof mass spectrometry

The complete characterization of polymers requires the evaluation of both the molecular weight and chemical composition distributions. Size exclusion chromatography and liquid chromatography at the critical condition (LCCC) are two methods often used to obtain these distributions. Recently, matrix-assisted laser desorption ionization (MALDI)–time of flight (TOF) mass spectrometry has been used for polymer characterization. The application of packed capillary SEC and LCCC is illustrated here for the characterization of poly(propylene glycol) based materials. Especially in the case of critical chromatography, packed capillary chromatography separations were achieved that were not possible using analytical-scale critical chromatography. In addition, the off-line combination of packed-capillary SEC and LCCC with MALDI-TOF is described. ©1999 John Wiley & Sons, Inc. J Micro Sep 11: 53–61, 1999

Improvements in Polymer Characterization by Size-Exclusion Chromatography and Liquid Chromatography at the Critical Condition by Using Enhanced-Fluidity Liquid Mobile Phases with Packed Capillary Columns

Microscale chromatography has found numerous applications in liquid chromatography. The combination of enhanced-fluidity liquid mobile phases with packed-capillary LC is evaluated for polymer characterization using size-exclusion chromatography (SEC) and liquid chromatography at the critical condition (LCCC) phase. Separations of polystyrene polymers and copolymers are completed using liquid chromatography at the critical condition. The critical conditions of polystyrene polymers were approached by changing the concentration of CO(2) in the mixture combined with temperature and pressure variation. Because the solvent strength of enhanced-fluidity liquid mixtures is affected by temperature and pressure variation, the solvent strength could be fine-tuned to accurately find the critical condition. Long packed capillaries could be used in this application because the enhanced-fluidity mobile phases have low viscosities. High efficiencies resulted. The performance of packed-capillary and analytical-scale analytical columns containing the same packing material was compared for a challenging separation at the critical condition.