Potential of Benchtop NMR for the Determination of Polymer Molar Masses, Molar Mass Distributions, and Chemical Composition Profiles by Means of Diffusion-Ordered Spectroscopy, DOSY.

The determination of molar masses and their distributions is crucial in polymer synthesis and design. This work presents the current performance and limitations of diffusion-ordered spectroscopy (DOSY) on a low-field (benchtop) NMR spectrometer (at 90 MHz) as an alternative to size exclusion chromatography (SEC) for determining diffusion coefficient distributions (DCDs) and molar mass distributions (MMDs). After optimization for narrowly distributed homopolymers, MMDs obtained with inverse Laplace transformation (ILT) and log-normal distribution are compared with average molar masses obtained with mono- and bi-exponential fits, as well as MMDs obtained from SEC. This approach enables ILT to determine DCDs and MMDs even for bimodal homopolymers with fully spectrally overlapping signals and block copolymers with various chemical compositions, for which chemical composition profiles are determined. The feasibility of low-field diffusion NMR with samples dissolved in non-deuterated solvents is further demonstrated and methods for solvent suppression are discussed.

Exploiting Addition-Fragmentation Reactions to Produce Low Dispersity Poly(isobornyl acrylate) and Blocky Copolymers by Semibatch Radical Polymerization.

Solution radical homopolymerization of isobornyl acrylate (iBoA) under starved-feed higher temperature conditions unexpectedly leads to polymer product with low dispersity (<1.3) compared to the polymerization of butyl acrylate (BA) under identical conditions. Both backbiting and ß-scission reactions occur, as the poly(iBoA) product contains close to 100% terminal double bond (TDB) functionality. However, the addition of monomer to the midchain radicals formed by backbiting is sterically hindered, greatly reducing both short and long-chain branching. The poly(iBoA) macromonomer functions as an excellent addition-fragmentation agent, not only lowering dispersity but also providing a means to efficiently produce blocky acrylate copolymers through sequential monomer feeding in the starved-feed semibatch process.

Ball Milling's Effect on Pine Milled Wood Lignin's Structure and Molar Mass.

The effect of ball milling expressed as the yield of milled wood lignin (MWL) on the structure and molar mass of crude milled wood lignin (MWLc) preparation is studied to better understand the process' fundamentals and find optimal conditions for MWL isolation (i.e., to obtain the most representative sample with minimal degradation). Softwood (loblolly pine) MWLc preparations with yields of 20⁻75% have been isolated and characterized based on their molar mass distribution (by Size Exclusion Chromatography (SEC)), hydroxyl groups of different types (31P NMR), methoxyl groups (HS-ID GC-MS), and sugar composition (based on methanolysis). Classical MWL purification is not used to access the whole extracted lignin. The results indicate that lignin degradation during ball milling occurs predominantly in the high molar mass fraction and is less pronounced in the low molar mass fraction. This results in a significant decrease in the Mz and Mw of the extracted MWLc with an increase in the yield of MWLc, but has only a very subtle effect on the lignin structure if the yield of MWLc is kept below about 55%. Therefore, no tedious optimization of process variables is necessary to achieve the required MWLc yield in this range for structural studies of softwood MWL. The sugar composition shows higher amounts of pectin components in MWLs of low yields and higher amounts of glucan and mannan in high-yield MWLs, confirming that lignin extraction starts from the middle lamella in the earlier stages of MWL isolation, followed by lignin extraction from the secondary wall region.

Control of Molar Mass Distribution by Polymerization in the Analytical Ultracentrifuge.

Molar mass distributions are of high interest in macromolecular chemistry because they directly determine the physical and chemical properties of polymers. A principal approach to obtain and control the shape of broad molar mass distributions is adjusting the initiator concentration in free radical polymerizations. A controlled gradient of the initiator concentration should potentially lead to tailored molar mass distributions. Here we use analytical ultracentrifugation (AUC) to adjust and measure a macroinitiator's concentration gradient. Subsequent photopolymerization of a uniformly distributed monomer leads to desired chain length distributions. Resulting distributions are described and calculated by a Schulz-Flory approach. The desired concentration profiles are simulated in advance and can be detected anytime by the optical systems in the centrifuge. Therefore, tailored broad molar mass distributions can now be produced using predictions from simulations using the established theory of AUC.

Separation and identification of oligomeric ethyl silicates by liquid chromatography with electrospray ionization mass spectrometry.

Reversed-phase liquid chromatography coupled with electrospray ionization mass spectrometry was used to study the molecular structures of components and molar mass distributions in ethyl silicate-40, a versatile liquid precursor for silicon-based materials. Identity testing by standard spectroscopic techniques showed that a commercial sample of ethyl silicate-40 was composed of linear/branched ethoxysiloxane oligomers with the silicon atoms ranging from 2 to 12 together with minor monocyclic species. Analysis of the sample by liquid chromatography coupled with evaporative light scattering detection resulted in an elution profile consisting of a series of peak clusters. Peak identification showed that the linear/branched homologous series of oligomers were eluted in the order of increasing number of silicon atoms in the molecules and the time duration (width) of the resulting peak clusters increased in the same fashion corresponding to increasing number of geometric isomers. In addition, small amounts of monocyclic oligomers present in the sample were found to be less retained than each linear/branched counterpart. Finally, the molar mass distribution parameters for ethyl silicate-40 determined by the developed method were in good agreement with the literature values. Overall, this work demonstrates that reversed-phase liquid chromatography coupled with electrospray ionization mass spectrometry is an indispensable tool for the comprehensive characterization of complex mixtures of this type.

Overview of Methods for the Direct Molar Mass Determination of Cellulose.

The purpose of this article is to provide the reader with an overview of the methods used to determine the molecular weights of cellulose. Methods that employ direct dissolution of the cellulose polymer are described; hence methods for investigating the molecular weight of cellulose in derivatized states, such as ethers or esters, only form a minor part of this review. Many of the methods described are primarily of historical interest since they have no use in modern cellulose chemistry. However, older methods, such as osmometry or ultracentrifuge experiments, were the first analytical methods used in polymer chemistry and continue to serve as sources of fundamental information (such as the cellulose structure in solution). The first part of the paper reviews methods, either absolute or relative, for the estimation of average molecular weights. Regardless of an absolute or relative approach, the outcome is a molecular weight average (MWA). In the final section, coupling methods are described. The primary benefit of performing a pre-separation step on the molecules is the discovery of the molecular weight distribution (MWD). Here, size exclusion chromatography (SEC) is unquestionably the most powerful and most commonly-applied method in modern laboratories and industrial settings.

Structural analyses of supernatant fractions in TEMPO-oxidized pulp/water reaction mixtures separated by centrifugation and dialysis.

Side reactions occurring on cellulose during 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TMEPO)-catalyzed oxidation have not been considered to be significant. Then, TEMPO-oxidized hardwood and softwood bleached kraft pulps (HBKP and SBKP) were prepared with an excess NaOCl·5H2O. Supernatant fractions (SFs) were obtained in the aqueous reaction mixtures of TEMPO-oxidized pulps by centrifugation and dialysis. The SFs with carboxyl contents of 5.0 and 4.2 mmol/g were obtained in the yields of 19 % and 30 % from HBKP and SBKP, respectively. These carboxy contents are much higher than those (2.6-2.7 mmol/g) of the precipitate fractions in the TEMPO-oxidized pulps. Solid-state 13C NMR spectra and other analyses revealed that the water-soluble ß-(1 â†’ 4)-polyglucuronic acids were predominantly present in the SFs. In addition, water-insoluble TEMPO-oxidized cellulose nanocrystals were present in the SFs, but they constituted less than ~10 % of the SFs. The mass-average degrees of polymerization (DPw) of the SFs obtained from HBKP and SBKP were 166 and 155, respectively, whereas the original HBKP and SBKP had DPw values of 1990 and 2140, respectively. These substantial depolymerization and formation of the water-soluble ß-(1 â†’ 4)-polyglucuronic acids occur on cellulose and oxidized cellulose molecules as side reactions during TEMPO-catalyzed oxidation, which should be considered for structural analyses of TEMPO-oxidized products.

Asymmetrical Flow Field Flow Fractionation for Molar Mass Characterization of Polyethylene Oxide in Abuse-Deterrent Formulations.

High molar mass polyethylene oxide (HM-PEO) is commonly used to enhance the mechanical strength of solid oral opioid drug products to deter abuse. Because the properties of PEO depend on molar mass distribution, accurately determining the molar mass distribution is a necessary part of understanding PEO's role in abuse-deterrent formulations (ADF). In this study, an asymmetrical flow field-flow fractionation (AF4) analytical procedure was developed to characterize PEO polymers with nominal molar masses of 1, 4 or 7 MDa as well as those from in-house prepared placebo ADF. The placebo ADF were manufactured using direct compress or hot-melt-extrusion methods, and subjected to physical manipulation, such as heating and grinding before measurement by AF4 were performed. The molar mass distribution characterized by AF4 revealed that PEO was sensitive to thermal stress, exhibiting decreased molar mass with increased heat exposure. The optimized AF4 method was deemed suitable for characterizing HM-PEO, offering adequate dynamic separation range for PEO with molar mass from 100 kDa to approximately 10 MDa.

Evolution of Molar Mass Distributions Using a Method of Partial Moments: Initiation of RAFT Polymerization.

We describe a method of partial moments devised for accurate simulation of the time/conversion evolution of polymer composition and molar mass. Expressions were derived that enable rigorous evaluation of the complete molar mass and composition distribution for shorter chain lengths (e.g., degree of polymerization, Xn = N < 200 units) while longer chains (Xn ≥ 200 units) are not neglected, rather they are explicitly considered in terms of partial moments of the molar mass distribution, µxN(P)=∑n=N+1∞nx[Pn] (where P is a polymeric species and n is its' chain length). The methodology provides the exact molar mass distribution for chains Xn < N, allows accurate calculation of the overall molar mass averages, the molar mass dispersity and standard deviations of the distributions, provides closure to what would otherwise be an infinite series of differential equations, and reduces the stiffness of the system. The method also allows for the inclusion of the chain length dependence of the rate coefficients associated with the various reaction steps (in particular, termination and propagation) and the various side reactions that may complicate initiation or initialization. The method is particularly suited for the detailed analysis of the low molar mass portion of molar mass distributions of polymers formed by radical polymerization with reversible addition-fragmentation chain transfer (RAFT) and is relevant to designing the RAFT-synthesis of sequence-defined polymers. In this paper, we successfully apply the method to compare the behavior of thermally initiated (with an added dialkyldiazene initiator) and photo-initiated (with a RAFT agent as a direct photo-iniferter) RAFT-single-unit monomer insertion (RAFT-SUMI) and oligomerization of N,N-dimethylacrylamide (DMAm).

Analysis of celluloses, plant holocelluloses, and wood pulps by size-exclusion chromatography/multi-angle laser-light scattering.

Size-exclusion chromatography with multi-angle laser-light scattering and refractive index detection (SEC/MALLS/RI) provides the number- and weight-average molar masses, molar mass distributions, conformations, and linear/branched structures of polymers. In the case of pure celluloses including highly crystalline tunicate and alga celluloses, and hemicellulose-rich plant holocelluloses, soaking in ethylene diamine (EDA) and subsequent solvent exchange to N,N-dimethylacetamide (DMAc) though methanol is effective for complete dissolution in ∼8% (w/w) LiCl/DMAc. SEC/MALLS/RI analysis can, therefore, be applied to pure celluloses, chemical wood pulps, and plant holocelluloses after dissolution in ∼8% (w/w) LiCl/DMAc, dilution to 1% (w/v) LiCl/DMAc and membrane filtration. All pure celluloses and the high-molar-mass cellulose fractions of hardwood and grass holocelluloses have linear and random-coil conformations and various average molar masses and molar mass distributions depending on the cellulose and holocellulose resources. In contrast, Japanese cedar (i.e., softwood) holocellulose and softwood bleached kraft pulp have alkali-stable cellulose/glucomannan branched structures in the high-molar-mass fractions.

Characterization of ultrahigh molar mass polyelectrolytes by capillary electrophoresis.

High to ultrahigh molar mass (above 1 million g/mol) anionic poly(acrylic acid-co-acrylamide)s are widely used industrial polymers for water treatment and oil drilling. Their properties are strongly related to their charge density and molar mass distributions. However, due to inherent separation limits of SEC with currently available columns (< 5 ×106 g/mol) and possible occurrence of chain breakage, and/or adsorption leading to abnormal elution, characterization of unusually high molar masses polyelectrolytes is challenging. In this work, we investigate the use of polymer sieving capillary electrophoresis for the size-based characterization of these high to ultrahigh molar mass polyelectrolytes. By optimizing the operating conditions (electric field, ionic strength, injected polyelectrolyte concentration, nature of the polymer sieving), it has been possible to considerably reduce polyelectrolyte aggregation and to get sufficient size-based selectivity, allowing to obtain the size distribution of the polyelectrolytes over a large range of molar mass from 105 up to ~10×106 g/mol. The data processing of the raw electropherograms is a key step in the analytical protocol leading to the molar mass distribution. The polyelectrolyte effective mobility in sieving conditions has to be normalized to its free-draining electrophoretic mobility in free solution conditions to take into account possible variability in the charge density between the different samples.

Improvement of starch films for food packaging through a three-principle approach: Antioxidants, cross-linking and reinforcement.

This study uses sunflower hulls, a by-product from the sunflower snack industry, to recover both, valuable phenolic compounds and cellulose fibers, for the production of antioxidant reinforced starch films as potential food packaging material. The phenolic extract provided antioxidant properties to the films with EC50 values of 89 mg film/mg DPPH. The cellulose fibers reinforced the starch films with a threefold increase in Young´s modulus. Furthermore, citric acid was added to induce cross-linking of the starch polymers and improve film integrity. The addition of citric acid induced both, starch polymer hydrolysis and cross-linking, seen in a shift in chain-length distribution after debranching with iso-amylase. This is the first study that focuses on a three-principle approach to improve edible starch films, and follows UN goals on sustainability to reduce waste and increase value in by-products as a step forward to functionalize packaging material.

Molar mass distributions of linear homopolymers by size exclusion chromatography with light scattering detection: A method for automatic band broadening correction.

Size exclusion chromatography (SEC) equipped with a differential refractometer (DR) and a light scattering (LS) detector is a well-known technique for determining the molar mass distribution (MMD) of many polymers. In the case of narrow polymers, correction of the band broadening (BB) effect is necessary; but unfortunately, the available BB correction methods are rather impractical for most SEC users. This work proposes an automatic BB correction method for determining the MMD of narrow linear homopolymers (or multimodal homopolymers that include narrow modes) on the basis of SEC/(DR + LS) measurements. The required data are: the baseline-corrected DR and LS chromatograms, the inter-detector volume (IDV), and a molar mass calibration independently determined from narrow standards. In comparison to other available alternatives for BB correction, the here-proposed method has the following key advantages: a) no previous knowledge on the BB function is required; b) the detectors gain constants are unnecessary; and c) no numerical regularization method is required. Moreover, if the IDV is unknown, then the proposed method could also be used for estimating the IDV from the knowledge of the dispersity index of a narrow homopolymer. The proposal was experimentally assessed by analyzing narrow standards of poly(styrene) and poly(methyl-methacrylate). The proposed method estimated the dispersity indexes of the standards with errors lower than 0.9% with respect to values reported by manufacturers (between 1.015 and 1.044); while the classical approaches based on SEC/DR and SEC/(DR + LS), produced errors of up to -11% and 3%, respectively.

Aqueous acetone fractionation of kraft, organosolv and soda lignins.

Technical lignins are structurally heterogeneous and polydisperse. This work describes the use of a simple and green method for lignin fractionation, using different proportions of acetone (40 and 60%) in water. Lignins from three different sources (wheat straw organosolv lignin, wheat straw soda lignin and softwood kraft lignin) were used in this fractionation protocol. The obtained fractions showed different molar mass and functional groups. The lower molar mass fractions showed more phenolic hydroxyl groups and carboxylic acid moieties than higher molar mass fractions, which also possessed much higher amounts of carbohydrates. The chemical characterization of these fractionated lignins showed that the PREC fraction was exceptionally pure and homogeneous lignin. Its total lignin content was >96% for all three lignins and it was practically free from carbohydrates and inorganics (ash). Furthermore, PREC fraction possessed the highest carbon content for the three lignin samples (63.05-69.26%). These results illustrate that the proposed aqueous acetone fractionation protocol could indeed produce pure and uniform lignin fraction and it was applicable for lignins from different sources.

Accurate molar masses of cellulose for the determination of degradation rates in complex paper samples.

Complex cellulosic samples are often difficult to analyse with size-exclusion chromatography. The strong molecular associations of hemicelluloses and lignin with cellulose produce multimodal molar mass distributions (MMD) that are difficult to interpret. More reliable ways of calculating the molar masses of cellulose are thus necessary. This is particularly relevant when studying the kinetics of paper degradation, as the number average molar mass is the most precise indicator. In this study various data handling methods based on the deconvolution of bimodal and multimodal MMDs of complex cellulosic samples after SEC-MALS-DRI analysis are examined in order to propose more accurate paper degradation rates. Two deconvolution methods, which do or do not rely on polymer calibration curves were developed and were applied to several kraft and groundwood pulp papers unaged and hygrothermally aged. The deconvolution methods are discussed and evaluated in light of calculated cellulose activation energies, degradation rates and paper usable lifetime predictions.

Use of Capillary Electrophoresis for Polysaccharide Studies and Applications.

CE applications to charged polysaccharides are briefly reported. A simple procedure is presented to determine the esterification degree of a hyaluronan derivative. In this case the degree of substitution was as low as 14 %.The molecular weight distribution of mannuronic oligosaccharides mixture produced by hydrolysis of native polymannuronic is readily calculated from peak area of the species resolved by CE on the basis of a specific degree of polymerization.The influence of the applied electric field strength on the free solution mobility of hyaluronan samples is briefly addressed for molar masses of the order of 10(5) and 10(6) g/mol. The data are compared with the results obtained for a 50 % galactose substituted HA.Mobility data obtained as a function of buffer pH for a native HA sample as well as for two galactose-amide HA derivatives, having slightly different degrees of substitution, are presented and discussed in terms of the polymer charge density parameters ξ.In most cases, more questions than answers arise from the application of CE to charged polysaccharides. However, perspectives are disclosed for a further understanding of the reliability of CE applied for the structural elucidation of such macromolecules.

Separation and identification of an oligomeric light stabilizer Chimassorb 944 by gradient elution chromatography coupled with matrix-assisted laser desorption/ionization mass spectrometry.

A non-aqueous reversed-phase high-performance liquid chromatographic (HPLC) method has been developed to separate a light stabilizer Chimassorb 944 into individual oligomers, which are further identified using pre-column fluorescent derivatization and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Consistent with previous studies, we find that the Chimassorb 944 product is a complex mixture consisting of a homologous series with the amine end groups and the number of repeat units (n) span from 1 to 26. In addition to the dominant linear species, cyclic oligomers are present at relatively high levels in the low-mass range. Their concentration decreases rapidly with the length of the oligomer backbone and becomes undetectable when n>7. Moreover, comparison of the HPLC and MALDI-MS molar mass distributions of Chimassorb 944 shows that the HPLC analysis produces greater molar mass averages and thus offers an effective means for accurate measure of the relative abundances of the oligomers.

Characterisation of cationic potato starch by asymmetrical flow field-flow fractionation. Influence of ionic strength and degree of substitution.

The properties of a paper sheet depend on the absorption together with the physico-chemical properties of additives used in the paper processing. The effect of ionic strength and degree of substitution of cationic potato starch on the elution pattern of asymmetrical flow field-flow fractionation was analysed. The effect of starch derivatisation, in either dry or wet phase, was also investigated. Average molar mass showed no difference between the starches obtained from the two derivatisation processes. Apparent densities showed that dry cationic starch had higher density than wet cationic starch for a hydrodynamic radius between 50 and 100 nm. Elution times of native and three cationic starches increased when the ionic strength increased from 50 to 100mM. No differences in the molar mass among cationic starches with different degree of substitution suggested no degradation due to a derivatisation process. Large sample loads can be used at 100mM without overloading.

"Polymeromics": Mass spectrometry based strategies in polymer science toward complete sequencing approaches: a review.

Mass spectrometry (MS) is the most versatile and comprehensive method in "OMICS" sciences (i.e. in proteomics, genomics, metabolomics and lipidomics). The applications of MS and tandem MS (MS/MS or MS(n)) provide sequence information of the full complement of biological samples in order to understand the importance of the sequences on their precise and specific functions. Nowadays, the control of polymer sequences and their accurate characterization is one of the significant challenges of current polymer science. Therefore, a similar approach can be very beneficial for characterizing and understanding the complex structures of synthetic macromolecules. MS-based strategies allow a relatively precise examination of polymeric structures (e.g. their molar mass distributions, monomer units, side chain substituents, end-group functionalities, and copolymer compositions). Moreover, tandem MS offer accurate structural information from intricate macromolecular structures; however, it produces vast amount of data to interpret. In "OMICS" sciences, the software application to interpret the obtained data has developed satisfyingly (e.g. in proteomics), because it is not possible to handle the amount of data acquired via (tandem) MS studies on the biological samples manually. It can be expected that special software tools will improve the interpretation of (tandem) MS output from the investigations of synthetic polymers as well. Eventually, the MS/MS field will also open up for polymer scientists who are not MS-specialists. In this review, we dissect the overall framework of the MS and MS/MS analysis of synthetic polymers into its key components. We discuss the fundamentals of polymer analyses as well as recent advances in the areas of tandem mass spectrometry, software developments, and the overall future perspectives on the way to polymer sequencing, one of the last Holy Grail in polymer science.

Hydrolysis of concentrated raw starch: A new very efficient α-amylase from Anoxybacillus flavothermus.

A new α-amylase from Anoxybacillus flavothermus (AFA) was found to be effective in hydrolyzing raw starch in production of glucose syrup at temperatures below the starch gelatinization temperature. AFA is very efficient, leading to 77% hydrolysis of a 31% raw starch suspension. The final hydrolysis degree is reached in 2-3h at starch concentrations lower than 15% and 8-24h at higher concentrations. AFA is also very efficient in hydrolyzing the crystalline domains in the starch granule. The major A-type crystalline structure is more rapidly degraded than amorphous domains in agreement with the observed preferential hydrolysis of amylopectin. Amylose-lipid complexes are degraded in a second step, yielding amylose fragments which then re-associate into B-type crystalline structures forming the final α-amylase resistant fraction. The mode of action of AFA and the factors limiting complete hydrolysis are discussed in details.