Charles M. Guttman

Continuous flow enzyme-catalyzed polymerization in a microreactor.

Enzymes immobilized on solid supports are increasingly used for greener, more sustainable chemical transformation processes. Here, we used microreactors to study enzyme-catalyzed ring-opening polymerization of ε-caprolactone to polycaprolactone. A novel microreactor design enabled us to perform these heterogeneous reactions in continuous mode, in organic media, and at elevated temperatures. Using microreactors, we achieved faster polymerization and higher molecular mass compared to using batch reactors. While this study focused on polymerization reactions, it is evident that similar microreactor based platforms can readily be extended to other enzyme-based systems, for example, high-throughput screening of new enzymes and to precision measurements of new processes where continuous flow mode is preferred. This is the first reported demonstration of a solid supported enzyme-catalyzed polymerization reaction in continuous mode.

Modelling the amorphous phase and the fold surface of a semicrystalline polymer—the Gambler's Ruin method

Abstract A semicrystalline polymer with lamellar morphology consists of alternating amorphous and crystalline regions. If sufficiently long, each molecule in this system traverses both the crystalline and amorphous zones. The amorphous portion is comprised of portions of a molecule that form loops that re-enter the same lamella at some distance from the point of emergence, and bridges that form connections between two different crystal lamellae. (A tight fold is not considered to be a loop). The statistics of loops and bridges are shown to be identical to the classical Gamblers Ruin problem in mathematical statistics. This is a useful observation because the extensive existing literature on the Gamblers Ruin problem allows us immediately to transcribe results to the polymer system. Using this approach, the ratio of the number of loops to the number of bridges is determined to be M , the thickness of the amorphous zone in unit statistical steps. Also, the average number of steps comprising the amorphous run is determined to be 3 M +1 for a simple cubic lattice in three dimensions. This modelling leads to a calculation of the minimal fraction of crystal stems involved in tight folding in a semicrystalline polymer. For a simple cubic lattice this is found to be 2 3 . The effects of crystal structure and stiffness of the chain in the melt on this bound are discussed.

Polymeric Silsesquioxanes: Degree-of-Intramolecular-Condensation Measured by Mass Spectrometry

Abstract The degree-of-intramolecular-condensation, defined as the number of residual silanol (Si–OH) groups per oligomer, for a variety of silsesquioxane polymers was measured by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Condensation of the Si–OH groups of a given silsesquioxane oligomer leads to the formation of intramolecular Si–O–Si bridges accompanied by the loss of water. This elimination reaction is easily identified via high resolution time-of-flight mass spectrometry. Various polymeric silsesquioxanes were prepared from the corresponding trialkoxyorganosilanes by hydrolysis–condensation reactions at moderate temperatures (25–65°C) in aqueous acetone with catalytic amounts of formic acid. An n -decyl-silsesquioxane was found to have the highest degree-of-intramolecular-condensation with lower degrees found in n -propyl and in 3-methacryloxypropyl silsesquioxanes. An n -propyl silsesquioxane synthesized without a catalyst was found to have the lowest degree-of-intramolecular-condensation of all as well as the lowest overall molecular mass. In all cases the number of intramolecular Si–O–Si bridges per oligomer was found to be a linear function of the number of repeat units in the oligomer. This linear relationship is discussed in light of theories of intramolecular interactions and bonding in isolated chains.

Molecular structure of silsesquioxanes determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was used to deduce the three-dimensional structure of a complex silsesquioxane polymer. Four distinct levels of structure were observed in the mass spectrum. The overall shape of the peak distribution was typical of polymers formed by condensation reactions. The mass separation between major clusters of peaks, each major cluster corresponding to an oligomer with a unique number of repeat units, confirmed that the synthesis proceeded as expected with no side reactions. The mass separation between peaks within a major cluster showed that intramolecular reactions during synthesis resulted in the elimination of water. The loss of water was ascribed to the formation of closed loops in the polymer structure. A simple arithmetic algorithm is presented for identifying these peaks. Autocorrelation techniques were used to determine the number and distribution of intramolecular closed loops per oligomer. This knowledge was used to deduce whether a particular oligomer is branched-linear, ladder, polyhedral, or some combination of these. The single-oligomer isotopic distribution was used to determine that cationization was present from both sodium and potassium ions.

Laser desorption ionization and MALDI time-of-flight mass spectrometry for low molecular mass polyethylene analysis

Polyethylene’s inert nature and difficulty to dissolve in conventional solvents at room temperature present special problems for sample preparation and ionization in mass spectrometric analysis. We present a study of ionization behavior of several polyethylene samples with molecular masses up to 4000 Da in laser desorption ionization (LDI) time-of-flight mass spectrometers equipped with a 337 run laser beam. We demonstrate unequivocally that silver or copper ion attachment to saturated polyethylene can occur in the gas phase during the UV LDI process. In LDI spectra of polyethylene with molecular masses above ∼1000 Da, low mass ions corresponding to metal—alkene structures are observed in addition to the principal distribution. By interrogating a well-characterized polyethylene sample and a long chain alkane, C94H190, these low mass ions are determined to be the fragmentation products of the intact metal—polyethylene adduct ions. It is further illustrated that fragmentation can be reduced by adding matrix molecules to the sample preparation.

On the problem of crystallization of polymers from the melt with chain folding

It is shown that the “reptation” process proposed by de Gennes allows molecules to be reeled from the melt onto the crystal surface with chain folding by the force associated with crystallization at a rate that is comparable with that demanded by the observed crystallization kinetics for polyethylene fractions n= number of C atoms = 1290–5310. Hence, the rate of transport in the melt is sufficient to permit a considerable amount of chain folding, and an objection due to Flory and Yoon is thereby countered for the range of n noted. The deductions of Yoon and Flory from the neutron scattering data of Schelten and co-workers on PEH + PED mixtures (nped≅ 3750) quench-crystallized from the melt are considered next. It is shown that Yoon and Florys favoured pes= 0.3 model, which gives a probability of adjacent re-entry par close to zero, is deficient despite the good fit of the scattering data, since it exhibits a large density anomaly in the region between the crystal lamellae. This opposes their own view that the material in the interlamellar region has essentially normal amorphous state properties. A “central core” model is proposed that does not possess a density anomaly, and which predicts the scattering curve, characteristic ratio and crystallinity with fair accuracy. This and certain other models give par≈ 0.65, indicating that the adjacent position is by a considerable margin the most probable site for re-entry, in contrast to the analysis of Yoon and Flory. The core model exhibits a mean throw distance of ≈ 22 A for the non-adjacent re-entry loops. This is comparable with the mean “niche” distance calculated from nucleation theory. The number of ties between the lamellae is less than one per chain. Hence the connections of this type between the lamellae are less profuse than have sometimes been depicted.

Three statistical mechanical arguments that favour chain folding in polymer systems of lamellar morphology

Abstract Three arguments are given that imply substantial chain folding in crystalline-amorphous polymer systems of lamellar morphology. These arguments assume random coil behaviour in the amorphous region. (1) The random walk character of the polymer chain portions comprizing the amorphous regions topologically constrains the number of crystalline stems that connect covalently with the amorphous regions to be small. Quantitative estimates are made and lower bounds on the amount of chain folding are given. Estimates of (2) the tightness of loops and (3) the average spatial separation between consecutive crystal stems (along a given chain) are made. The results favour substantial amounts of adjacent and near-adjacent re-entry. Existing experimental measurements on diblock copolymers consisting of crystallizable and non-crystallizable blocks provide a measure of the random walk character of the chains in the amorphous region. It is argued that such systems form lamellar structures with large amounts of chain folding in the crystalline regions as a condition of thermodynamic equilibrium .

Post-source decay in the analysis of polystyrene by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

Various secondary series are observed in matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectra of polystyrene. The number and positions of the series depend on the choice of matrix and added cation. For a given treatment, series observed in linear mode are not necessarily observed in reflectron mode, and vice versa. Post-source decay analysis was used to determine that the secondary series arise at least in part from formation and decay of adducts of polystyrene with matrix species. There is some treatment-to-treatment variation, but adduct formation and decay were observed for all tested treatments. The multiplicity of secondary series makes it unclear whether post-source decay occurs for the main series (polystyrene + cation)+ ions under the conditions normally used for polystyrene analysis. Such ions do undergo post-source decay at laser fluences greater than normally used. Although only polystyrene was investigated in this work, other polymers may also produce adduct and decay series in MALDI analysis. Their presence can mask the presence of minor components in a sample, but at least as observed here, do not have a strong influence on molecular mass determinations.

“Covalent cationization method” for the analysis of polyethylene by mass spectrometry

Polyethylene and other polyolefins have not been amenable to mass spectrometric characterization of molecular mass distribution due to the ineffectiveness of conventional methods of cationization. The lack of polar groups, unsaturation, and aromaticity excludes cationization methods that are used commonly for polymers. A new method is introduced in which an organic cation is covalently bonded to the polymer to produce the necessary ionization for successful matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. A strong MALDI signal results from modified polymers that give no response in their unmodified form.

Calculation of SANS intensity for polyethylene: effect of varying fold planes and fold plane roughening

Abstract The intensity of the small angle neutron scattering (SANS) for polyethylene crystallized in the lamellar habit from the melt at large supercoolings is calculated for μ = 0.01 to μ = 0.14 [ μ = ( 4π λ ) sin ( θ 2 ) ]. Computations are made on models which allow various amounts and types of chain folding and varying degrees of ‘tight’ or ‘regular’ folds. The models that fit the SANS data best have folding along lattice planes in which the stem separation is larger than 0.5 nm (5 A) or which allow for fold plane roughening on a variety of fold planes. the ‘leapfrog’ type folds mentioned by Sadler were also considered, and a possible cause for their existence suggested. As an example, the variable cluster model gives a good account of the SANS data with the surface roughening suggested by nucleation theory with fold planes [110], [200], and [310], or a mixture. Even though the conditions of crystallization used in preparing the SANS specimens (large supercoolings) were conducive to the maximum surface disorder, the probability of ‘tight’ or ‘regular’ folding, ptf, was found to be ∼0.7 for the best models. This corresponds closely to the theoretical lower bound p tf = 2 3 which is rigorous for the case of non-tiled stems. The probability of strictly adjacent re-entry in a single specified fold plane, par, was ∼0.4 to ∼0.7 depending on the particular model chosen. The best models fit not only the SANS data, but also the liquid and crystal density, degree of crystallinity, and characteristic ratio (or radius of gyration). None of the models show the density anomaly inherent in the switchboard or random re-entry models of Yoon and Flory.