Multimodal Molecular Weight Distributions Research Articles

Diffusion-Separated Nuclear Magnetic Resonance Spectroscopy of Polymer Mixtures

The potential of diffusion-ordered 2D NMR spectroscopy (DOSY) for the analysis of solutions of polymer mixtures and polymers with complex molecular mass distributions is investigated. Diffusion coefficient labeling in NMR is generally achieved by stepwise ramping up of the amplitudes of pairs of pulsed field gradients (PFGs). After Fourier transformation in the acquisition dimension and an inverse Laplace transform (ILT) with respect to the square of the gradient strength, 2D spectra are obtained that show the chemical shift along one dimension and the translational diffusion coefficient along the other. Since polymers may have broad, nonsymmetric or complex (bi- and multimodal) molecular weight distributions (MWDs), the diffusion coefficient distribution should follow the main features of the MWD. However, the calculation of the diffusion coefficient distribution involves a numerically unstable data inversion (ILT), which limits the resolution in the diffusion dimension. The applications of DOSY NMR techniques to the study of polymer blends and for the determination of MWDs of polymers with PFG NMR are assessed. DOSY spectra recorded from industrial polypropylene and polystyrene samples and mixtures are shown and interpreted to illustrate the features of this technique.

Multiplicity of molecular weight distribution of polyethene produced with modified‐polypropene‐supported Ziegler catalyst systems

The molecular weight and molecular weight distribution of polyethene prepared with a modified-polypropene-supported Ziegler catalyst were investigated in terms of the variation of the polymerization conditions, such as the kind of co-catalyst, its concentration, and polymerization temperature. Ethene polymerization with the catalyst in the presence of triethylaluminium proceeded in heterogeneous system to give a polyethene with extremely broad molecular weight distribution. The result was completely changed upon replacing the triethylaluminium by other alkylaluminium compounds, such as diethylaluminium chloride. Polyethenes with very broad, narrow, bimodal, and multimodal molecular weight distributions were obtained by simply varying the co-catalyst. The molecular weight distributions of polyethenes obtained were also very sensitive to co-catalyst concentration and polymerization temperature. From the results obtained, it is suggested that the modified-polypropene-supported Ziegler catalyst is feasible as a novel candidate for a catalyst system giving polyethenes with controlled molecular weight distributions in a wide range of molecular weight.

Coupling reaction of polyisoprenyllithium with 1,2‐dibromoethane

The coupling reaction of polyisoprenyllithium with 1,2-dibromoethane (DBE) gives a multimodal molecular weight distribution (MWD). Species with molecular weights three and four times (P3, P4) higher than the base polymer (P) are present beside the main expected one with double molecular weight (P2). The relative abundancies of P1, P2, P3, and P4 depend on the experimental conditions. Gas-phase chromatographic analysis carried out during the coupling reaction show the presence of ethylene whose formation is related to a lithium–bromine exchange reaction competing with direct alkylation. The lithium–bromine exchange reaction is more effective at T 100°C and yields alkyl bromide chain ends. The allylic and alkylic bromides react differently with the remaining polyisoprenyllithium: the former adds only to polyisoprenyllithium yielding P2, while the later also yields P3 and P4 through radical reactions. © 1997 John Wiley & Sons, Inc.

Coupling reaction of polyisoprenyllithium with 1,2‐dibromoethane

The coupling reaction of polyisoprenyllithium with 1,2-dibromoethane (DBE) gives a multimodal molecular weight distribution (MWD). Species with molecular weights three and four times (P3, P4) higher than the base polymer (P) are present beside the main expected one with double molecular weight (P2). The relative abundancies of P1, P2, P3, and P4 depend on the experimental conditions. Gas-phase chromatographic analysis carried out during the coupling reaction show the presence of ethylene whose formation is related to a lithium–bromine exchange reaction competing with direct alkylation. The lithium–bromine exchange reaction is more effective at T < 80°C and results in the formation of allyl bromide-terminated polyisoprene, while direct alkylation is effective at T > 100°C and yields alkyl bromide chain ends. The allylic and alkylic bromides react differently with the remaining polyisoprenyllithium: the former adds only to polyisoprenyllithium yielding P2, while the later also yields P3 and P4 through radical reactions. © 1997 John Wiley & Sons, Inc.

Multimodal molecular weight distributions of guayule rubber from high rubber yielding lines

Molecular weight distribution (MWD) of guayule rubber extracted from well characterized, high rubber yielding plants, grown under controlled and natural conditions were observed to be multimodal (especially in young plants). The MWD curves apparently depend on genetic and environmental inter-relations, which have not been established. We believe that multimodal curves may reflect a “batch type” polymerization process which could be triggered by rubber-synthesizing enzyme activities, depending on genetic and environmental factors.

Anionic Polymerization of (Meth)acrylic Monomers. 22. Anionic Polymerization of tert-Butyl Methacrylate in Toluene/THF Mixtures: A Preliminary Approach to the Role of Solvation/Ligation Equilibria

Although the anionic polymerization of tert-butyl methacrylate (tBMA) induces a narrow and unimodal molecular weight distribution (MWD) when performed in either pure THF or pure toluene at -78 o C in the presence of a lithium counterion, that in THF/toluene mixtures, keeping other conditions unchanged, surprisingly gives rise to broad bi- and even multimodal MWD's. Moreover, with increasing polymerization temperature from -78 to 0 o C, that multimodal MWD gets narrower, atypical of termination reactions, and indeed at 0 o C, a very narrow unimodal MWD is again obtained. These results have been satisfactorily interpreted in terms of a multiple solvation equilibria mechanism: there coexist several types of THF-solvated and nonsolvated active species and the exchange among them is slow at -78 o C but fast at 0 o C compared to the monomer propagation rate, thus resulting in a multimodal MWD in the former case and a narrow one in the latter. Furthermore, this solvation mechanism also dominates the stereoregulation of the tBuMA polymerization in mixed solvent. It has indeed been demonstrated that addition of various types of ligands, i.e., lithium chloride (LiCl) and lithium tert-butoxide (LiOtBu) (both μ-ligands), 12-crown-4 (12CE4) and cryptand 211 (K211) (both α-chelating ligands), and lithium 2-(2-methoxyethoxy)ethoxide (LiOE 2 M) (a μ/ dual type of ligand), affects the tBMA anionic polymerization in a way which can be accounted for by these so-called solvation/ligation multiple equilibria among THF-nonsolvated, THF-solvated, and ligated ion pairs. Once again, the fact that, in comparison with 12CE4 and LiOtBu, such ligands as LiOE 2 M, K211, and LiCl can effectively promote a much narrower MWD is nicely related to their high propensity to coordinate with a lithium ester enolate and at the same time to either promote a fast-exchanging ligation equilibrium or form a single type of active complex, in good agreement with the NMR investigation of corresponding model systems as previously reported by us

Inifer surfactants in emulsion polymerization

Thiol-ended non-ionic surfactants, used in combination with t-butyl hydroperoxide are components of a redox system able to initiate the emulsion polymerization of styrene, as well as control the molecular weight by transfer. Most of the surfactants remain as side products in the water phase, while multimodal molecular weight distributions of polymer are observed depending on the structure of the surfactant, the conversion of the monomer and the process used for feeding the reactor. The maximum incorporation yield of these reactive surfactants in the polymer is around 40%.

Synthesis of aromatic polyethers by the Scholl reaction. Part 9.—Cation–radical polymerization of 4,4′-bis(2-naphthoxy)diphenyl sulphone

The synthesis and cation–radical polymerization of 4,4′-bis(2-naphthyloxy)diphenyl sulphone (3) are presented. For long reaction times, the soluble fractions of the polymers resulting from 3 exhibit a multi-modal molecular-weight distribution consisting of three peaks of very different molecular weights. The highest molecular-weight fraction usually has a number-average molecular weight (n) of ca. 106 g mol–1 relative to a polystyrene calibration, while the n of the lowest molecular-weight fraction is usually < 10 000 g mol–1. The multimodal molecular-weight distribution of the polymers synthesized from 3 was suggested to be due to the participation in reactions of several reactive positions on the 2-naphthyloxy rings. The reaction between the most reactive C1 positions takes place during the early stage of the polymerization. However, once the concentration of C1 positions decreases at a number-average molecular weight of ca. 10 000 g mol–1, intermolecular reactions between the less reactive positions such as C5, C8 and C4 of the low-molecular-weight polymer chains causes the formation of a branched polymer fraction of very high molecular weight and also a small amount of an insoluble fraction. A polymer fraction of intermediary molecular weight is also obtained.

Anionic Polymerization of α-Methylstyrene. IX. Nature of the Propagating Species

Abstract The nature of the initiating and propagating species involved in the anionic polymerization of α-methylstyrene has been explored. The earlier hypothesis that multimodal GPC molecular weight distributions in polymers arise solely out of different reaction steps or different ion-pair mechanisms being involved has been modified for poly-α-methylstyrene. Multimodal GPC molecular weight distributions in poly-α-methylstyrene initiated with potassium at 25°C and polymerized at 25°C or higher in THF, p-dioxane, or cyclohexane as solvents have been ascribed to the presence of two different types of tetramers which grow simultaneously but at different rates, each responding to its own well-defined thermodynamic equilibrium and yielding dormant and living polymers. Reaction schemes describing the initiation (at 25°C) and propagation reactions (between -25 and 60°C) in the polymerization (in solution of THF as well as in bulk) of α-methylstyrene initiated with potassium-naphthalene, butyl-lithium, and butyllithium-tetramethylethylenediamine (TMEDA) have been presented. The role of coordinating agents naphthalene and TMEDA in changing irreversible propagations into reversible ones has been emphasized.

Multimodal molecular weight distribution of polydienyllithium compounds caused by thermolytic side reactions

Multimodal molecular weight distribution of polydienyllithium compounds caused by thermolytic side reactions