Porous dipeptide crystals as polymerization nanoreactors

Channeling a good polymer: Dipeptide crystals have been used for solid-state polymerization reactions. The porosity of the crystals enables monomer absorption, followed by γ-irradiation to produce polymers with high molecular weights. The crystal channels induce stereoselective reactions to yield 1,4-trans polydienes and isotactic poly(acrylonitrile) with retention of fibrillar morphology.     

Raman spectroscopic studies of the solid-state polymerization of diacetylenes. I. Thermal polymerization of 1,6-di-p-toluenesulfonyloxy-2,4-hexadiyne

The resonance Raman spectra of polymer chains in partially polymerized crystals of 1,6-di-p-toluenesulfonyloxy-2,4-hexadiyne are reported. The polymer chain distortion is deduced using the results obtained previously for fully polymerized samples under tensile strain. Changes in crystal lattice dimensions both parallel and lateral to the polymer chains are found to be important in interpreting the variations in frequency of the Raman-active vibrational modes. Further evidence is found for the resonant interaction of backbone and side-group vibrations reported previously. This interaction is affected by the lateral dimensional changes and is also sensitive to residual strain fields in the monomer crystals. It is not necessary in the interpretation of the Raman spectra to take any account of changes in polymer chain length during polymerization.     

Preparation of polymeric nanocapsules by radiation induced miniemulsion polymerization

In this research, polystyrene (PSt) nanocapsules with liquid cores were prepared by ⁶⁰Co γ-ray radiation induced miniemulsion polymerization, in which N-vinyl pyrrolidone (NVP) was used as the polar monomer. The characterization of polymer was carried out by ¹H NMR. It was verified that during polymerization, graft copolymerization between poly (pyrrolidone) (PVP) and PSt had taken place instead of random copolymerization. The interfacial tension between polymer and water was reduced because of the grafting reaction that had occurred, which was helpful to form nanocapsules. The influence of the ratio of St to NVP, the type and amount of the surfactant and the monomer/dodocane ratio on the particle morphology was studied by TEM. Finally, the releasing process of the synthesized nanoparticles was monitored by UV-vis measurement.     

Solid-state polymerization of oxetanes. I. Lattice parameters and packing properties of the monomers

The crystallographic unit cells of melt-crystallized 3,3-bischloromethyloxetane and 3,3-bisbromomethyloxetane were determined by the Weissenberg method. The two isomorphous lattices are triclinic with two molecules in the unit cell. 3,3-Bisfluoromethyloxetane forms plastic crystals in the temperature range between ?36°C and +22°C, as shown by differential calorimetry and NMR broad-line spectroscopy. The Debye-Scherrer diagram and the general physical properties indicate the formation of a face-centered cubic lattice. No correlation between the lattice parameters of the monomer and polymer can be found On the basis of these results, the question is raised as to whether a topochemical polymerization of bishalomethloxetanes, i.e., a solid-state polymerization without destruction of the crystal lattice, can take place at all. The halomethyl side groups of the oxetanes can be shown to possess different conformations in monomer and polymer crystals, so that a conformational change of the groups and rearrangement of the molecules must take place during polymerization. Therefore, a topochemical mechanism for the solid-state polymerization of bishalomethyloxetanes seems to be impossible.     

Coordination copolymerization of tetrahydrofuran and oxepane with oxetanes and epoxides

The chelate catalyst, as typified by the Et₃Al-0.5 H₂O-0.5 acetylacetone product, usually prepared with Et₂O or tetrahydrofuran (THF) present, has all the known characteristics of a coordination catalyst for polymerizing epoxides and uniquely for oxetanes. We have found that the chelate catalyst gives fairly good copolymerization of THF (54% in monomer charge) with 3-(trimethylsilyloxy) oxetane which, after hydrolysis, is a water-soluble, moderate molecular weight copolymer of THF (36%) with 3-hydroxyoxetane (HO). This apparent coordination copolymerization of THF has been extended to trimethylene oxide (TMO), 3,3-bis(trimethylsilyoxymethyl) oxetane, 3,3-bis(chloromethyl)oxetane (BCMO), trans-2,3-epoxybutane (TBO), and propylene oxide, listed in order of decreasing copolymerizability with THF. Presumably, this is the first known coordination copolymerization of THF which hitherto has only been polymerized with cationic catalysts. Oxepane also copolymerizes coordinately with TMO and BCMO, but less readily than THF, with the chelate catalyst. TBO polymerizes slowly with the chelate catalyst to form stereoregular polymer which can be separated into an acetone-insoluble, highly stereoregular fraction and an acetone-soluble, somewhat less stereoregular fraction. The soluble fraction can be eliminated by using 1.0 acetyl acetone per Al in the catalyst or by adding a small amount of a very strong base (0.09 quinuclidine per Al). The copolymerization of TBO with THF (39%) gives insoluble stereoregular homopolymer and soluble copolymer containing about 23% THF, reflecting the varied steric hindrance of the sites. Some anomalous results appear to be related to the mechanism: (1) steric and electronic factors of the monomers and of the polymerization site. For example, the fourth coordination position of Al is needed to achieve homopolymerization of BCMO and TMO-THF copolymerization. (2) The aggregation state of the catalyst, since a nonpolar diluent as toluene is unfavorable for coordination copolymerization of THF. (3) The greater ring strain of epoxides causes a greater ease of polymerization, compared to oxetanes. Thus, Et₂O often present in the chelate catalyst lowers the molecular weight of the polymer considerably with oxetanes compared to epoxides where Et₂O has little or no effect.     

Cationic polymerization of 4-methyl-1,3-dioxene-4 and copolymerization with tetrahydrofuran

A new monomer, 4-methyl-1,3-dioxene-4 was synthesized from allyl chloride and paraformaldehyde. The monomer was polymerized at room temperature or ?78°C. by boron trifluoride etherate catalyst, and the structure of the obtained polymer was determined by infrared, nuclear magnetic resonance spectra, and chemical analysis. It was ascertained that the polymerization process proceeded through a ring-opening mechanism at the dioxane ring. In the presence of tetrahydrofuran, the polymerization of 4-methyl-1–1,3-dioxene-4 led to copolymer. The mechanism of the copolymerization is described in detail.     

Dielectric absorption in polyoxymethylene obtained from tetraoxymethylene crystal by radiation induced polymerization

In order to study the dielectric behavior of polyoxymethylene prepared by radiation-induced solid-state polymerization, large crystals of tetraoxymethylene (2 cm in diameter) were prepared by Bridgman's method and polymerized by γ-rays. The x-ray diffraction pattern of the polymer did not reveal the existence of a so-called amorphous region. In dielectric measurements, only one dielectric absorption was observed in the low-temperature region, while in the high-temperature region ε″ did not change up to about 120°C, where thermal decomposition started. When the specimen was stabilized by acetylating the endgroups in the solid state, ε″ did not change up to about 150°C. This dielectric absorption showed a significant anisotropy for the direction of an applied field. The dielectric absorption was much larger when the electric field was perpendicular to the fiber axis than when the field was parallel. The dielectric absorption was larger in a specimen which was estimated to be more imperfect according to DSC analysis. This leads to the conclusion that the dielectric absorption is attributable to defect regions. On the other hand, the dielectric absorption became larger with increasing numbers of terminal OH groups, and hence it is attributable to the response of the terminal OH groups. Moreover, the dielectric absorption was depressed by the acetylation of the OH groups in the solid state. It is, therefore, concluded that the dielectric absorption observed in polyoxymethylene prepared by solid-state polymerization of tetraoxymethylene is due to the response of terminal OH groups localized in defect regions. Polyoxymethylene crystallized from the melt gives an asymmetric low-temperature absorption. This asymmetry can be ascribed to the superposition of two relaxation processes. When its absorption curve is compared with that of the solid-state polymerized polymer, the low-temperature component can be assigned to crystal defects and the high temperature one to amorphous regions.     

Mechanism of Ziegler-Natta polymerization of propylene and α-d-propylene

Rates of propylene homopolymerization and α-d-propylene-propylene copolymerization were determined by using constant-pressure polymerization conditions. It could be demonstrated that the rate of propylene homopolymerization was constant under the conditions used. However, the initial rate of copolymerization was faster and decreased with time to the rate obtained for propylene homopolymerizations. The higher initial copolymerization rate was attributed to the stabilization of potentially active centers in solution when the deuterated monomer was present. These active centers are assumed to be formed by reactions of tetravalent titanium with monomer. These active centers, which are formed in solution, are said to be destroyed by isotopically controlled reactions, i. e., abstraction of the hydrogen or the α-deuterium atom from these monomer-alkylated species in solution or at the interface. These active centers are believed to be adsorbed and/or chemisorbed onto the precipitated catalyst surface and to be responsible for a polymer of considerably lower steric order. This scheme predicts a stereoregular polymer of high molecular weight produced by polymerization on a Ti(III) surface and a largely amorphous polymer of lower molecular weight produced by adsorbed and/or chemisorbed species. This prediction was verified by fractionation of the deuterated polymers into crystalline and amorphous portions.     

Polymerization of para-xylylene derivatives (parylene polymerization). IV. Effects of the sublimation rate of di-p-xylylene on the morphology and crystallinity of parylene N deposited at different temperatures

The effect of the sublimation rate of di-p-xylylene on the crystallinity and morphology of Parylene N deposited on stainless steel was studied as a function of substrate temperature. For a given rate of dimer sublimation, the deposition rate increases with decreasing substrate temperature. Increasing the sublimation rate of the dimer increases the deposition rate 10-fold, decreases the crystallinity, and shifts the appearance of the hexagonal β structure towards higher substrate temperature for samples synthesized from room temperature (RT) to ?60°C. Solution annealing resulting from solvent extraction, and isothermal annealing, increase the crystallinity of the polymers and result in structures containing both α and β polymorphs. The surface topology, as revealed by scanning electron microscopy (SEM), for polymers synthesized from RT to ?40°C shows a globular structure, whereas low temperature samples exhibit a rod-type morphology. For higher sublimation rates of the dimer, SEM micrographs show that oligomeric species start appearing on the polymer films after a period of 4–5 days. Solvent extraction removes the oligomeric crystals, and GPC analysis of the resulting extract indicates that most of the oligomers range in molecular weight from 100 to 900. The cross-sectional morphology for fractured low temperature samples, however, reveals different morphologies as polymerization proceeds. It is postulated that in the temperature range ?50 to ?78°C, both surface condensation and surface adsorption of monomer occurs, leading to different morphologies and lower crystallinity. The polymer synthesized at liquid nitrogen temperature shows the presence of voids along with different morphologies. X-ray diffractograms of polymers synthesized at liquid nitrogen reveal a considerable amount of amorphous phase in the films. Hence, it is inferred that, although the liquid nitrogen polymerization is a solid state polymerization of the crystalline monomer, it does not lead to 100% crystalline material, and the reasons for this are discussed.     

Stereospecific polymerization of methacrylonitrile. II. Influence of polymerization conditions on polymerization and on properties of polymers

Stereospecific polymerization of methacrylonitrile with diethylmagnesium has been studied. Polymerization temperature has an important effect on polymerization. The conversion, stereoregularity, and intrinsic viscosity of the polymer increased significantly with increasing polymerization temperature. Stereoregularity of the polymer improved with increasing the polymerization time and the monomer concentration, but it is independent of the catalyst concentration. Intrinsic viscosity of the crystalline polymer increased with increasing monomer concentration but is independent of the polymerization time and the catalyst concentration. It is suggested that two mechanisms are involved in this polymerization: coordinated anionic polymerization to from the crystalline polymer, and probably conventional anionic polymerization to form the amorphous polymer. It is found that crystalline polymer can also be obtained in homogeneous phase such as in tetrahydrofuran solvent.     

Effect of polymerization temperature on the original morphology of polyethylene prepared by γ-ray-induced polymerization

Polyethylene was prepared by γ-ray-induced polymerization in the temperature range 0–180°C. The morphology and the physical properties of the polymer as polymerized were studied by electron microscopy, differential scanning calorimetry, and gel permeation chromatography. Aggregates of small lamellar crystals with irregularly growing faces were produced below 55°C. Aggregates of large spherical particles were formed above 60°C together with hemispherical particles which adhered to the substrate. A few lamellar crystals of triangular or amoeba-like shapes were also found above 55°C. The polymers formed below 55°C showed a sharp single endothermic DSC peak and a bimodal molecular-weight distribution, while the sample above 60°C had a double endotherm and a unimodal molecular-weight distribution. These facts suggest that the mechanism of crystallization during polymerization below 55°C is different from that above 60°C. The melting point, however, decreased continuously with increasing polymerization temperature and was much lower than that of extended-chain crystals. The results show that the polyethylene, as polymerized, is composed of folded-chain crystals irrespective of the reaction temperature.     

Solid-state polymerization of hexaphenylcyclotrisiloxane

The anionic solid-state polymerization of triclinic crystals of hexaphenylcyclotrisiloxane (HPhTS) initiated by KOH and potassium oligo(methylphenylsiloxane) has been studied. It was found that this reaction can yield high molecular weight poly(diphenylsiloxane) (PDPhS) with a specific viscosity up to 5 (1 wt % diphenyloxide solution at 145°C). The main features of the process are as follows: (a) this is a heterogeneous reaction that proceeds inward from the surface of HPhTS crystals; (b) the crystalline polymer is obtained from the crystalline trimer; (c) OPhTS simultaneously forms along with the polymer; (d) the specific viscosity of the resulting polymer remains constant or decreases with polymerization time and, consequently, with the conversion of HPhTS; and (e) the crystallinity of polymerized PDPhS samples depends inversely on its specific viscosity. Together, these features suggest that polymerization and crystallization proceed successively. The morphologies of the resulting PDPhS phase revealed by means of scanning electron microscopy are consistent with this mechanism. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1973–1984, 1997     

Cationic polymerization of cyclic dienes. V. Polymerization of the methylcyclopentadiene

Methylcyclopentadiene (MCPD) has been polymerized with cationic catalysts in toluene solution at ?78°C. to a white powdery polymer, whose intrinsic viscosity in benzene solution at 30°C. ranged from 0.1 to 0.5. The comparison of the rate of homopolymerization of MCPD with that of cyclopentadiene (CPD) and the copolymerization of MCPD and CPD indicated that MCPD is much more reactive than CPD. It was suggested that the high stability of cycloalkenyl cation is responsible for the high reactivity of cyclic dienes. Infrared and NMR spectroscopy on polymethylcyclopentadiene showed that almost all of the monomer units in polymethylcyclopentadiene produced under the present conditions have a trisubstituted double bond. The mechanism of the initiation reaction in the polymerization of cyclic dienes is also discussed.     

On the reaction mechanism of gamma-ray solid-state copolymerization

The gamma-ray induced solid-state polymerization of binary mixtures consisting of a maleimide derivative as the first component and acenaphthylene or trans-stilbene as the second component was investigated regarding the occurrence of copolymerization. The binary solid mixture of these compounds exhibited a phase-equilibrium diagram including a simple eutectic mixture, and the irradiation caused the formation of a random copolymer together with that of homopolymers of component comonomers. The reaction rates and the composition of products suggested that the mobility of monomer molecules in the crystals affected the solid-state polymerization significantly. The solid-state copolymerization was supposed to take place by the diffusion of comonomer molecules via a vapor phase to the nuclei where the copolymerization proceeded.     

Radiation-induced solid-state copolymerization of maleic anhydride and acenaphthylene

Radiation-induced solid-state copolymerization of the maleic anhydride–acenaphthylene system was carried out for the purpose of studying the solid-state polymerization of vinyl compounds in a binary system. Melting point measurement confirmed that this binary monomer system forms a eutectic mixture in the solid state. The solid-state polymerization of these monomers proceeds at maximum rate at the eutectic composition, and the polymerization products consist of a mixture of polyacenaphthylene and 1:1 maleic anhydride–acenaphthylene alternating copolymer. Since the 1:1 copolymer was obtained in solution polymerization also and maleic anhydride did not homopolymerize in solid state, it is considered that the solid-state copolymerization of maleic anhydride and acenaphthylene occurs in a liquidlike state at the boundary of the two monomer crystals.     

Stereospecific polymerization of methacrylonitrile. VI. Effect of esters as a complexing agent with diethylmagnesium catalyst

The stereospecific polymerizations of methacrylonitrile with diethylmagnesium were carefully studied by using various ethers as complexing agents. The complexed ethers exhibit a beneficial effect on the stereoregularity of the resulting polymer, namely, the crystallinity increased by using ethers as a complexing agent. The polymerization rate and the molecular weight of the polymer also increased by using ether-complexed catalysts. The polymerization behavior was studied with the dioxane–diethylmagnesium complex as a typical complexed catalyst. The behavior was mostly similar to that of the diethymagnesium alone, that is, the rate of the polymerization increased in proportion to monomer concentration, and the solubility index increased with increasing monomer concentration. Interestingly, the viscosity of the acetone-insoluble fraction increased with increasing monomer concentration, while that of the acetone-soluble fraction was independent of monomer concentration. This is explained by considering that the catalyst has at least two kinds of catalytic species, one being the species that produces the crystalline polymer by a coordinated anionic polymerization, another being the one from which an amorphous polymer is obtained by a conventional anionic mechanism. The fact that the viscosity of the polymer decreased with increasing the initiator concentration is explained in terms of chain trasfer to the initiator. In case of diethylmagnesium alone, the viscosity of the polymer is independent of the initiator concentration.     

Equilibrium anionic polymerization of α-methylstyrene in p-dioxane

The equilibrium anionic polymerization of α-methylstyrene in p-dioxane, with potassium as initiator, has been investigated at 5, 15, 25, and 40°C by using high-vacuum techniques. The comparison of these results with those obtained previously for the equilibrium polymerization of α-methylstyrene in tetrahydrofuran revealed that, although the values of ΔG_1c, the free-energy change upon the polymerization of 1 mole of liquid monomer to 1 bases-mole of liquid amorphous polymer of infinite chain length, are the same for both systems, there is a distinct effect of the solvent. This effect is reflected in the value of monomer equilibrium concentration and its variation with polymer concentration and is explained in terms of a solvent–monomer and solvent–polymer interaction parameter.     

Polymerization of coordinated monomers. I. Stereoregulation in the free-radical polymerization of methyl methacrylate-zinc chloride or methyl methacrylate–stannic chloride complexes

Stereoregulation in free-radical polymerization was studied for the polymerization of the 2:1 or 1:1 complex of methyl methacrylate with ZnCl₂ or SnCl₄. The complexes were polymerized with the use of a free-radical initiator or γ-ray irradiation either in the liquid or solid state at various temperatures ranging from ?196 to 110°C, and the tacticities of the resulting polymers were determined by NMR spectroscopy. The polymers had different and characteristic values of tacticities depending upon the complex species, i.e., the kind of metal chloride and the stoichiometry. The tacticities were found to be independent of the polymerization temperature in both the liquid and solid states, in contrast with the fact that tacticities of the polymer from pure monomer changed markedly with the temperature. A temperature dependence appeared in the polymerization system, which contained more monomer than that corresponding to the 2:1 complex. The effect of the viscosity or the solid phase on the stereoregulation was examined in comparison with the polymerization of a mixture of methyl methacrylate and liquid paraffin. Two possible explanations regarding the stereoregulation mechanism are offered in relation to the structures of the complexes.     

Cationic polymerization of formaldehyde in liquid carbon dioxide. Part I

A cationic polymerization of formaldehyde which gave a high molecular weight polymer was studied in liquid carbon dioxide at 20–50°C. In the polymerization without any catalyst both the rate of polymerization and the molecular weight of the resulting polymer increased rapidly with a decrease in the loading density of the monomer solution to the reaction vessel, and also increased with an increase in the initial monomer concentration. From these results it was concluded that the initiating species could be ascribed to an impurity contained in the monomer solution. Both the rate of polymerization and the degree of polymerization of the polymer also increased with rising temperature. The carboxylic acid added acted as a catalyst in the polymerization because of increase in the polymer yield, the molecular weight of polymer formed, and the number of moles of polymer chain with increasing dissociation constant of acid used. It was concluded that the polymerization in liquid carbon dioxide proceeded by a cationic mechanism. Methyl formate had no influence on the polymerization, but methanol and water acted as a chain-transfer agent.     

Four-center type photopolymerization in the crystalline state. V. X-ray crystallographic study of the polymerization of 2,5-distyrylpyrazine

Changes in crystal structure during polymerization and oligomerization of 2,5-distyrylpyrazine have been investigated by x-ray crystallography. The polymer and the oligomer as obtained are three-dimensionally oriented, and the directions of the three axes of the resultant crystals coincide with those of the original crystal. The space group of the products also agrees with that of the monomer. It is concluded that the polymer and the oligomer crystals approximately duplicate the molecular arrangement in the monomer crystal. The polymerization mechanism is discussed on the basis of the crystal structures.