Reactive Chain Ends Research Articles

Stereoregular Polysiloxanes via Ring-Opening Polymerization, a Review

Although polysiloxanes are not ordinarily considered stereoregular, a few references describe ring-opening polymerizations of cyciosiloxanes that can produce polymers enriched in stereoregularity. Most are made from the cis-isomers of unsymmetrically substituted strained cyclosiloxanes by methods where ring–chain and chain–chain equilibria are suppressed. The monomers insert randomly at the reactive chain ends with equal probabilities of forming new meso (m) or raceme (r) siloxane links while preserving the stereoconfiguration of the original monomer. The resulting polysiloxanes are enriched in isotactic configurations but are limited in their run lengths by the new randomly configured siloxane links formed between the original monomer units. One citation, however, describes a ring–chain equilibrium where the isotactic conformation was the preferred conformation. Another illustrates the profound influence of stereoregularity on the mechanical properties of a silicone elastomer.

Reaction rate enhancement during swollen-state polymerization of poly(ethylene terephthalate)

Experimental data are obtained for the extent of swelling and progress of the step-growth swollen-state polymerization (SwSP) of polyethylene terephthalate (PET). The SwSP is carried out in biphenyl and diphenyl ether mixture (26 : 74 w/w) solvent under appropriate conditions designed to understand the factors responsible for enhanced reaction rates. The kinetics rate constants, evaluated in terms of simple model, are found to be 2.5–5 times higher for SwSP as compared to the solid-state polymerization (SSP). As the diffusional/mass transfer effects are eliminated in our experiments, this increase in rate constants can be attributed to increased mobility of reactive chain ends. Polymerization rate is found to be further enhanced by addition of a polycondensation catalyst (Sb2O3) to the solvent during SwSP. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1589–1595, 1998

Mechanism of Exchange in PBT/PC and PET/PC Blends. Composition of the Copolymer Formed in the Melt Mixing Process

Mechanisms operating in the exchange reactions occurring in the melt mixing processes of Bisphenol A polycarbonate (PC) with poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) blends have been investigated making use of appropriate polymer samples, capped or containing reactive chain end groups. The exchange process may proceed by two different mechanisms: a direct exchange reaction between inner functional groups located inside the polymer chains, i.e., inner−inner, or by attack of reactive chain ends functional groups (outer) on inner groups, i.e., outer−inner. It is shown that the distinction between the two processes can be conveniently made by determining the composition of the copolymer formed in the exchange reaction. The inner−inner mechanism occurs only in the reaction between end-capped or high molar mass PBT/PC or PET/PC samples, and it was found that the molar composition of the copolymer formed is always equal to the feed ratio of the two homopolymers and independent from the reaction time. The outer−inner mechanism occurs in the presence of hydroxyl or carboxyl reactive chain ends in PBT and PET samples. The reaction proceeds by the attack of the reactive end groups on the PC chains, originating block copolymers of PC and PBT and low molar mass PC with phenol end groups which are unreactive. The reaction stops right after the reactive end groups are consumed. The amount and the composition of the copolymers generated in the reactions are found to be constant as a function of time. The copolymer composition shows an excess of PBT or PET units with respect to the feed molar ratio. These results indicate that monitoring the composition of the copolymer formed in each case is diagnostic for establishing the mechanism of the reaction. The approach used here allows control of the composition and yield of the copolymer to be produced, and it is applicable to other systems where exchange reactions occur.

Engineering of zinc finger and MHC motifs to locked-in tertiary folds

The assembly of helical and β-sheet peptide blocks containing reactive chain ends results inhighly branched chain architectures (‘locked-in folds’) mimicking native tertiary structures.This molecular kit strategy allows to bypass the protein folding problem in protein de novodesign and gives access to protein mimetics of high thermodynamic stability. The validity ofthis concept is exemplified for the design and synthesis of locked-in folds mimicking the zincfinger and MHC folding motifs.

Dendrimers and Hyperbranched Polymers: Two Families of Three-Dimensional Macromolecules with Similar but Clearly Distinct Properties

Abstract Dendrimers and hyperbranched polymers are globular macromolecules that are characterized both by a highly branched structure, in which all bonds converge to a focal point or core, and a multiplicity of reactive chain-ends. Because of the obvious similarity of their building blocks, many assume that the properties of these two families of dendritic macromolecules are almost identical and that the terms “dendrimer” and “hyperbranched polymer” can be used interchangeably. This assumption is incorrect because only regular dendrimers have a precise end-group multiplicity and functionality and exhibit properties that are totally unlike those of all other families of macromolecules. For example, regular dendrimers display a maximum in the relationship between their intrinsic the applications that are contemplated for the dendritic polymers. Using living systems as a model, one must anticipate that the regular placement of reactive groups at a precise location, as opposed to throughout a structure, has great importance in terms of ultimate performance when surface and interfacial properties are involved. Therefore, just like Langmuir-Blodgett films, dendrimers belong to a special class of well-defined molecular architectures where exact structure and ultimate performance are intimately related. It is likely that regular dendrimers will continue to draw much attention in specialized applications that take advantage of their precise architecture: drug delivery, catalysis, nanoreactors, molecular devices, etc. In contrast, hyperbranched polymers will join other interesting functional polymers in applications that involve commodity materials: from coatings and adhesives to lubricants, compatibilizers, and carriers. Today, the frontiers in the chemistry of dendritic polymer are in the search for rapid methods of synthesis involving, for example, vinyl polymerizations [46, 50], the preparation of “engineered” dendritic structures, and the discovery of new properties and applications [1, 47].

Synthesis and copolymerization of macromonomers based on 2‐nonyl‐ and 2‐phenyl‐2‐oxazoline

AbstractSeveral macromonomers were prepared by cationic ring‐opening polymerization of 2‐nonyl‐ and 2‐phenyl‐2‐oxazoline using different techniques of functionalization. Introduction of the unsaturated functional group directly via a suitable termination agent proved to be superior to the use of a phenol‐functionalized initiator and to the introduction of hydroxyl end groups via hydrolysis of the reactive cationic chain end and subsequent esterification with methacryloyl chloride. The macromonomers were characterized by spectroscopic techniques as well as GPC. One of the 4‐vinylbenzyl‐terminated macromonomers was copolymerized with MMA in different ratios. The copolymerization parameter r1 was determined to be 0.75. The resulting graft copolymers were characterized regarding number of grafts per chain, molar mass, and glass transition temperature.

Chemical Assays of End-groups Displayed on the Surface of Poly(ethylene terephthalate) (PET) Films and Membranes by Radiolabeling

Poly(ethylene terephthalate) BET) films and track-etched microporous membranes naturally display, on their surfaces, reactive chain-ends, i.e. carboxyl and hydroxyl functions. These were assayed by suitable activation (reaction with carbodiimide and tosyl chloride, respectively), followed by coupling with H-3-lysine and liquid scintillation counting of the sample-associated radioactivity. Values ranging between 5 and 30 pmol/cm(2) (open surface) of labeled end-groups were obtained, depending on the physico-chemical nature of the samples. Basic hydrolysis enriched the PET films with both types of endings (15-25 pmol/cm(2)). Reduction of films with the NaBH4-catechol complex in tetrahydrofuran enriched their surfaces with hydroxyl groups. However, this procedure was not readily applicable to the surface modification of membranes; we observed an erosion effect that was confirmed by scanning electron microscope analyses. In contrast with the reduction process, the oxidation with KMnO4 in 1.2 N H2SO4 could be easily applied to the modification of either films or membranes; their surfaces were significantly enriched with carboxyl groups (15-50 pmol/cm(2)). This surface modification strategy has been used for the covalent coupling of adhesive proteins on PET membranes developed as supports for cell cultivation.

Functionalized Poly-Imide–Amides: Molten State Reaction Mechanisms and Kinetics

Molten state polymerization reaction mechanisms and kinetics of telechelic polyimide–amides (PIA) were investigated. We conducted a basic study of the reactions of these functionalized systems as a function of the chemical structure of the polymer. Since reaction products very rapidly became insoluble, the study was carried out on model compounds characteristic of reactive chain ends. The data obtained were applied to the study of industrial prepolymers. Different model compounds were synthesized and studied in the temperature range of 100 to 300 °C using such physicochemical methods as HPLC, 1H and 13C NMR (liquid and solid), FTIR and DSC. We were thus able to determine a number of precise parameters characteristic of the different reactions occurring (isomerization, crosslinking, co-reactions and side reactions). The results were applied to the study of 1500 molecular weight prepolymers and preimpregnated carbon fibres.

Simulation of diffusion-limited step-growth polymerization in 2D: Effect of shear flow and chain rigidity

Multimolecule Brownian dynamics simulation results for diffusion controlled polymerization of bead–rod chain molecules in 2D solution are presented. Reaction between any two molecules undergoing Brownian diffusion takes place if the reactive chain ends approach each other to within a certain reaction radius, and if the chain end carrying segments are collinear within certain specified limits. The second order reaction rate constant is found to decrease with time as the molecular lengths increase and the diffusivities decrease. Application of a shear flow is seen to result in alignment of the molecules along the flow direction, thereby enhancing the concentration of molecular pairs with parallel orientation of reactive-end carrying chain segments, and hence the overall reaction rate. This effect is found to be more pronounced in the case of long rigid molecules as compared to flexible molecules because of the slow rotation and high level of orientation under flow of the former. Even the molecular weight distribution (MWD) obtained during polymerization may be affected. For example, longer molecules have lower diffusivities and hence lower reactivities, resulting in a narrower MWD in the absence of flow, as compared to the results with the usual assumption of molecular reactivity being independent of chain length. Furthermore, in the presence of an external flow, the longer molecules orient to a higher degree and hence display a higher enhancement in reactivity. This results in a wider MWD of the polymer. The simulation results are in qualitative agreement with previous experimental data for solution polymerization of rod-like molecules.

Modelling of polyethylene terephthalate reactors—X. A comprehensive model for solid-state polycondensation process

A new comprehensive model for solid-state polycondensation (SSP) of polyethylene terephthalate (PET) has been developed, which interprets a number of hitherto unexplained observations. Degradation and other side reactions which were neglected in the earlier models for SSP process of PET are included. The diffusivity changes due to crystallisation of PET, the true influence of nature of carrier gas and the increase in the reactive chain end group mobility due to the change in the free volume in the presence of an inert gas have been considered. The model predictions have been validated with experimental data reported in the literature. The model can be used for a quantitative prediction of the influence of a variety of parameters on the degree of polymerisation as well as side products formation. The model correctly brings out the true influence of the carrier gas which had remained unexplained so far. It also suggests additional directions for improving the SSP process.

Approaches to controlled polymerization of methyl acrylate through functional anionic initiators

Controlled ‘living’ polymerization of methyl acrylate has been achieved at room temperature using functional carbanionic initiators in THF solvent. Methyl acrylate oligomers with reactive chain ends have been synthesized.

Theory of π-electron states in polydiacetylene molecules

We report on π-electron states in polydiacetylene chains using Hückel and self-consistent-field methods (SCF). The Hückel band-structure calculation for the infinitely long chain yields two valence and two conduction bands. The Peierls gap between the upper valence and the lower conduction band is dependent on the bond alternation within the conjugated π system described by a bond alternation parameter −1<q<1. It turns out that the acetylene structure (q>0) is energetically more favorable than the butatriene structure (q<0). Performing Hückel calculations for short chain polydiacetylene intermediates and including the σ electrons of the polymer backbone by a harmonic potential we obtain the chain length dependency of the bond alternation parameter q. The intermediates with two reactive chain ends (diradicals and dicarbenes) show a transition from the butatriene to the acetylene structure. To determine the spin multiplicity of the ground state of these intermediates we perform a SCF calculation for the singlet and the triplet state of the conjugated π system. According to the experiments we find a singlet ground state for chain lengths of less than six monomer units (diradicals) and a triplet ground state for longer chain lengths (dicarbenes). Optical excitation energies of the short-chain intermediates are determined using the method of Pariser, Parr, and Pople. The qualitative tendencies of the chain length dependency agree well with the experimental results of optical absorption measurements, the absolute values are too high as compared to the experiments.

Kinetics of the UV-Photopolymerization in TS Diacetylene Single Crystals as Studied by Time Resolved Optical Spectroscopy

Abstract The intermediate products A, B, C, D, E and V of the UV initiated solid state polymerization in single crystals of 2,4 hexadiynylene bis (p-toluene sulfonate), particulary their kinetics, have been studied by time resolved optical absorption spectroscopy in the temperature range between 180 and 300 K. The description by a simple kinetic model states that A to E react in a direct sequence by monomer addition, which is thermally activated. The activation energies range from 0.25 to 0.30 eV per monomer. At 300 K the reaction rate constants are of the order of 106 s-1. - Product V is interpreted as a superposition of several long chain oligomers with reactive carbene chain ends. The reaction V → Polymer is thermally activated with an activation energy of 0,4 eV per monomer. - The fluorescence spectrum of the monomer crystal has been measured for the first time by use of a fast detection system. It is discussed with respect to its significance for the polymerization initiating step. - Deuterium isotope effects on the polymer absorption energy and on reaction kinetics are reported.

Kinetic study on reactions between polymer chain-ends—II. Reactions between chlorosulphonyl-ended and primary amino-ended polyoxyethylenes followed by fluorometry

Rates of the reaction of 1,5-naphthalenedisulphonyl dichloride, a bifunctional fluorescent reagent, with primary amino-ended polyoxyethylene were measured by fluorometry in dilute solution. The second-order rate constants obtained are equivalent to those for the reaction between polyoxyethylenes with chlorosulphonyl and primary amino groups as the reactive chain-ends respectively (polymer-polymer reaction). Rates of the reaction of 5-dimethylamino-1-naphthalenesulphonyl chloride, a monofunctional fluorescent reagent, with primary amino-ended polyoxyethylene were also measured as model (small molecule-polymer) reactions. In both cases the rates of reaction in cloroform (good solvent) are substantially independent of the degree of polymerization over the range of about 20 to 20,000. These and previous results show clearly that, if there is a “kinetic excluded volume effect”, it is very small and Flory's principle of equal reactivity of a functional group holds. Recent theories of the kinetic excluded volume effect are discussed briefly.

Isotactic polymerization of .alpha.-olefins: stereoregulation for different reactive chain ends

Isotactic polymerization of .alpha.-olefins: stereoregulation for different reactive chain ends

Kinetic study on reactions between polymer chain-ends. Coupling reactions of living polystyrene with chloro-ended polystyrene

The reaction of living polystyrene with chloro-ended polystrene was studied to examine “the kinetic excluded volume effect” on the intermolecular reaction between reactive chain-ends of two monodisperse polymers. The reaction of living polystyrene with 1-chloropentane was also studied as a model reaction (small molecule-polymer reaction). The second-order rate constants, k 2, for the polymer polymer reaction in benzene (with a small amount of tetrahydrofuran to break the association of living-ends) is independent of the degree of polymerization, DP, for the range of DP studied (up to 400). The ratios of the rate constants for the polymer-polymer and the polymer-small molecule reactions, k 2/ k 2 0, are the same in benzene and in cyclohexane (good and poor solvent for polystyrene respectively), showing that the effect of the coil expansion is not large enough to be detected. These results confirm the Flory basic concept that the reactivity of a functional group attached to an inert polymer is not affected by the presence of the polymer chain in activation-controlled reactions.

Crystallization of polypeptides during polymerization

In order to pursue the possibilities of formation of extended chain crystals in the course of polymerization, kinetics of the heterogeneous polymerization of L- and DL-alanine N-carboxy anhydride (NCA) initiated by butylamine in acetonitrile and the morphologies of the resultant polymers were studied. The fibers formed in the beginning of polymerization thickened as the polymerization proceeded. The growing chain molecules, lost solubility in the system when they attained a degree of polymerization of 3 to 4 and crystallized in the form of ribbon-like fibers in which molecules with the β-conformation were aligned perpendicularly to the fiber axis. As the growing chains attained a critical length by further polymerization, they began to take the α-helical conformation. The crystals grew successively thereafter, taking the α-helical conformation through addition of the monomer molecules onto the reactive chain ends. Possible mechanisms are proposed for the nucleation and the subsequent growth of the crystals in the course of polymerization. The polymerization of NCA's of glycine and β-alanine has also been studied. The polymer precipitated in the form of spherulites consisting of ribbon-like crystalline units. The molecules in the ribbon-like crystals were arranged antiparallel, forming a hydrogen-bonded sheet. Homopolymerizations of other NCA's and copolymerizations were also examined in the same heterogeneous system.

Kinetics of the topochemical solid‐state polymerization of diacetylenes as studied by a flash photochemical experiment

AbstractThe kinetics of an organic solid‐state reaction is studied by a flash‐photochemical experiment. A set‐up is described which allows the detection of a photoreaction monitoring the change in reflectivity of a polycrystalline sample reacting upon photoinitiation with a laser light pulse. The time scale of the measurement extends from 5 ns to several hundred ms. The topochemical polymerization of two different diacetylenes is investigated measuring the formation of the polymer as a function of time after the initiating light pulse of wavelength λ = 337.1 nm. The first polymer is formed after a timelag of 300 ns. The total conversion per light pulse is less than 1 percent. The time conversion curve is described by an exp √t‐law for virgin samples suggesting a kinetic model which includes a strongly scattering random walk of photo‐excited states to randomly distributed reaction centers as the rate determining factor of the photopolymerization. The average life time of the photoexcited state is 758.5 ps. The overall quantum yield of the polymerization is approximately 10 and the frequency of addition of a monomer unit to a reactive chain end is va, ≤ 3.3 · 107 s−1.

Epitaxial crystallization of polyoxymethylene during cationic polymerization of trioxane

AbstractEpitaxial crystallization of polyoxymethylene (POM) during cationic polymerization of trioxane in ethane dichloride was studied. Nascent POM crystals grew epitaxially on the surface of drawn, high‐molecular‐weight POM and on the (001) surface of KCl single crystals. Half‐hexagonal crystals several thousand angstroms in thickness were formed on these substrates. No discrete diffraction maximum could be observed in small‐angle x‐ray scattering patterns, but diffuse streaks were present on both the equator and the meridian of the pattern. These diffuse streaks are due to inhomogeneities in the nascent crystals. It is proposed that the crystals contain voids due to occlusion of reactive chain ends in the crystal. The optical diffraction pattern from a model made up of rectangles agreed qualitatively with the actual x‐ray pattern.

Crystallization during polymerization with special reference to synthetic polypeptides

In order to pursue the possibilities of the extended chain crystals formed in the course of polymerization, kinetics of the heterogeneous polymerization of L- and DL-alanine N-carboxy anhydrides (NCA) in acetonitrile and the morphologies of the resultant polymers were studied. It was revealed that the growing chain molecules, when they attained a certain degree of polymerization (DP = 3 to 4), crystallized into the form of ribbon-like fibers in which molecules, with the β-conformation, were aligned perpendicularly to the fiber axis. As the growing chains attained a critical length by further polymerization, they began to take the α-helical conformation. The crystals grew successively thereafter, taking the α-helical conformation through addition of the monomer molecules onto the reactive chain ends. The grown polypeptide chains were extended α-helices in the crystals throughout the remainder of the polymerization. Possible mechanisms are proposed for the nucleation and the subsequent growth of the crystals in the course of polymerization. The polymerization of NCA's of glycine and β alanine has also been studied.