Initial Stage Of Polymerization Research Articles

(Pyridylamido)Hf(IV)-Catalyzed 1-Octene Polymerization Reaction Interwoven with the Structural Dynamics of the Ion-Pair-Active Species: Bridging from Microscopic Simulation to Chemical Kinetics with the Red Moon Method.

We performed the atomistic simulation of 1-octene polymerization reaction catalyzed by the ionic pair (IP) consisting of the cationic active species of (pyridylamido)Hf(IV) catalyst, HfCatPn+, and different counteranions (CAs), B(C6F5)4- and MeB(C6F5)3-, at different monomer concentrations. Using a hybrid Monte Carlo/molecular dynamics method, that is, the Red Moon (RM) method, the reaction progress measured by the "RM cycle" was transformed into effective real time using the time transformation theory. Then, the degree of polymerization was found to be consistent with that in the chemical kinetics, a macroscopic theory, and experimental ones. Remarkably, the current simulation has revealed the different dynamical features in the polymerization behavior originating from the CA. Namely, the HfCatPn+-B(C6F5)4- IP mainly forms an outer-sphere IP (OSIP) throughout the polymerization. The HfCatPn+-MeB(C6F5)3- IP, on the other hand, forms an inner-sphere IP (ISIP) in the initial stage of polymerization, and the ratio of ISIP steeply drops after the first monomer insertion because the IP interaction is reduced by the steric hindrance between the inserted monomers and the CA. In conclusion, we have shown that the microscopic IP dynamics interwoven with the polymerization reaction can be computationally observed in the real-time domain by using the RM method. Therefore, our current work demonstrates the promising potential of the RM method in studying catalytic olefin polymerization and complex chemical reaction systems.

Diffusion controlled stereoregular polymerization in granule reactors - Reaction kinetic of Pr/Bt sequence polymerization with TiCl4/MgCl2 Ziegler-Natta catalyst

Diffusion controlled stereoregular polymerization was studied to understand the relations between fragmentation and growing of catalyst granules and its catalytic features for olefin polymerizations. In this work, propylene (Pr) polymerization kinetics and Pr/butene-1 (Bt) sequence polymerization kinetics with spherical TiCl4/MgCl2 Ziegler-Natta catalyst were investigated by monitoring catalytic activity (CA), the number of active centers ([C*]/[Ti]) and microstructure of polymers. It was found that the increase in Pr polymerization time benefiting to the catalyst granule fragmentation, had different influences on the [C*]/[Ti] and catalytic efficiency (CE) for Bt-stage polymerization in different stage. Anyway, the isotacticity and weight-average molecular weight (Mw) of isotactic polybutene-1 (iPB) fractions decreased gradually with the increase in Pr polymerization time. It was deduced that the easy solubility of polybutene-1 (PB) brought difficulties in further fragmentation of catalyst particles to expose more active centers, and Bt was more sensitive to the chemical and steric environment around the active species. This work provides further understanding of the catalytic characteristics of granule reactors in the initial polymerization stage, which will guide the novel polyolefin alloy production.

INFLUENCE OF FIBRIN D AND DD FRAGMENTS ON FIBRINOGEN AND FIBRINOGEN FRAGMENT X POLYMERIZATION INITIATED BY THROMBIN OR ANCISTRON

Aim. Study of the role of the complex between the αC region and the BβN domain in the initial stages of fibrin polymerization has been investigated. Materials and Methods. Method of turbidimetry to study the influence of fibrinogen fragments D and DD on the polymerization and methods of isolation, purification, fragmentation for fibrinogen, monomer and cross-linked fibrin, fibrinogen X-fragment, Glu -plasminogen were used. Results. It was shown that fragment DD completely inhibited polymerization process in all the systems examined (“Fg + Thr”, “Fg + Anc H”, “X + Thr”, “X + Anc H”). Fragment D inhibited fibrin polymerization at all stages in the system “Fg + Thr”, but in the system “Fg + Anc H” it almost did not influence fibrin polymerization. In the both systems “X + Thr” and “X + Anc H” fragment D weakly inhibited the self-assembly of fibrin molecules into protofibrils, but accelerated the process of lateral association in the second system. Conclusions. The data obtained indicated that the complex between the αC region and the BβN domain of fibrin desA, on the initial stage of polymerization supported the rate of self-assembling and lateral association of fibrin desA protofibrils, protecting the oligomers against the depolymerizing influence of fibrinogen.

Flow-accelerated polycondensation reaction to prepare rigid rodlike poly(p-phenylene-cis-benzobisoxazole)

Under the conditions specified in this work, the polycondensation reaction to prepare rigid rodlike poly(p-phenylene-cis-benzobisoxazole) (PBO) is diffusion-controlled. This work aims to study the flow-driven polycondensation reaction of PBO. The effect of mixing efficiency through changing the stirring speed on the polycondensation reaction of PBO was first investigated in a paddle impeller reactor (PIR). In the initial stage of polymerization, the increase of stirring speed is obviously conducive to accelerating the polycondensation reaction. However, the promoting effect of stirring speed on the polycondensation reaction is greatly reduced in the middle and late stages of polymerization. The effect of the flow pattern with different proportions of shear flow, elongational flow and rotational flow on the polycondensation reaction of PBO was further investigated by comparing three reactors including Couette-Taylor reactor (CTR) with a complete shear flow, PIR with strong shear and elongational flows, and double helical ribbon reactor (DHR) with strong shear and rotational flows. Result shows that the improvement of proportions of rotational flow and elongational flow is beneficial to accelerate the polycondensation reaction of PBO in the middle and late stages of polymerization, especially for the rotational flow.

Dispersion polymerization of styrene/acrylonitrile in polyether stabilized by macro-RAFT agents

A series of poly(acrylate polyether)s (RPA) was designed and synthesized by reversible addition-fragmentation chain transfer (RAFT) copolymerization, which were then used as dispersants for the dispersion copolymerization of styrene/acrylonitrile in polyethers in a semicontinuous approach to prepare graft polyether polyol (POP) with high solid content. The effects of structure, dosage, and feed ratio for macro-RAFT agent RPA on the stability, viscosity, and particle size of POP were examined and discussed. The dispersion polymerization process was also characterized by gradient elution polymer chromatography, and the results indicated that RPA was rapidly consumed and block-type stabilizer formed in the initial stage of polymerization. A portion of RPA added with monomer feeding provided better stability for the semicontinuous polymerization process.

Stability Enhancement of a π-Stacked Helical Structure Using Substituents of an Amino Acid Side Chain: Helix Formation via a Nucleation-Elongation Mechanism.

Molecular design involving the incorporation of an α-amino acid residue into the side chain or main chain of a polymer is often used to stabilize artificial molecular architectures through intramolecular hydrogen bonding. However, this molecular design strategy rarely considers the importance of interactions between substituents at the α-position of amino acid moieties, as found in nature. Herein, we report the synthesis of a novel series of π-stacked helical poly(quinolylene-2,3-methylene) with amino acid derivatives bearing different substituents at the α-position. We found that the thermal stability of π-stacked helical poly(quinolylene-2,3-methylene) is significantly improved by packing the substituents in the empty spaces between the side chains. In particular, when a bulky cyclohexyl alanine derivative was used as the side chain, the π-stacked helical structure maintained its stability even in dimethylsulfoxide, a hydrogen bond competitor. The stabilization of the π-stacked structure by the amino acid substituents resulted in a unique polymerization behavior involving nucleation-elongation steps. In the case of derivatives with leucine and cyclohexyl alanine, which form stable π-stacked helical structures, metastable structures with entangled main chains were formed in the initial polymerization stage. These structures subsequently underwent an irreversible structural change to achieve a thermodynamically stable helical π-stacked conformation as a nucleus for subsequent polymerization. Thereafter, the polymerization reaction proceeded with the elongation of the π-stacked helical structure. Differences in the stability of these systems indicated that the amino acid substituents on the side chains determine the most thermodynamically stable π-stacked helical structure.

Reactivity of short radicals at the initial stages of radical polymerization of allyl chloride: Chain growth versus addition to fullerene

AbstractA quantum‐chemical study of the C60 interaction with various radicals generated from monomers in radical polymerization is significant in the fundamental aspects. However, the interactions between fullerene and radicals formed from allyl monomers has been poorly studied. Using the modern density functional theory methods, we performed quantum‐chemical simulation on the C60 interaction with the radicals arising upon the initial stages of the allyl chloride polymerization. We have found that the addition of growth radicals (with 1–4 monomer units) to C60 are more favorable than the chain growth reactions. Herewith, the radicals with 2–4 monomer units are most reactive in the addition reaction. Our quantum‐chemical findings explain the features of the initial stages of the allyl chloride radical polymerization in the presence of C60, namely, the increasing inhibition degree of the radical copolymerization of allyl chloride with methyl methacrylate and increasing induction period of copolymerization with styrene.

Synthesis of amine-capped Trans-1, 4- polyisoprene

Polymer chain-end functionalization with polar groups can significantly improve the hydrophilicity of the polymers. In this work, a facile strategy was proposed for synthesis of amine-functionalized trans-1,4-polyisoprenes (F-TPI) with heterogeneous TiCl4/MgCl2 type Ziegler-Natta catalyst by using dicyclohexylamine (DCHA) as chain-transfer agents (CTA). The microstructure and molecular weight of F-TPI were analyzed by 1H NMR and GPC. With the increase in DCHA/isoprene (IP), the amine-capped efficiency (CE) of the obtained F-TPI increased and the number-average molecular weight (Mn) decreased accordingly, accompanied with the reduced polymerization activity. The F-TPI sample was separated into low-molecular-weight (LMW) fractions with about 98% CE and high-molecular-weight (HMW) fractions with about 30% CE by using precipitation fractionation method. The isoprene polymerization kinetics in the presence of DCHA revealed that the CE in the polymers reached 98.5% in the initial polymerization stage (about 1.8 wt% conversion) and then decreased gradually with the polymerization time extention. The chain end structures and possible chain transfer reactions were proposed to explain the fraction formation in F-TPI. The water contact angle of the obtained F-TPI decreased with increased CE, indicating higher hydrophilicity of the F-TPI membranes.

Chemical Oxidative Polymerization of Methylene Blue: Reaction Mechanism and Aspects of Chain Structure.

The kinetic regularities of the initial stage of chemical oxidative polymerization of methylene blue under the action of ammonium peroxodisulfate in an aqueous medium have been established by the method of potentiometry. It was shown that the methylene blue polymerization mechanism includes the stages of chain initiation and growth. It was found that the rate of the initial stage of the reaction obeys the kinetic equation of the first order with the activation energy 49 kJ · mol−1. Based on the proposed mechanism of oxidative polymerization of methylene blue and the data of MALDI, EPR, and IR spectroscopy methods, the structure of the polymethylene blue chain is proposed. It has been shown that polymethylene blue has a metallic luster, and its electrical conductivity is probably the result of conjugation over extended chain sections and the formation of charge transfer complexes. It was found that polymethylene blue is resistant to heating up to a temperature of 440 K and then enters into exothermic transformations without significant weight loss. When the temperature rises above 480 K, polymethylene blue is subject to endothermic degradation and retains 75% of its mass up to 1000 K.

Trans‐1,4‐stereospecific copolymerization of isoprene and butadiene catalyzed by TiCl4/MgCl2 Ziegler–Natta catalyst: III Effect of alkylaluminium on monomer reactivity ratios

AbstractCopolymerizations of isoprene (Ip) and butadiene (Bd) in different Bd/Ip feed ratios were carried out using the heterogeneous TiCl4/MgCl2 type Ziegler–Natta (Z‐N) catalysts activated by trimethylaluminium (AlMe3), triethylaluminium (AlEt3), triisobutylaluminium (Al(i‐Bu)3) or tri‐n‐octylaluminium (AlOct3). Monomer reactivity ratios of Ip and Bd (rIp, rBd) in the copolymerizations were calculated by the Kelen–Tödüs method and then instantaneous compositions of the copolymers were theoretically acquired based on the Mayo–Lewis equation. The effects of alkylaluminiums on copolymerization activity, copolymer microstructure, comonomer incorporation, monomer reactivity ratios and copolymer instantaneous composition were investigated. Using AlEt3 led to higher copolymerization activity and trans‐1,4 stereoselectivity compared with other trialkylaluminiums. AlOct3 with bulky n‐octyl groups showed a higher Bd monomer reactivity ratio. The theoretical copolymer composition drift based on the Mayo–Lewis equation was in good coincidence with the experimental data measured by real‐time 1H NMR during the steady polymerization stage. The nature of the discrepancy between the theoretical and measured copolymer compositions obtained in the initial polymerization stage is discussed in detail. © 2021 Society of Industrial Chemistry.

The morphology evolution of polyethylene produced in the presence of a Ziegler‐type catalyst anchored on the surface of multi‐walled carbon nanotubes

AbstractData are presented on the evolution of the morphology of polyethylene (PE) formed via in situ polymerization with different polymer yield over a Ziegler‐type titanium‐magnesium catalyst anchored on the CNT surface. Individual polymer microparticles are formed on the CNT surface at the initial polymerization stage (the yield of 2.5–10 g PE/g CNT) with the formation of PE/CNT composites having a shish‐kebab structure. As the polymer yield increases above 10 g PE per g CNT, the size of microparticles increases and the CNT surface gets totally covered with the polymer. We have found also a great effect produced by the morphology of initial CNT particle aggregates of individual nanotubes on the morphology of macroparticles in PE/CNT composites and the uniformity of CNTs distribution in PE/CNT composites. In the case of CNT samples with a loose structure of macroparticles (aggregates of entangled nanotubes), it is possible to obtain a homogeneous distribution of nanotubes in the polymer matrix of composites and increase the electrical conductivity of composites by 1–8 orders of magnitude by varying the CNT content in the composites from 0.9 to 2.8 wt%.

Isoprene polymerizations catalyzed by TiCl4/MgCl2 type Ziegler-Natta catalysts with different titanium contents

The isoprene polymerization kinetics by using TiCl4/MgCl2 type Ziegler-Natta catalysts (TMC) with different titanium contents were studied for better understanding the possible active-specie structures of heterogeneous Ziegler-Natta (Z-N) catalysts for conjugated diene polymerizations. The catalytic features including the catalytic activity (CA), number of active centers ([C*]/[Ti]), rate constant for polymerization (kp), stereoregularity of these TMC catalysts were studied. The low titanium content catalyst TMC-1 (Ti = 0.09 wt.%) with more isolated mononuclear titanium species showed higher CA and yielded cis-1,4-/trans-1,4- mixed microstructure polyisoprene with low molecular weight (Mw) in the initial polymerization stage. The medium high titanium content catalyst TMC-2 (Ti = 2.38 wt.%) with more clustered mononuclear titanium species showed lower CA and yielded cis-1,4-/trans-1,4- mixed microstructure polyisoprene with high Mw in the early stage of polymerization. The high titanium content catalyst TMC-3 (Ti = 3.76 wt.%) presented lowest CA and yielded high trans-1,4- polyisoprene with higher Mw.

Optical and mechanical factors in the temporal development of tooth–composite bond

ObjectivesTo obtain an accurate picture of the temporal development of bond strength between resin composites and tooth structures during cure for assessing debonding at the tooth–composite interface. MethodsAn assembly of uncured composite sandwiched between a glass block and a dentin slab with a layer of pre-cured adhesive was used in this study. A conventional composite was compared against a bulk-fill composite. The rate of bond formation was determined by measuring the tensile bond strength of specimens of different thicknesses at different time points during cure. The changing light irradiance exiting the composite as it cured was also recorded. Mode of fracture was analyzed by examining the fracture surfaces. ResultsPhoto-bleaching occurred in both resin composites. The development of the dentin–composite bond strength was initially dictated by the developing cohesive strength of the resin composite, and its final value was capped by the strength of the preformed dentin–adhesive bond. The higher interfacial irradiance in the bulk-fill composite did not lead to faster development of the overall bond strength. This was caused by its slower rate of cohesive strength development as reflected in the longer time for its interfacial irradiance to plateau and the greater proportion of cohesive failure seen in the initial stage of polymerization. The law of reciprocity did not hold for the development of dentin bond strength. SignificanceThe results from this study, when compared with the development of shrinkage stress, can be used as a basis for ensuring the integrity of the dentin–composite interface during cure.

Porosity Development in Silica Particles during Polymerization: Effect of Solvent Reactivity and Precursor Concentration

The porous morphology drives the application of nano- and mesoporous materials in a variety of areas. However, limited information is available on the porosity development during their synthesis at various times due to challenges in experimental and simulation techniques. In this work, we have probed the porosity development in silica particles using Monte Carlo simulation techniques. We have developed an algorithm to measure the porosity of small, irregular shaped, finite-size particles formed during the polymerization process. We observed that smaller and denser clusters are formed initially, which later aggregate to form porous clusters in the range of 500–1000 h for the system having a concentration of silica precursor = 104 mg/cm3 in a reactive solvent. In the case of nonreactive solvents, the porosity development is significantly different and occurs from initial stages of polymerization and lasts until the aggregation stage, in the range of 2–2000 h for similar concentrations. This significant chan...

Some Specific Features in the Applying the Method of Raman Spectroelectrochemistry while Studying Polyaniline Electrosynthesis in Polymeric-Acid Medium

Initial stages of aniline galvanostatic polymerization at platinum electrode in aqueous solutions of poly-2-acrylamido-2-methyl-1-propansulfonic acid and polystyrenesulfonic acid are studied by the method of Raman spectroelectrochemistry. A laser with wavelength of 532 nm excited the Raman scattering. The very presence of intermolecular associates able to luminescence in the polyacid solution (in the case of polystyrenesulfonic acid) was shown to result in the Raman-spectrometer photodetector overload if normal incidence of laser beam at the electrode (the angle 0°) was used. The Raman-spectrometer photodetector overload can be avoided by the varying of the incidence angle over the 0°–20° range, even without using other techniques (leading to a decrease in the reliability of the studied Raman band registration, such as the lowering of the integration time, intensity or energy of the excitation). Comparative study of aniline electropolymerization in the presence of poly-2-acrylamido-2-methyl-1-propansulfonic acid, polystyrenesulfonic acid, and HCl revealed some characteristic bands in the Raman spectra of the polyaniline–polyacid complexes in the Raman frequency range from 2000 to 3000 сm–1; these bands relate to the polyacid’s backbone and are absent in the Raman spectra of the polyaniline–HCl film. The dynamics of changes in the contribution to the integral Raman spectrum of the band of radical-cations (1330–1350 сm–1) characterizing the highly conductive emeraldine form of polyaniline was compared for the polymerization media. In the course of the electrosynthesis it was found, that the accumulation rate of these species in the film decreased in the series poly-2-acrylamido-2-methyl-1-propansulfonic acid > HCl > polystyrenesulfonic acid.

Deactivation Effect Caused by Catalyst-Cocatalyst Pre-contact in Propylene Polymerization with MgCl2-supported Ziegler-Natta Catalyst

Propylene slurry polymerization with a MgCl2-supported Ziegler-Natta catalyst containing internal electron donor was conducted after different durations of pre-contact of the catalyst with triethylaluminum cocatalyst. The number of active centers ([C*]/[Ti]) was determined by quenching the polymerization with 2-thiophenecarbonyl chloride and measuring sulfur content in the polymer. The pre-contact treatment caused selective deactivation of a part of active centers with low stereoselectivity and much lower activity in the initial stage of polymerization as compared with the polymerization run without the pre-contact stage. The active center concentration and polymerization activity decreased with prolonging of the pre-contact stage. The proportion of stereoselective active centers was increased by prolonging the pre-contact stage, so the isotacticity of produced polypropylene was enhanced. Release of active centers through catalyst particle fragmentation was significantly retarded, and the polymerization rate curve changed from decay type to induction type by the precontact treatment. In the induction period both non-stereoselective and stereoselective active centers were released and activated, resulting in gradual reduction of the polymer’s isotacticity in the first 5–10 min of polymerization. Selective deactivation of non-stereoselective active centers also took place in propylene polymerization using the catalyst without pre-contacting with the cocatalyst. In this case, the polymerization rate decayed with time after a short induction period of 2–5 min. Over reduction of the active center precursors with low stereoselectivity by triethylaluminum was considered as the reason for their deactivation during the pre-contact or the polymerization processes.

On the initial stages of lignin polymerization through spin-polarized density functional theory

Spin-polarized Fukui functions in the density functional theory were applied to a set of molecular structures of oligolignols, considering only their closed shell configurations without open-shell models to avoid chemical perturbations. The results describe how the specific atoms from a set of monolignols and some oligolignols, drive the lignin polymerization in its initial stages because of their redistribution of electronic density and simultaneously where their spin-orbit coupling effects are at work. Also indicate that some atoms are activated after the formation of specific dilignols or trilignols, discerning between the continuity of lignification or when its endwise occurs.

Effect of alkylaluminium on the regio- and stereoselectivity in copolymerization of isoprene and butadiene using TiCl4/MgCl2 type Ziegler-Natta catalyst

Alkylaluminums as one of the most important components of the heterogeneous MgCl2-supported (TiCl4/MgCl2) type Ziegler-Natta catalysts, play significant roles in yielding active species with varied regio- and stereoselectivity. Studying the contribution of alkylaluminums on the conjugated diene isoprene (Ip) and butadiene (Bd) copolymerizations with TiCl4/MgCl2 - alkylaluminum catalysts system is little reported. In this work, effects of various alkylaluminums including triethylaluminium (TEA), triisobutylaluminium (TiBA), diisobutylaluminium hydride (DiBAH) and diethylaluminum chloride (DEAC) on the copolymerization behaviors of Ip and Bd using the TiCl4/MgCl2 - alkylaluminum catalysts system, such as catalytic activity, active center numbers, butadiene incorporation, trans-1,4 stereoselectivity, molecular weight Mw as well as molecular weight distribution Mw/Mn of copolymers were first investigated. TEA with smaller size ethyl groups showed higher copolymerization activity, higher reducing ability and then yielding more active center numbers ([C*]/[Ti]) when compared with TiBA, DiBAH and DEAC. Microstructure of copolymers gradually transformed from the cis-1,4/trans-1,4 mixed structure in the initial polymerization period to the finally high trans-1,4 structure, indicating a transformation of the active center nature during the early polymerization stage, which can be accelerated effectively through the addition of DEAC into the TiCl4/MgCl2-TEA catalyst system. TEA with smaller size ethyl group showed higher transformation rate from the mixed structures of trans-1,4 and cis-1,4 to highly trans-1,4 structure. The active center numbers of the copolymerization systems increased gradually with increasing time until reaching a stable level. The Mw/Mn of copolymers showed interesting evolution with increasing polymerization time from broad distribution with low to medium molecular weight fractions in the initial polymerization stage to relatively narrow distribution with high molecular weight fractions. The contribution of alkylaluminium compounds on the structure of active species which determining the regio- and stereoselectivity of Ip and Bd copolymerization using heterogeneous TiCl4/MgCl2 type catalysts were discussed in details

Synthesis, chain structures and phase morphologies of trans-1,4-poly(butadiene-co-isoprene) copolymers

High trans-1,4-poly(butadiene-co-isoprene) (TBIR) copolymers from the stereo-selective and chemo-selective copolymerization of butadiene (Bd) and isoprene (Ip) are reported with MgCl2 supported Ziegler-Natta catalyst (TiCl4/MgCl2Al(i-Bu)3). Three major components in the copolymers, separated by stepwise isothermal crystallization fractionation, consist of crystalline trans-1,4-polyisoprene (TPI) blocks and trans-1,4-polybutadiene (TPB) blocks but in varied sequence length. Unique phase morphologies with different crystalline lamellar structures are observed in these TBIR copolymers for the first time. The phase morphologies of TBIR reflecting as the discrete lamellar stacks of TPB blocks and dendrimer-shaped lamellae of TPI blocks become diverse in different components with the varied statistical monomer and comonomer unit sequence distribution. The copolymers with TPB blocks are produced in the initial polymerization stage from the active species with relatively low stereospecificity. With the stereospecificity transformation of the active species from low to high, TBIR copolymers with crystalline TPB and TPI blocks and copolymers with TPI blocks are successively formed.

Variation in the Conductivity of Polyaniline Nanotubes During Their Formation

It is shown that the method for the growth of conducting polyaniline nanotubes, based on the direct polymerization of aniline on the surface of channels in track membranes, can be used to produce nanotubes with a given conductivity. An island-type film with a channel resistance of ~1019 Ω is formed during the initial stage of polymerization (up to 2 min). As the polymerization duration increases to 3 min, the channel resistance falls by more than 10 orders of magnitude. This is attributed to the formation of a continuous film on the channel surface, i.e., a nanotube is formed. With the polymerization duration increasing further, the channel (nanotube) resistance gradually decreases to ~1019 Ω at 10 min. The conductivity of polyaniline during the formation of a hollow nanotube is estimated to be 0.01–0.04 S/cm. If the nanotube is completely filled with polyaniline, the conductivity increases to ~0.2 S/cm.