Chain-length dependent termination in pulsed-laser polymerization, 1. Interaction with propagation

Using a new simulation procedure in which each individual propagation step is subjected to a Poisson process it was proved that in case of chain-length dependent termination the apparent rate of propagation no longer coincides with the true one. This is caused by the polydispersity of the chain-length distribution of the growing chains: shorter chains are removed preferentially. This effect is comparatively small although significant. The consequences for the determination of the rate constant of chain propagation k_p are nearly negligible.     

Structural and kinetic continuum in the butadiene polymerization with titanium catalysts

The short-time polymerization of butadiene induced by heterogeneous titanium catalysts was studied for the first time. In the 0.1?0.3 s time interval, polymerization is characterized by a considerable decrease in the chain propagation rate constant and 1,4-stereospecificity of the catalysts. A structure kinetic continuum model was proposed in which the initiation step and the first propagation steps are kinetically continuous. The violation of this continuity changes the catalyst stereospecificity and creates conditions for chain transfer reactions.     

Polymerization of vinylcyclopropanes. IV. Radical copolymerization of 1,1-dichloro-2-vinylcyclopropane with maleic anhydride

Radical copolymerization between 1,1-dichloro-2-vinylcyclopropane (M₁) and maleic anhydride (M₂) was studied. Rearrangement of the radical caused from monomer M₁ and cyclization of growing chain, which was suggested from a consideration of composition and structure of the obtained copolymer, complicate the propagation step in this system. A peculiar copolymer composition equation containing four reactivity ratio parameters was developed, and these parameters were determined.     

Synthesis of aromatic polyethers by Scholl reaction. VII. Oxidative polymerization of 2,2-bis[4-(1-naphthoxy)phenyl]propane and 2,2-bis [4-(1-naphthyl)phenyl]propane

The synthesis and the mechanism of oxidative polymerization of 2,2-bis[4-(1-naphthoxy)phenyl]propane ( 4 ) and 2,2-bis[4-(1-naphthyl)phenyl]propane ( 9 ) are presented. Both monomers polymerize by two different propagation steps. The first one represents a cation-radical dimerization of the naphthyl groups to dinaphthyl structure. H⁺[FeCl₄]^? generated from the first propagation step initiates a transalkylation reaction which provides structural units containing isopropylidenic groups inserted between phenyl and naphthyl, and between two naphthyl groups, respectively. Since the phenyl groups resulted from the second propagation reaction are unreactive in both the oxidative coupling and the transalkylation steps this polymerization reaction leads to polymers with low molecular weights containing phenyl chain ends.     

A combined density functional theory and molecular mechanics (QM/MM) study of single site ethylene polymerization catalyzed by [Cp{NC(tBu)2}TiR+] in the presence of the counterion,CH3B(C6F5)3−

A density functional study was conducted on the approach and insertion of ethylene monomer into the Ti-C_α bond of the catalyst system, CpNC(^tBu)₂RTi-μ-Me-B(C₆F₅)₃ (R= methyl, propyl). A validated QM/MM model was used to represent the counterion. Solvent effects were incorporated with single point solvent calculations done with cyclohexane (ϵ = 2.023) as the solvent. For R=Me (the initiation step), approach and insertion of the ethylene was found to be endothermic, with the barrier for insertion being 12.7 kcal/mol for the most favourable case. For R=Pr (the propagation step), the insertion barrier was found to decrease slightly (11.5 kcal/mol for the most favorable case), corroborating experimental evidence of decrease in insertion barrier with increase in chain length. Termination by chain transfer to monomer was also considered, and found to be unfavourable, in comparison to insertion, by 8.6 kcal/mol for the propagation step. Solvent effects were found to be significant for the propagation step, changing the rate determining step from insertion to uptake for the most favorable case of insertion.     

Quantum-chemical investigation of the mechanism of ionic polymerization in crystals

The results of quantum-chemical investigations of radiation-induced polymerization in molecular crystals are presented. The detailed calculations of the potentials energy curves characterizing the chain generation and propagation reactions, with explicit account of the reacting system interaction with the crystalline environment, provide a reasonable interpretation of the experimental observations. The basic conclusion is that the addition of the growing polymer ion to the monomer molecule placed at a lattice point needs no activation energy. The polymer chain is spontaneously moving in the interlayer space of the crystalline lattice at every propagation step, the overall displacement reaching a macroscopic value. Several termination mechanisms are briefly discussed. The possibility of molecular tunneling cannot be completely eliminated, however, it is expected to occur at the nonchemical preliminary stage of the reaction.     

Absolute propagation constants in vinyl polymerization

The determination of the individual rate constants in a reaction involving more than a single step is part of the basic knowledge required to understand the process itself. The history of the chain mechanism of vinyl polymerization is presented briefly. The techniques needed to measure the chain propagation step are discussed for the three basic mechanisms: free-radical, cationic, and anionic polymerization. Illustrative examples of the rate constants obtained are given, with stress placed on the monomers styrene and methyl methacrylate, which have the advantage of being able to be polymerized by all three or two of the mechanisms, respectively. This allows a comparison of propagation constants between mechanisms. Some factors influencing the magnitudes of the constants are mentioned, and some problems involved in specific cases are discussed. © 1999 Government of Canada. Exclusive worldwide publication rights in the article have been transferred to John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4467–4477, 1999     

The Anionic Polymerization of Trimethylvinylsilane

Trimethylvinylsilane was found to polymerize anionically in cyclo-hexane much like styrene or dienes, except for the occurrence of a termination step. The propagation and initiation reactions were found to be first order with respect to trimethylvinylsilane and one-half order with respect to the initiator and chain ends initially, but first order after the initiator was consumed. Rate constants for initiation, propagation, and termination were found to increase in the presence of ethers, with the termination rate constant increasing the fastest. In the absence of ethers, the propagation rate constant is small, 1.4 mL^1/2/(mol^1/2 min) at 22°C.     

The influence of short‐chain branching on the morphology and structure of polyethylene single crystals

The influence of short‐chain branching on the formation of single crystals at constant supercooling is systematically studied in a series of metallocene catalyzed high‐molecular‐weight polyethylene samples. A strong effect of short‐chain branching on the morphology and structure of single crystals is reported. An increase of the axial ratio with short‐chain branching content, together with a characteristic curvature of the (110) crystal faces are observed. To the best of our knowledge, this is the first time that this observation is reported in high‐molecular‐weight polyethylene. The curvature can be explained by a continuous increase in the step initiation—step propagation rates ratio with short‐chain branching, that is, nucleation events are favored against stem propagation by the presence of chain defects. Micro‐diffraction and WAXS results clearly indicate that all samples crystallize in the orthorhombic form. An increase of the unit cell parameter a₀ is detected, an effect that is more pronounced than in the case of single crystals with ethyl and propyl branches. The changes observed are compatible with an expanded lattice due to the presence of branches at the surface folding. A decrease in crystal thickness with branching content is observed as determined from shadow measurements by TEM. The results are in agreement with additional SAXS results performed in single crystal mats and with indirect calorimetry measurements. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1751–1762     

3/2-order chain kinetics involving a postulated dicationic intermediate in the isomerization of a p-isocyano to a p-cyano azaphosphatrane monocation

Kinetic evidence suggests the possibility of a dicationic intermediate in the title reaction. Thus the linkage isomerization reaction, PNC+ = PCN+, is described by the rate law, nu = 3/2k[PNC+]3/2, which can be interpreted by a chain mechanism with the propagation reaction PNC+ + P2+ --> P2+ + PCN+. Such propagation is unusual in that the intermediate regenerates itself in this single step rather than forming a different intermediate for a second propagation step. Cyanide ions inhibit the rate because they participate in the termination step, P2+ + CN- --> PCN+. The rate constant in CD3CN at 100 degrees C is 3/2k = 7.2 +/- 0.6 x 10-5 L1/2 mol-1/2 s-1; 3/2k represents the composite (kinit/kterm)1/2 kprop. When the reaction is carried out in the presence of PBr+, however, the reaction becomes much faster and is described by the rate law, nu = kBr[PBr+][PNC+]; because [PBr+] remains at constant concentration, the time-course experiments follow first-order kinetics.     

Carbenoid chain reactions: substitutions by organolithium compounds at unactivated 1-chloro-1-alkenes

The deceptively simple "cross-coupling" reactions Alk(2)C=CA-Cl + RLi --> Alk(2)C=CA-R + LiCl (A = H, D, or Cl) occur via an alkylidenecarbenoid chain mechanism in three steps without a transition metal catalyst. In the initiating step 1, the sterically shielded 2-(chloromethylidene)-1,1,3,3-tetramethylindans 2a-c (Alk(2)C=CA-Cl) generate a Cl,Li-alkylidenecarbenoid (Alk(2)C=CLi-Cl, 6) through the transfer of atom A to RLi (methyllithium, n-butyllithium, or aryllithium). The chain cycle consists of the following two steps: (i) A fast vinylic substitution reaction of these RLi at carbenoid 6 (step 2) with formation of the chain carrier Alk(2)C=CLi-R (8), and (ii) a rate-limiting transfer of atom A (step 3) from reagent 2 to the chain carrier 8 with formation of the product Alk(2)C=CA-R (4) and with regeneration of carbenoid 6. This chain propagation step 3 was sufficiently slow to allow steady-state concentrations of Alk(2)C=CLi-Aryl to be observed (by NMR) with RLi = C6H5Li (in Et2O) and with 4-(Me3Si)C6H4Li (in t-BuOMe), whereas these chain processes were much faster in THF solution. PhC[triple bond]CLi cannot perform step 1, but its carbenoid chain processes with reagents 2a and 2c may be started with MeLi, whereafter LiC[triple bond]CPh reacts faster than MeLi in the product-determining step 2 to generate the chain carrier Alk(2)C=CLi-C[triple bond]CPh (8g), which completes its chain cycle through the slower step 3. The sterically congested products were formed with surprising ease even with RLi as bulky as 2,6-dimethylphenyllithium and 2,4,6-tri-tert-butylphenyllithium.     

再活化链式聚合

基于传统的链式聚合和逐步聚合二种高分子链增长过程,提出了再活化链式聚合。按此聚合机理,高分子的链增长是通过将一个非活性或睡眠状态的链(Mm)重新活化为活性种(Mm*),活性种再和一个单体(M)反应,生成一个较大分子量的休眠产物(Mm 1)来实现的。再活化链式聚合主要例子包括苯胺和或许其它芳香族单体的氧化聚合,活性自由基聚合,以及核酸和蛋白质合成中的生物聚合。     

Co-oligomerization of ethylene and higher linear alpha olefins. II. Olefin reactivities in various reaction steps of co-oligomerization with nickel ylide-based system

Part I described co-oligomerization reactions of ethylene and various linear α-olefins (propylene, 1-butene, 1-hexene, 1-heptene, 1-octene, and 1-decene) in the presence of the homogeneous catalyst consisting of sulfonated nickel ylide and diethylaluminum ethoxide. The present article analyzes olefin reactivities in various reaction steps of the co-oligomerization reactions as well as reactivities of various catalytic species. Chain propagation reactions (insertion into the Ni? C bonds) with participation of α-olefins exhibit poor regioselectivity, primary insertion being ca. 60% more probable than the secondary insertion. Ethylene is significantly more reactive in chain propagation reactions: 50–70 times compared to olefin primary insertion and 100–120 times compared to olefin secondary insertion. Reactivities of α-olefins in chain propagation reactions decrease slightly for higher olefin alkyl groups. Reactivities of Ni? C bonds in chain propagation and chain termination reactions strongly depend on the structure of the alkyl group attached to the nickel atom. The Ni? CHR? CH₂? R bond has very low reactivity in ethylene insertion reaction and usually decomposes in the α-hydrogen elimination process. Kinetic analysis of olefin co-oligomerization reactions provides numerous analogies with olefin copolymerization reactions in the presence of Ziegler–Natta catalysts.     

Kinetics of Ziegler propylene polymerization

Rate studies were done on the polymerization of propylene with the TiCl₃–diethyl aluminum chloride catalyst system. The polymerization is initially first-order with respect to propylene concentration. There is a rapid rate decline in the initial period, during which time the reaction becomes functionally second-order. A physical explanation for this behavior has been adapted from the Avrami equation for crystal growth kinetics. A yield equation was developed which fits experimental data closely. Rate correlations show that the initial rate is exponentially related to the TiCl₃/alkyl ratio. Water and other active hydrogen compounds reduce rate; hydrogen increases rate. A “bimetallic” mechanism is proposed which views catalyst activation as consisting of three equilibria, followed by a propagation step where an alkyl group is transferred to the growing chain, and a realkylation of the hydride that remains after the propagation step.     

在稀土体系催化下的丁二烯聚合动力学

本文应用键谷勤聚合动力学公式进行动力学研究。求出了活性中心数目,有关的动力学常数和聚合速度方程(R_p=k_p[C~*][M])并着重研究了聚合物活性链性质。 在链引发阶段,发现20℃和50℃聚合,属于迅速引发类型,在0℃时则是缓慢引发类型。聚合过程中,存在明显的链转移反应。Al(i-C_4H_9)_3是主要的链转移剂。在50℃聚合时,有活性中心失活现象发生,并表明其为双基终止。     

γ-Radiation-initiated post-polymerization of methacrylic acid in the solid state

The solid-state postpolymerization of slowly crystallized methacrylic acid was studied at 0°C with ⁶⁰Co γ-radiation as the initiator. The yield, molecular weight, molecular weight distribution, and stereosequencing of the polymer product were determined as a function of polymerization time. The narrow molecular weight distribution and the linear dependence of molecular weight on polymer yield were attributed to a polymerization mechanism characterized by both independent chain propagation and essentially no termination step. The overall polymerization rate was substantially faster than that reported previously for shock-crystallized monomer, a result which was attributed to termination by the occlusion of propagating radicals at defects in the shock-crystallized monomer. Although largely atactic, the polymer synthesized in the solid state contained a secondary kind of stereosequencing; the meso triad probability was highest at the end of the chain, where propagation had initiated and decreased continuously with chain growth. The gradient in stereosequencing along the chains was attributed to defects that were introduced into the monomer crystals by the growing polymer chains.     

Electron Transfer Chain Catalysis

Electron-transfer chain (ETC) catalysis belongs to the family of chain reactions where the electron is the catalyst. The ETC mechanism could be initiated by chemical activation, electrochemistry, or photolysis. If this pathway is applied to the preparation of organometallic complexes, it utilizes the greatly enhanced reactivity of organometallic 17e and 19e radicals. The chemical propagation is followed by the cross electron-transfer while the electron-transfer step is also followed by the chemical propagation, creating a loop in which reactants are facilely transformed into products. Interestingly the overall reaction is without any net redox change.     

Anionic polymerization initiated by lithium diethylamide in organic solvents. II. Investigation of the polymerization of isoprene

The polymerization of isoprene, initiated by lithium diethylamide has been investigated in the presence of a number of additives. Kinetic results are interpreted on the basis of simultaneous initiation and propagation reactions. The effect of additives, particularly diethyl either, has a profound effect on both the rate of initiation and propagation. The active centers are believed to be ion-pairs with the lithium counterions solvated by both ether and monomer molecules, and the actual propagation reaction is believed to involve a rearrangement of the monomer, complexed to the lithium, and the growing polymeric chain.     

Studies in ring-opening polymerization. II. Cyclohexane spiro-5-1,3,2-dioxathiolan-4-one 2-oxide

The effect of spiro cyclohexane substitution on the polymerizability of the 1,3,2-dioxathiolan-4-one-2-oxide ring in various solvents has been examined. The steric hindrance of the cyclohexane ring inhibits the bimolecular chain propagation reaction which involves direct attack by a terminal hydroxyl group on the ring and which has been shown to occur in simpler dioxathiolan systems. The conjoined cyclohexane ring does not, however, markedly affect the “thermal” polymerization which occurs in nonhydroxylic solvents and in which chain propagation is thought to involve a reactive α-lactone intermediate. The rate-determining step in the sequence of reaction leading to polymer formation is a ring-scission process in which sulfur dioxide is evolved and the α-lactone intermediate formed. The values of the activation energy (25–30 kcal/mole) and frequency factor (10¹¹–10¹³sec^?1) associated with this reaction are, therefore, those which govern the the overall polymerization, since the subsequent steps are sensibly instantaneous. In the presence of adventitious traces of water the resultant polymer, poly(1-hydroxycyclohexanecarboxylic acid) has one carboxyl and one hydroxyl endgroup per chain. Polymers having M?_n ～ 15,000 are readily obtained; these are amorphous materials, in contrast to the analogous poly-β-ester and dialkyl-substituted poly-α-esters which are crystalline. At temperatures in excess of 120°C a competitive first-order fragmentation reaction leading to the formation of cyclohexanone, carbon monoxide, and sulfur dioxide was observed. Kinetic studies demonstrated that this reaction, which is characterized by an activation energy of ～40 kcal/mole is unimportant, in the sense that it does not interfere with polymer formation at temperatures below 100°C.     

Coupling vs surface-etching reactions of alkyl halides on GaAs(100). 2. CH2I2 reactions

Surface reactions of CH2I2 on gallium-rich GaAs(100)-(4 x 1), studied by temperature programmed desorption and X-ray photoelectron spectroscopy (XPS), show CH2I2 adsorbs dissociatively at liquid nitrogen temperatures to form surface chemisorbed CH2(ads) and I(ads) species. Controlled hydrogenation of a fraction of the CH2(ads) species in the chemisorbed layer by the background hydrogen radicals results in a surface layer comprising both CH3(ads) and CH2(ads) species. This hydrogenation step initiates a plethora of further surface reactions involving these two species and I(ads). Thermal activation leads to three sequential methylene insertions (CH2(ads)) into the CH3-surface bond to form three higher alkyl (ethyl (C2), propyl (C3), and butyl (C4)) species, which undergo beta-hydride elimination to evolve the respective higher alkene (ethene, propene, and butene). In competition with beta-hydride elimination, reductive elimination of the ethyl and propyl species with I(ads) occurs to liberate the respective alkyl iodide. Beta-hydride elimination in the alkyls, in the temperature range 420-520 K, is the more dominant pathway, and it is also the rate-limiting step for further chain propagation. The evolution of the alkyl iodides represents the only pathway for the removal of surface iodines in this study and is different from previous investigations where gallium and arsenic iodide etch products (GaI(x), AsI(x) (x = 1-3)) formed instead. The desorption of methane and methyl iodide, formed from surface CH3(ads) species at high temperatures by the reaction between surface methylenes and hydrogens eliminated from the surface C2-C4 alkyls, terminates the chain propagation. We discuss the reaction mechanisms by which the observed reaction products form and postulate reasons for the reaction pathways adopted by the surface species.