Kinetics of the Polymerization of Permethylcyclosiloxanes Initiated by Tetrakis(pentafluorophenyl)borate Protic Complex

The kinetics of ring opening polymerization of hexamethylcyclotrisiloxane, D₃, and octamethylcyclotetrasiloxane, D₄, in toluene initiated by protic borate complex H⁺ (H₂O)₃B(C₆F₅)₄ have been studied. The reaction is first order in monomer and first order in initiator. Rate constants and activation parameters were determined. The formation of cyclic oligomers was followed. The kinetics of the formation of decamethylcyclopentasiloxane, D₅, during the polymerization of D₄ was in agreement with a back-biting mechanism. D_3n cyclics (n = 2, 3,...) are exclusively formed during D₃ polymerization by intramolecular reactions between the chain ends. The amount of generated D₆ increases linearly with the monomer conversion in the range of the D₃ conversion between 0 and 60% and is almost independent of temperature. Kinetic results are interpreted in terms of a mechanism involving cyclic tertiary trisilyloxonium ion transitory intermediates.     

Influence of rare earth compounds on the anionic ring-opening polymerization of cyclosiloxanes

The influence of rare earth compounds on the polymerization of cyclosiloxanes has been studied for hexamethylcyclotrisiloxane (D₃) and octamethylcyclotetrasiloxane (D₄). In both cases, it has been shown that rare earth compounds alone do not lead to well defined polymers: yields are never quantitative and the formation of cyclic side-products is always observed. The influence of these compounds has also been studied in the anionic polymerization of the same cyclosiloxanes initiated by organolithium reagents or fluorides. In the case of D₃, it has been shown that back-biting reactions can be slowed down in the presence of rare earth compounds whereas for D₄, both polymerization and back-biting reactions get slower. Received: 22 May 1997/Revised: 8 September 1997/Accepted: 8 September 1997     

Cationic ring opening polymerization of octamethylcyclotetrasiloxane initiated by solid superacid

Kinetics of the cationic ring opening polymerization of octamethylcyclotetrasiloxane (D₄) in bulk initiated by solid superacid was studied. The optimalizing experimental conditions were based on preliminary experiments. Higher temperature was beneficial to equilibrium conversion while stirring intensity beyond a certain level displayed no obvious effect on the rate of the reaction. A kinetic model was elicited and kinetic parameters were obtained through optimization. The calculated activation energies were 32.6 and 34.1 kJ mol^–1 for D4 and all other annuluses (D_x), respectively.     

Kinetic study on the ring-opening polymerization of octamethylcyclotetrasiloxane (D4) in miniemulsion

A kinetic study on the ring-opening polymerization (ROP) of octamethylcyclotetrasiloxane (D4) in miniemulsion is presented in this work. The polymerization is initiated by dodecylbenzenesulfonic acid (DBSA) which also serves as the surfactant (inisurf). The influence of the size of monomer droplet and the concentration of DBSA on the polymerization rate were studied. Since the main place for the ROP of D4 was confirmed to be at the oil/water interface, a three-layer interface model was proposed to analyze the distribution of molecules at the interface and the effects of DBSA. A kinetic equation is then developed based on this model. In the equation, the polymerization rate (R_p) is related to the initial monomer concentration ([D4]₀), the droplet radius (r), the coverage of DBSA on monomer droplets surface (x). The polymerization rate can be predicted from the kinetic equation feeding all parameters with their experimental values. Finally, the equation gave a good accordance between the predicted polymerization rate data and the experimental results under different polymerization conditions.     

The kinetics of hydrolytic polymerization of ε-caprolactam. V. Equilibrium data on cyclic oligomers

The concentrations of the cyclic oligomers (C_i; i = 3, 4, 5, and 6) in the polymeric products of ε-caprolactam were determined by high-performance liquid chromatography. The equilibrium data on the oligomers were obtained as a function of the polymerization temperature and initial water concentration. The concentration of each oligomer in the equilibrated polymer was found to increase with the temperature and/or initial water concentration. A set of the kinetic equations to express the oligomer formation during the polymerization was also proposed.     

Kinetics of the anionic ring‐opening polymerization of octamethylcyclotetrasiloxane initiated by potassium isopropoxide

Kinetics of the anionic ring‐opening polymerization of octamethylcyclotetrasiloxane (D₄) in bulk initiated by potassium isopropoxide was studied. Several promoters including N‐methyl‐2‐pyrrolidinone (NMP), N,N‐dimethylformamide (DMF), and diglyme were used. It is indicated that the reactions are first‐order in D₄ during the initial stage of polymerization. The polymerization rate of D₄ is influenced by a number of factors, such as the nature of promoters, the molar ratio of promoter to initiator (C_p/C_i ratio), and the reaction temperatures. With the use of NMP as promoter, the polymerization rate constant at 30°C is 10.482 h^?1 with the C_p/C_i ratio equal to 3.0. As the C_p/C_i ratio increases, the polymerization rate constant increases sharply and the cyclic oligomers content in polymer decreases evidently. The back‐biting reaction that leads to the formation of decamethylcyclopentasiloxane (D₅) occurred in the polymerization of D₄. The rate of the D₅ formation relatively to the rate of D₄ conversion increases with the conversion of D₄. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3510–3516, 2006     

Poly(2,2-dimethyltrimethylene carbonate)-block-poly(dimethylsiloxane)-block-poly(2,2-dimethyltrimethylene carbonate). Synthesis and thermal properties

Lithium salts of hydroxytelechelic poly(dimethylsiloxane) [poly-(DMS)] were used as initiators for the anionic ring-opening polymerization of 2,2-dimethyltrimethylene carbonate (DTC). Triblock copolymers were obtained in high yields. Complexation of the active site in DTC polymerization by the poly(DMS) chain leads to a decrease in polymerization rate. The thermal properties of the copolymers of different compositions were determined. Catalysed thermal degradation of the block copolymers leads to cyclic oligomers D_n and DTC upon ring-closing depolymerization.     

Kinetic study of radical polymerization. III. Solution polymerization of acrylamide by 1H‐NMR

Solution and radical polymerization of acrylamide in the presence of potassium persulfate in D₂O was investigated up to high conversion by high‐field ¹H‐NMR spectroscopy. The kinetics of reaction was studied according to the data obtained from the corresponding spectra at various times during the polymerization reaction progress. Processing of the data led us to derive the rate equation of this polymerization reaction and determine the reaction order of each component in the rate equation. The order, with respect to initiator, was consistent with the classical kinetic rate equation (0.45), whereas the order with respect to monomer was greater than unity (1.49). The effect of temperature on the polymerization rate was also investigated and the activation energy of 48.4 kJ mol^?1 was obtained over the temperature range of 60–75°C. Also some mechanistic studies were discussed. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2007–2013, 2004     

A Novel Method to Model Emulsion Polymerization Kinetics: The Explicit Radical-Particle Size Distribution Approach

Abstract:

An alternative approach to model emulsion polymerization is presented that is capable of rigorously solving both particle and radical kinetics for emulsion polymerization: the explicit radical-particle size distribution approach. The method is based on a direct solution of all population balances and fully covers the strong influence of compartmentalization on rates of reactions between macroradicals and, consequently, on chain length averages. An essential and new feature is the compartmentalization factor (D_f), which accounts for compartmentalization in a transparent manner. The generic approach allows for studying the complete emulsion polymerization conversion range, including gel-effect, and the effect of various parameters on both chain length and particle size distribution. Well-known kinetic regimes for emulsion polymerization naturally arise as limiting cases from our model. The dynamic behavior of the model was studied by simulating several realistic seeded emulsion polymerization reactions for styrene. The model dealt with compartmentalization accurately and was able to correctly reproduce the dynamic behavior known to be typical for emulsion polymerization.     

Ring-Opening Polymerization of Siloxanes Using Phosphazene Base Catalysts

Phosphazene bases such as {(NMe₂)₃P=N–)₃P=NBu^t} have been reported in the literature to be strongly basic materials with basicities up to 1×10¹⁸ times stronger than that of diazabicycloundecene (DBU) a strong hindered amine base used in organic reactions. A study of these phosphazene bases as catalysts revealed that they can be activated by small amounts of water, which all silicone feed stocks contain, to form an active ionic base catalyst [(NMe₂)₃P=N–)₃P–NHBu^t]⁺[OH]^–. This paper discusses the use of these types of base catalysts, and their analogues, as ring-opening polymerization catalysts for cyclosiloxanes. Phosphazene base catalysts can be used at low concentrations to make high molecular weight polydimethylsiloxanes with short reaction times over a wide temperature range. Molecular weight can easily be controlled in the presence of suitably functionalized endblockers. Water and carbon dioxide have been shown to have a significant impact on the polymerization rates. Polymers prepared show excellent thermal stability by thermogravimetric analysis (TGA), following neutralization of the catalyst, with decomposition onset temperatures >500°C in some cases. As a result of the extremely low levels of catalyst used, the polymers often do not require filtration.     

The effect of octamethylcyclotetrasiloxane (D4) addition on the structure and properties of film‐forming polyacrylate/silica core–shell composite particles

The film‐forming polyacrylate/silica core–shell nanocomposite particles with octamethylcyclotetrasiloxane (D₄) were successfully synthesized via aqueous emulsion polymerization in the presence of a glycerol‐functionalized nano silica sol. The ring‐opening polymerization of D₄ and the reaction with the glycerol‐functionalized nano silica particles before emulsion polymerization was the key procedure in this process. Transmission electron microscopy results showed that more nano silica particles tended to coat on the polyacrylate particles surface after the nano silica sols were modified with D₄. The silica aggregation efficiency was increased from 90.9 to 98.6% when the amount of D₄ used in the system was varied from 0 to 8.0 wt %. The transparency of the nanocomposite films was not compromised after D₄ was incorporated into the system. The films of the nanocomposite particles with or without D₄ both exhibited superior abrasive resistance. Furthermore, the water resistance and hydrophobicity of the films of these particles with D₄ were also improved significantly. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42003.     

Anionic non‐equilibrium ring‐opening polymerization of octamethylcyclotetrasiloxane (D4) initiated by silazyl‐lithiums

The initiation and propagation reactions in anionic non‐equilibrium polymerization of octamethylcyclotetrasiloxane (D₄) initiated by silazyl‐lithiums are investigated. It is found that: both the steric hindrance effect and the electronic effect of the substitutes on the Si atom Si influences the initiation activity of silazyl‐lithiums; temperature effect on the propagation rate of D₄ increases strongly above 40 °C; the molecular weight distribution of the prepared polysiloxanes widens with time. © 2003 Society of Chemical Industry     

Kinetics of the Anionic Ring Opening Polymerization of Cyclosiloxanes Initiated with a Superbase

Kinetics of the anionic ring opening polymerization of octamethylcyclotetrasiloxane, D₄, and hexamethylcyclotrisiloxane, D₃, in toluene solution initiated by hexapyrrolidinediphosphazenium hydroxide, P₂Pyr₆ ⁺OH^– were studied. Reactions are first order both in monomer and in initiator. The specific rate of the D₃ polymerization is higher than that of D₄ by about 2–3 orders of magnitudes. Activation energies are 18.1 kcal mol^–1 for D₄ and 11 kcal mol^–1 for D₃. The back-biting reaction leading to decamethylcyclopentasiloxane, D₅, was followed in the polymerization of D₄. This reaction is retarded by the presence of monomer. The kinetics is interpreted in terms of a mechanism in which the active propagation center appears mostly as a monomer separated ion pair, which is also the intermediate in the propagation step.     

Mechanism of hexamethylcyclotrisiloxane polymerization in the presence of siloxanediols

Polymerization of hexamethylcyclotrisiloxane (D₃ ) in CH₂Cl₂ at 30°C, initiated by triflic acid (TfOH) was studied in the presence of siloxanediols (HD₂OH or HD_xOH) which polycondense giving water, or in the presence of water alone for comparison. D represents a siloxane unit OSiMe₂. In the second case, the relative amounts of cyclic oligomers (D₆, D₉) and of linear high polymer (HP) vary strongly with the molar ratio r =[H₂O]/[TfOH]. When r increases from 1 to 100 (in homogeneous phase), D₉ /D₆ increases from 0.2 to 2 and D₆ /HP decreases from 1 to 0.3. The large decrease of D₆ amount and the increase in D₉/D₆ are attributed to the suppression of their formation through oxonium ions and to their exclusive formation, for r>100, by cyclization of silanol-esters, which is more rapid for HD₉OTf than for HD₆OTf. For polymerization of D₃ in the presence of HD₂ OH, there is a fast and limited ring-opening of D₃ (about 10% conversion over 6 min) at the beginning, without formation of cyclics (D₆, D₉ , etc). Then reaction with D₃ stops. Polymerization of HD₂OH takes place simultaneously, at the same rate as in the absence of D₃, with slow formation of high polymer. At the end of the polycondensation (after, eg, 4 h) D₃ polymerization starts again, giving D₆ , D₉, etc, and HP. The inhibition period for D ₃ is attributed to the complexation of triflic acid hydrates by silanol groups. These activated silanol groups do not react with D₃. A comparison of D₃ polymerization with the addition of water, HD₂OH or HD₁₅OH leads to the conclusion that when r is not too large, propagation involves silylester end-groups, but that for large r ratios (100 or higher), it probably occurs mainly on silanol groups reacting with an activated monomer. © 1999 Society of Chemical Industry     

Mechanism of octamethylcyclotetrasiloxane polymerization in the presence of siloxanediols

Polycondensation of tetramethyldisiloxanediol (HD ₂OH) by triflic acid (TfOH) in the presence of octamethylcyclotetrasiloxane (D₄) has been studied at 30°C in CH₂Cl₂ solution and in bulk at 80°C. The condensation follows a similar pathway, at a similar rate, in the presence or in the absence of D₄. Larger oligosiloxanediols and D₄ are formed first, and high polymer only after several hours. At 30°C, no polymerization of D₄ occurs, even 24 h after the total conversion of HD₂OH. In bulk at 80°C, polymerization of D₄ takes place when silanol concentration becomes very low at the end of the polycondensation, and is assumed to result from a reaction of D₄ with ester end-groups activated by hydrated acid at this temperature. Indeed, these results show that silanol groups activated by TfOH are not responsible for D₄ polymerization. Polymerization made in the presence of longer-chain oligosiloxanes HD_xOH (x ∼ 15) indicates that the presence of silyltriflates is a prerequisite of the propagation reaction. Polymerization of D₄ alone at 80°C also occurs in the presence of water in situ (0.9 mol litre⁻¹) after addition of pure triflic acid, and water cocatalyses the propagation reaction. However, the initiation reaction is almost completely inhibited when triflic acid is prehydrated.     

Fischer–Tropsch Synthesis: Deuterium Kinetic Isotopic Effect for a 2.5 % Ru/NaY Catalyst

The isotopic effect of D₂ was investigated for a 2.5 % Ru/NaY catalyst for Fischer–Tropsch synthesis at industrially relevant conditions (493 K, 1.92 MPa, H₂/CO = 2). An inverse kinetic isotopic effect was found for CO conversion (r_D/r_H = 1–1.4) and methane formation (1.2–1.4), suggesting that H is involved in the rate determining steps of both reactions. D₂ enhanced methane formation and inhibited C₂₊ formation and therefore decreased chain growth probability. The decline in r_D/r_H with carbon number is due to increasing dependence on the ratio (<1) of the chain growth probability. D₂ also favored paraffin formation over olefins. The apparent activation energy for methane formation was higher with H₂ (116 kJ/mol) than with D₂ (98 kJ/mol). The inverse kinetic isotopic effect arises from a combination of the kinetic isotopic effect of the rate limiting step and the thermodynamic isotopic effect of the equilibrium constant. The activity and product selectivity is consistent with a higher surface D coverage relative to H.     

Emulsifier‐free emulsion polymerization of acrylonitrile: Effect of in situ developed Cu(II)/glycine chelate complex initiated by monopersulfate

The catalytic effect of various Cu(II) salts and Cu(II) chelate complexes of certain amino acids on the emulsion polymerization of acrylonitrile in the absence of added emulsifier was investigated in experiments. The CuSO₄/glycine chelate complex was chosen for a detailed kinetic study of acrylonitrile polymerization. The polymerization was studied at varying concentrations of initiator, monomer, Cu(II), glycine, solvents, and TiO₂ over a temperature range of 30–60°C. The overall activation energy and the viscosity average molecular weight of the polymer were computed. From the kinetic and spectrophotometric studies, the mechanism of KHSO₅ decomposition by the Cu(II)/glycine complex and initiation of polymerization was suggested. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2785–2790, 1999     

A comparative study of tetraisobutyldialuminoxane and methylaluminoxane as cocatalysts for Cp2ZrCl2 catalyzed ethylene polymerization

Summary The effect of [A1]/[Zr] mol ratio and temperature on the cocatalytic effects of tetraisobutyldialuminoxane (TIBDAO) and methylaluminoxane (MAO) for ethylene polymerization using Cp₂ZrCl₂ catalyst were studied. The decay type kinetic profile was observed for both TIBDAO and MAO cocatalyzed ethylene polymerizations. Catalytic activity and rate of polymerization were found to be low for TIBDAO cocatalyzed ethylene polymerization when compared to MAO cocatalyzed ethylene polymerization. The differences in catalytic activity and rate of polymerization for ethylene polymerization catalyzed by Cp₂ZrCl₂-TIBDAO and Cp₂ZrCl₂-MAO were discussed with respect to the structures of MAO and TIBDAO. An active species for Cp₂ZrCl₂-MAO and Cp₂ZrCl₂-TIBDAO catalyzed ethylene polymerizations was also discussed. The polyethylene was characterized by intrinsic viscosity measurements.     

Preparation of poly(vinyl acetate) microspheres with narrow particle size distributions by low temperature suspension polymerization of vinyl acetate

To estimate influences of suspension polymerization conditions including conversion, polymerization temperature, stirring rate, initiator concentration, monomer concentration, and suspending agent concentration on the volume average diameter (D_avg) and particle size distribution (PSD) of poly(vinyl acetate) (PVAc) microspheres, vinyl acetate (VAc) was suspension‐polymerized at low temperature using 2,2′‐azobis(2,4‐dimethylvaleronitrile) as an initiator. The effects of each condition, on D_avg of PVAc microspheres, were expressed as follows, D_avg = [conversion]^a[temperature]^b[rpm]^c[ADMVN]^d[VAc]^f [suspending agent]^g. Logarithms of D_avg were linearly proportional to those of polymerization conditions, and their exponents, a, b, c, d, f, and g were calculated as 0.27, ?13.7, ?1.37, ?0.21, 0.58, and 0.29, respectively. Variations of PSDs, according to polymerization conditions, were examined by considering polymerization rate, droplet or suspension viscosity, and droplet break‐up/coagulation equilibrium. From these results, PVAc microspheres with various sizes and narrow PSDs were obtained effectively under carefully controlled polymerization conditions, which can be used as promising precursors of novel PVA microspheres through heterogeneous surface saponification. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4064–4070, 2006     

New method for evaluation of kinetic parameters and mechanism of degradation from pyrolysis–GC studies: Thermal degradation of polydimethylsiloxanes

Thermal degradation of polydimethylsiloxane (PDMS) polymers having hydroxyl (PS) and vinyl (PS‐V) terminals was studied by pyrolysis‐gas chromatography (PGC) in the temperature range from 550 to 950°C. The degradation products were primarily cyclic oligomers ranging from trimer (D₃) to cyclomer D₁₁ and minor amounts of linear products and methane. The product composition varied significantly with pyrolysis temperature and extent of degradation. A new method was developed to derive a mass loss‐temperature curve (pyrothermogram, PTG) and to determine the kinetic parameters of decomposition (k, n, and E_a) from sequential pyrolysis studies. It was shown that isothermal rate constants can be derived from repeated pyrolysis data. Good agreement between the rate constants derived from the two methods validates the methodology adopted. This was further confirmed from thermogravimetric studies. The E_a values for the decomposition of PS and PS‐V derived from sequential pyrolysis were 40 ± 2 and 46 ± 2 kcal mol⁻¹, respectively. Various mechanisms for the degradation of PDMS were reviewed and discussed in relation to the PGC results. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 441–450, 1999