Theoretical Investigation on the Thermal Rearrangement of Chloroalkyl Silanes and Polysiloxanes
|Course||Polymer Chemistry and Physics|
|Keywords||Chloroalkylsilane Chloroalkyl-polysiloxane Thermal Degradation Reaction Mechanism Density Functional Theory|
Chloroalkyl-functionlized organosilicon compounds and organic silicon polymers are of great value in applications. As important reactive intermediates, in addition to their excellent performance of organosilicon compound, they have high reactive activity and unique physical and chemical properties owing to the introduction of chloroalkyl group. They can especially improve the adhesion of two kinds of materials with different properties and achieve the elastic coupling so as to improve the mechanical properties, electrical properties and age resistance of the products. They have become a bridge that links organic materials with inorganic materials, and links organic silicon with non-organic silicon materials.However, studies indicate that in many cases the introduction of carbon functional groups will reduce the thermal stability of polysiloxane and affect the service performance of the materials. Thus their applications are limited to some extent. So understanding the degradation law of the polymers and the effects of functional groups on the degradation reaction mechanism has important significance in practice as well as in theory for better utilizing the compounds, improving their service performance and developing superior products.In this paper theoretical studies are first made on the thermal rearrangement reactions of H3SiCH2Cl and H3SiCH2CH2Cl by using ab initio quantum chemical calculations on the basis of density functional theory (DFT) at B3LYP/6-311G(d,p) level. The reaction mechanisms are revealed, kinetic and thermodynamic analyses are made and the effects of several substituents in different positions on the thermal rearrangement reactions are discussed. Then the chloropropyl terminated siloxane oligomer containing three siloxane-chains is used as a model and the thermal degradation reactions of chloropropylpolysiloxane and the effects of methyl and phenyl connected with the terminal carbon atom on the reactions are systematically studied at the levels of B3LYP/6-311++G(3df,3pd)//B3LYP/6-31G(d). The re-degradation of chlorine siloxane produced after the degradation is analyzed at the same levels. Besides the effect of end group on the thermal degradation reaction of polysiloxane, the catalyzers frequently employed in preparing and using carbon functional organic siloxane, and the reaction in which the soft polysiloxane chain under heating forms volatile annular small molecules are all the important factors that reduce the thermal stability of polysiloxane. The thermal rearrangement reactions of chloro-alkyl silane catalyzed by several Lewis acids, the mechanism of the degradation reaction in which the polysiloxane chain forms a ring by itself, the thermodynamics and the dynamics are also analyzed systematically in the paper.The important and valuable results are summarized as follows:1. The study on the thermal rearrangement reactions of a-chloromethylsilanes (H3SiCH2Cl) and the effects of alkyl-substituents on silicon atom or carbon atom indicates:(1). Rearrangement may occur when a-chloromethylsilane is heated. In the rearrangement the chlorine atom migrates from carbon atom to silicon atom, and the hydrogen atom migrates simultaneously from silicon atom to carbon atom through a double three-membered ring transition state in which silicon atom is pentacoordinate and the Si-C bond has part double bond features, forming methylcholosilane. The activation energy for the reaction calculated at B3LYP/6-311G(d,p)level is217.4kJ/mol. The kinetic and thermodynamic calculations indicate that this reaction is entropy reducing, exothermic and spontaneous, and that when the temperature reaches1000K, the reaction will proceed more completely at higher kinetic rates.(2). The methyl group or vinyl group on silicon atom can reduce the activation energy of the thermal rearrangement reaction. The thermal rearrangement activation energy of (CH3)2SiHCH2Cl and CH2=CHSiH2CH2Cl is201.6kJ/mol and208.7kJ/mol, respectively. The substituents do not affect the reaction mechanism, but have remarkable effects on the rate of the rearrangement reaction. When the temperature is above900K, very fast rates and a more complete rearrangement reaction of (CH3)2SiHCH2Cl will appear.(3). The methyl group or vinyl group on carbon atom can reduce the activation energy of the thermal rearrangement reaction. The thermal rearrangement activation energy of H3SiC(CH3)2Cl and H3SiCH(CH=CH2)Cl is158.3kJ/mol and160.7kJ/mol respectively. The presence of substituents does not affect the reaction mechanism but affects the rate of the rearrangement reaction obviously. When the temperature is above600K, the thermal rearrangement reaction of H3SiC(CH3)2Cl will proceed more completely at very fast rates with the sequence of H3SiC(CH3)2Cl>H3SiCH(CH=CH2)Cl>(CH3)2SiHCH2Cl>(CH2=CH)SiH2CH2Cl> H3SiCH2Cl.2. Systematic study on the thermal rearrangement of β-chloroethylicsilane (H3SiCH2CH2Cl) and the effects of different halogens shows:(1). Thermal rearrangement will occur when β-chloromethylsilane is heated. In the rearrangement, the chlorine group migrates from carbon atom to silicon atom, making Cp-Cl and Si-Ca bonds break and simultaneously forming a four-centered (Si-Ca-Cp-Cl) transition-state, and the molecular CH2=CH2and H3SiCI are produced. The activation energy is only177.0kJ/mol. The reason that rearrangement occurs is that at the degradation temperatures the entropy of volatile substance is higher than that of linear molecule and it is easy for the transition-state to form. Thermodynamic and kinetic calculations suggest that the reaction is entropy increasing, exothermic, with strong spontaneous tendency and great reaction extend. The reaction may proceed spontaneously at a low temperature and at a slow rate. When the temperature is above800K, the reaction will proceed at higher kinetic rates and proceed completely. Due to β effect, the rate constant of β-chloromethylsilane at800K is two or three orders of magnitude higher than that of α-chloromethylsilanes.(2). Thermal rearrangements will also occur when different halogen substituted H3SiCH2CH2F and H3SiCH2CH2Br are heated. The activation energy of β-fluoroethylicsilane and β-bromoethylicsilane is138.1and176.9kJ/mol, respectively. In the β-halogenethyl-silane series, rearrangement reaction of β-fluoroethylicsilane occurs most easily, and the reaction will proceed at higher kinetic rates and proceed more completely when the temperature reaches600K. The reaction equilibrium constant and rate constants of β-fluoroethylicsilane are two or three orders of magnitude higher than that of β-chloroethylicsilane.3. The results of the systematic theoretical studies on the thermal degradation reactions of y-chloropropyl polysioxanes (H3SiOSiH2OSiH2CH2CH2CH2Cl) and the effects of methyl or phenyl connected with the end carbon atom on the reactions show:(1). At least four possible thermal rearrangement reactions will occur when H3SiOSiH2OSiH2CH2CH2CH2Cl is heated, i.e.[1,5] backbiting reaction,[1,7] backbiting reaction,[1,9] backbiting reaction and H[4,6] C-O migration reaction. (a). The [1,5] backbiting reaction:Chlorine attacked the silicon atom three atoms apart from it, via a transition state with a three-membered ring and a linear siloxane chain structure, resulting in producing cyclopropane and chloride-terminated polysiloxane. The activation barrier of this reaction was277.0kJ/mol at B3LYP/6-311++G(3df,3pd)//B3LYP/6-31G(d) levels,(b). The [1,7] backbiting reaction:Chlorine attacked the silicon atom five atoms apart from it, via a transition state with a five-membered ring and a linear siloxane chain structure, producing a five-membered ring degradation product containing silicon oxygen carbon atom and chloride-terminated polysiloxane. The activation barrier of this reaction was259.1kJ/mol at B3LYP/6-311++G(3df,3pd)/B3LYP/6-31G(d) level,(c). The [1,9] backbiting reaction:Chlorine attacked the silicon atom seven atoms apart from it, via a transition state with a seven-membered ring and a linear chlorine siloxane structure, producing a seven-membered ring degradation product containing silicon oxygen carbon atom and chloride-terminated polysiloxane. The activation barrier of this reaction was280.4kJ/mol at B3LYP/6-31l++G(3df,3pd)//B3LYP/6-31G(d) levels,(d). H[4,6]C-O migration reaction:The hydrogen atom on the carbon atom which is connected with the silicon migrated to the oxygen atom in the end of polysiloxanes chain, producing hydroxy-terminated polysiloxane and chloroalkylsilane containing Si=C double bond. The activation barrier for this reaction was304.6kJ/mol at B3LYP/6-311++G(3df,3pd)//B3LYP/6-31G(d) levels.(2). Within the studied temperature scope (400-1500K), only [1,5] backbiting reaction was spontaneous and its equilibrium constant and rate constant were much greater than that of the other three reactions. So the thermal rearrangement reaction of y-chloropropyl polysioxanes H3SiOSiH2OSiH2CH2CH2CHCl was mainly [1,5] backbiting reaction, with the products of cyclopropane and chloride-terminated polysiloxane.(3). The presence of methyl group or phenyl group in the end of carbon atom reduced the activation energy of the reaction, thus increasing the reaction rate.4. The investigation on the thermal degradation reactions of chloride-terminated polysiloxane produced after degradation and the synergistic effect of phenyl connected with the cyclic degradation product on the degradation reactions indicates:(1). When the degradation product chloride-terminated polysiloxane was heated, the main reaction was the backbiting reaction in which the chlorine group migrated to the silicon eight atoms apart from it, via a transition state with a eight-membered ring structure, resulting in producing cyclobutasiloxane. The activation barrier of this reaction is161.5kJ/mol at B3LYP/6-311++G(3df,3pd)//B3LYP/6-31G(d) levels. Afterwards the degradation reaction continued in the unbutton mode.(2). The phenyl group connected to the cyclic degradation product accelerated the reaction5. Theoretical studies on the effects of Lewis acids AlCl3and BF3on the thermal rearrangements of chloromethylsilane indicates:AlCl3and BF3will not affect the product of the reaction, whereas with the chlorine group with negative charge, BF3can form complex in an umbrella structure and AlCl3can form complex in an approximate tetrahedron structure, greatly reducing the activation energy of the reaction and accelerating the formation of methylcholosilane. The activation energy of BF3-1and AICl3-1catalytic reactions is155.7and99.5kJ/mol, respectively,61.7and117.9kJ/mol lower respectively compared with the reactions without catalytic agents. The kinetic and thermodynamic calculations indicate that the reactions are spontaneous, and AlCl3and BF3have remarkable effects on kinetic rate constants, especially AlCl3.6. Theoretical studies on the degradation reaction in which the polysiloxane chain forms a ring indicate that pyrolysis products with a six-membered ring or an eight-membered ring will form when polysiloxane with a soft chain is heated.The activation barrier for the reactions is117.1and147.0kJ/mol, respectively at B3LYP/6-311++G(3df,3pd)//B3LYP/6-31G(d) levels. Within the studied temperature scope (400-1500K), both reactions are endoergic and entropy increasing. The dynamic and thermodynamic analyses show that the reaction in which the eight-membered ring forms is the main thermal degradation reaction, with its rate constants one order of magnitude higher and its equilibrium constants two or three orders of magnitude higher than that of the reaction in which the six-membered ring forms.