Study On The Reaction Mechanisms Of Phenolic Resin And Phenol-Urea-Formaldehyde Co-condensation Resin

Phenolic resin as an important synthetic polymer material,in a century of time,is used in many industries.From the middle of the last century to the last few years,some scholars have studied the mechanism of PF synthesis reaction to understand the mechanism of PF resin structure formation.But the results of the study are obvious differences.This is because the phenolic resin reaction system is more complex.In the phenolic resin synthesis process,the system occurred in a variety of basic reactions,and the final formation of the resin structure is due to the selectivity and competitiveness of the basic reaction.Therefore,to elucidate the mechanism of the formation of phenolic resin resin,it is necessary to explore the influence of reaction conditions on the microscopic process,reaction kinetics and thermodynamics of various elements.In this paper,the synthesis mechanism of phenolic resin and phenol-urea-formaldehyde copolyester was studied by means of quantum chemistry theory.The mechanism of synthesis reaction under different conditions of phenolic resin was elucidated by the combination of theoretical calculation and experiment,and the competition relation between elementary reaction was revealed,and the experimental results were well explained.The research results of the paper further provide theoretical reference for the realization of resin structure control and optimization in experiment,and perfect the theoretical system of resin synthesis.The quantum chemistry method is introduced into the field of phenolic resin synthesis research,and the research method of resin synthesis reaction is further expanded.The main research results and conclusions of this paper are as follows:1.Phenolic resin synthesis reaction system（1）Mechanism of methylation of phenol and formaldehyde under neutral conditions.The reaction mechanism of phenol-formaldehyde system in aqueous solution was theoretically studied.It was found that water molecules can catalyze the proton transfer from carbon to oxygen,and the effect can greatly reduce the energy barrier.The reaction is a concerted mechanism,but nucleophilic attack is ansynchronized with proton transfer processes.C-C bond is preceded by proton transfer,transfer proton from water molecules to the hydroxymethyl prior to extraction of protons from the benzene ring.The reaction barrier of para-position is lower than that of ortho-position.（2）Phenolic resin reaction system under acidic conditionsThe synthesis mechanism of benzyl carbocation and polycondensation reaction under acidic conditions were studied theory combined with experimental results.The results show that there are two paths to the formation of benzyl carbocation,one pathway is the proton of carbon of benzene ring transferred to the oxygen atom of hydroxymethyl group,protonated hydroxymethyl group was formed.After removing water molecule,benzyl carbocation intermediates generated.The reaction energy is higher,and the energy required for the water molecule leaving process is lower,the reaction mechanism is SN1 reaction mechanism;the other pathway is that proton transfer to hydroxymethyl group（WCP）catalyzed by water molecules.The presence of the molecule greatly reduces the energy barrier of the reaction,but The energy barrier of departure of water molecules which is hifher is a rate-determining step.The response mechanism is also SN1.Whether in the formation of bridge or ether bond reaction,the co-condensation reaction energy barrier is very low,some even negative,far less than the methylation of phenol and formaldehyde.So the methylation reaction rate of phenol and formaldehyde is far less than the rate of co-condensation reaction.Under the strong acidic conditions,the ortho-position of phenolic ring are more active,and the cocondensation reaction mainly occurred between the phenolic hydroxyl groups and phenol to increases the molecular chain.Therefore,in the phenol formaldehyde resin,the phenol ring is connected with the phenolic ring.（3）Alkaline reaction conditions of phenolic resin reaction systemThe mechanisms for the base-catalyzed condensation reactions in phenol-formaldehyde resin synthesis were investigated by using the density functional theory method.The structures of the intermediates and transition states,as well as the potential energy barriers of the involved reactions,were obtained.The hypothesis of quinine methide（QM）formation was theoretically confirmed.Two mechanisms were identified for QM formation,namely E1cb（elimination unimolecular conjugate base）and water-aided intra-molecular water elimination.The latter is energetically more favorable and is proposed for the first time in this work.Based on the QM mechanism,the condensation should be a unimolecular reaction because the following condensation between an ionized species（dissociated phenol or hydroxymethylphenol）with QM is much faster.The previously proposed SN2 condensation mechanism was found to be not competitive over the QM mechanism due to a much higher energy barrier.The condensation reaction between neutral phenol or hydroxymethylphenol and QM was also found to be possible.The energy barrier of this reaction is close to or higher than that of QM formation.Therefore,the overall condensation reaction may appear to be bimolecular if such a reaction is incorporated.The theoretical calculations in this work rationalized the discrepant results reported in previous kinetics studies well.（4）Glyceryl triacetate-phenolic resin reaction systemThe curing behavior of glyceryl triacetate on phenolic resin under room temperature and alkaline conditions was investigated by differential scanning calorimetry（DSC）,13 C nuclear magnetic resonance(¹³C-NMR)and electrospray ionization mass spectrometry（ESI-MS）methods.The results showed that:（1）After the addition of glyceryl triacetate,the exothermic peak of phenolic resin was divided into two parts,which showed that the curing of phenolic resin was promoted to a certain degree.The first exothermic peak was between 150-160℃,it was in advance and its exothermic capacity was increased with the increase amount of glyceryl triacetate.The temperature of the second exothermic peak was higher than that of the exothermic peak of the pure phenolic resin.With the increase amount of glyceryl triacetate,the second exothermic peak was gradually in advance.The etherification reaction of phenolic hydroxyl group was proved.（2）The existence of the quinone methides structure was proved,and it was suggested that quinone methide was a reaction product rather than an active intermediate.The possible reaction mechanisms of quinone methide in phenolic resin were speculated;it was pointed out that the glyceryl triacetate was only involved in the process of the quinone methide structures formation instead of involved in the condensation reactions of the phenolic resin.The calculated results show that the energy barrier of the intermediates of methylene quinone is far lower than that of hydroxymethylphenol,and the benzyl ester promotes the formation of methylene quinone intermediates.2.Phenol-urea-formaldehyde co-condensation resin synthesis reaction systemIn this study,the reaction systems of di-hydroxymethylurea（UF₂）with phenol and PUF systems under alkaline,weak and strong acidic conditions were investigated by means of ¹³C-NMR method.The main conclusions include: The self-condensations between urea formaldehyde（UF）or phenol formaldehyde（PF）monomers were observed to be dominant while the co-condensations were very limited under alkaline condition.This is because the intermediates produced from urea,methylolurea and phenol are less reactive in co-condensations with respect to self-condensations.Under weak acidic condition,the self-condensations exclusively occurred among UF monomers and the self-condensation structures of PF were not existed.In addition,co-condensations structures were not observed.The co-condensations were found to be much more competitive under strong acidic condition.The result can be rationalized by the higher reactivity of the carboncation intermediate produced from hydroxyphenol toward the nitrogen atoms in urea or methylolurea than toward the carbon atoms in phenol ring.The theoretical calculation of PUF resin under acidic condition was carried out.The results show that the reaction products of hydroxymethyl urea carbocation with urea is more stable than that of hydroxymethyl urea carbocation with phenol or hydroxymethyl phenol.The results showed that the reactivity of carbocations produced by hydroxylmethyl urea with the carbon of phenolic ring was lower than that of nitrogen atom of urea.When the hydroxymethyl urea carbocation is used as the reaction intermediate to participate in the co-condensation reaction,in the reaction system self-polycondensation of urea or phenol is dominant.The relative energy from reactants to products changes greatly,indicating that the reactants have relatively high relative energy and poor stability.Co –condensation reactions between hydroxylmethyl phenol and urea are more likely to occure,which is consistent with the experimental results.