Studies On The Polymerizayion Mechanism Of Styrene Radical Polymerization

This paper mainly studies the polymerization mechanism of styrene radical polymerization,studies the relationship between the chain initiation rate constant k_i and the chain propagation rate constant k_p,and studies the chain initiation and chain termination mechanism of styrene redox initiation system emulsion polymerization.In the system of styrene-AIBN-TEMPO with TEMPO as the inhibitor,there are several competitive reactions:the primary radicals are terminated by TEMPO,primary radicals initiate（k_i）styrene forming initiating-radical and the initiating-radicals are terminated by TEMPO,chain propagation occurs to a certain extent and the chain radicals are terminated by TEMPO.In order to obtain accurate and reliable analysis results,it is necessary to accumulate the product in the reaction system,so peristaltic pump was used to continuously add TEMPO and initiator into the system to maintain constant TEMPO and initiator concentration in the system.In order to get reassonable results,a series of model compounds（A-A,A-T,A-St₁-T）were prepared and characterized by using nuclear magnetic resonance spectrum（NMR）,high resolution mass spectrum,Fourier infrared spectrum（FT-IR）,and high performance liquid chromatography（HPLC）,including their retention time in HPLC column.For quantitatively study of the products in styrene-AIBN-TEMPO system,the peak area-substance quantitative correlation coefficients of the model compounds in HPLC column were also calibrated.Hydrogen peroxide and hydroperoxides with different substituents of different sizes were combined with ferrous sulfate to form redox initiation systems,which were used to initiate the polymerization of styrene in emulsion.Gas chromatography and size-exclusion chromatography were used to measure the monomer conversion and the molecular weight of the polystyrene.Nuclear magnetic resonance was used to study the structure for understanding of the polymerization mechanism.The results are shown below:1)In the system of styrene-AIBN-TEMPO,at higher TEMPO concentration,the content of A-T is dominating,and the content of A-St-T,especial the content of A-St₂-T is rare.Even though at very low TEMPO concentration,at which polymerization occurred,the content of A-T still accounted for 66.7%,and A-St₁-T and A-St₂-T accounted for 33%and0.23%respectively.Aslo quantitative estimation according to the degree of polymerization k_p/k_i=X_n[Tempo]_y/（[A-ST-T]_x/[A-T]_x）[Tempo]_x=163.All these resulrs prove that chain initiation rate constant k_i is much smaller than the chain propagation rate constant k_p.2)The initiation of the primary radicals directly depends on the size of the substitute,hydroxyl radical shows almost no selectivity between head-addition and tail-addition during initiation(F_hi=47.4%).But for primary radicals with big substitute groups,for example,t-butyl and cumyl hydroperoxides,tail-addition takes advantages over head-addition during initiation(F_hi≈80%).As for the termination mechanism,it mainly depends on the solubility of the peroxide in water,the interfacial area of the particle as well as the diffusion rate of the primary radical in aqueous phase.Primary termination dominates in the polymerization initiated by hydroperoxide with poor solubility in water,for example,F_pt=75–80%for t-butyl and cumyl hydroperoxides.But F_pt is only 18.3%in the polymerization initiated by hydrogen peroxide（H₂O₂）,indicating coupling termination predominately occurred because H₂O₂ has excellent solubility in wate,and the primary radical initiated the polymerization befire entering the micelle,the termination occurred between the propgating chain radical and the entering short chain radical.