Homo-and Copolymerizations Of Lactones Using Isothiourea/MgX₂ Dual Catalysis

Herein,three bicyclic isothioureas(ITUs),2,3,5,6-tetrahydroimidazo-[2,1-b]thiazole(ITU 1),2,3,6,7-tetrahydro-5H-thiazolo-[3,2-a]pyrimidine(ITU 2)and3,4,7,8-tetrahydro-2H,6H-pyrimido[2,1-b]-[1,3]thiazine(ITU 3)were prepared in one-step using cheap raw materials.And ITUs coupled with magnesium halide(Mg X₂)as the novel dual catalyst systems catalyzed the ring-opening polymertization(ROP)of lactones.The main work is as follows:(1)The ITUs/Mg X₂ dual catalyst systems were employed for ROP of?-caprolactone(?-CL)in dichloromethane(DCM)when benzyl alcohol(BnOH)was applied as a typical initiator.The effects of different polymerization conditions on the polymerization reactions have been systematically discussed.The results showed that the order of activities for ITUs was ITU 2>ITU 3>ITU 1,and for Mg X₂ was Mg I₂>Mg Br₂>Mg Cl₂.The ITU 2/Mg I₂ dual catalyst system exhibited much better activity and the optimal conditions were as follows:[e-CL]₀/[ITU2]₀/[Mg I₂]₀/[BnOH]₀=100:1:1:1,[e-CL]₀=2.0 M after 6 h at 25°C.Under the optimum conditions,the convision ofe-CL was as high as 99%,and the molecular weight(M_n)and polydispersity indice(PDI)of polycaprolactone(PCL)were 25.5kg/mol and 1.14,respectively.The kinetic and chain extension experiments showed that the ITUs/Mg I₂-cocatalyzed ROP of?-CL proceeded a living polymerization characteristics.The resulting polymers were characterized using GPC,¹H and ¹³C NMR,DSC,TGA and MALDI-To F MS.Finally,¹H and ¹³C NMR indicated that the proposed polymerization mechanism was through a dual catalytic mechanism.In this process,the monomer was activated by the coordination with Mg I₂,and ITU 2activated the alcohol hydroxyl which was attributed to hydrogen bonding.Then,the ring opening of?-CL was achieved through the nucleophilic attack of activated alcohol and generated the polymeric precursor.The polymeric precursor was activated by ITU 2 and attacked the incoming Mg I₂-activated monomers to generate linear polymers.(2)The ROP of?-decalactone(?-DL)was catalyzed by ITU 2/Mg I₂ dual catalyst system in the presence of BnOH initiator in toluene.The effects of different polymerization conditions(including the monomer concentration,reaction time,temperature,and monomer/initiator molar ratio)on the polymerization reactions were investigated in detail.The optimum conditions could be concluded as:[e-DL]₀/[ITU2]₀/[Mg I₂]₀/[BnOH]₀=100:1:1:1,[e-DL]₀=2.0 M,3 h,50°C in toluene.The monomer conversion was nearly complete within 3 h and the M_n and PDI of polydecalactone(PDL)were 28.1 kg/mol and 1.18,respectively.¹H NMR and MALDI-To F MS spectra showed that the polymerization produced linear PDL having predictable M_n,narrow PDIs and defined end groups,revealing the expected control for ROP.The living characteristics of ITU 2/Mg I₂-cocatalyzed polymerization ofe-DL was supported by chain extension experiment.The thermal properties of PDL were measured by DSC and TGA.(3)In the presence of BnOH,the copolymerizations ofe-DL ande-CL were conducted using ITU 2/Mg I₂ dual catalyst system.Firstly,the conditions of random polymerization were optimized and the optimum conditions were described as:[e-DL+?-CL]₀/[ITU 2]₀/[Mg I₂]₀/[BnOH]₀=100:1:1:1,[e-DL+?-CL]₀=2.0 M,50°C,40 min,in toluene.The random copolymers P(DL-co-CL)s with predictable M_n were prepared by adjusting the monomer/initiator molar ratios.The compositions of the obtained P(DL-co-CL)s were modified by the initial feeding ratios.And ¹H NMR analysis showed that the percentage of two monomers in P(DL-co-CL)s was close to the initial feeding ratios.In addition,we conducted the diblock copolymerization by polymerizinge-DL first with[e-DL]₀/[ITU 2]₀/[Mg I₂]₀/[BnOH]₀=50:1:1:1([e-DL]₀=1.0 M,50°C)without quenching.To the solutions of the PDL precursors,the same equivalent ofe-DL was added directly as the second monomer.The block copolymer PDL-b-PCL with well-defined end group and blocky structure were prepared.(4)To gain further insight into the applicability of proposed catalytic system,ITU 2/Mg I₂ dual catalysis system was used for the ROP of?-valerolactone(?-VL)in the presence of BnOH,producing polyvalerolactone(PVL)with predictable M_nvalues and narrow PDIs.In the further investigations,random copolymer P(VL-co-DL)and block copolymer PDL-b-PVL were prepared by one-pot addition and step-wise addition ofe-DL and?-VL,respectively.The structures of the obtaining copolyemers were proved by ¹H and ¹³C NMR.