Effect of solvent on the initiation mechanism of living anionic polymerization of styrene:A computational study

For living anionic polymerization(LAP),solvent has a great influence on both reaction mechanism and kinetics.In this work,by using the classical butyl lithium-styrene polymerization as a model system,the effect of solvent on the mechanism and kinetics of LAP was revealed through a strategy combining density functional theory(DFT)calculations and kinetic modeling.In terms of mechanism,it is found that the stronger the solvent polarity,the more electrons transfer from initiator to solvent through detailed energy decomposition analysis of electrostatic interactions between initiator and solvent molecules.Furthermore,we also found that the stronger the solvent polarity,the higher the monomer initiation energy barrier and the smaller the initiation rate coefficient.Counterintuitively,initiation is more favorable at lower temperatures based on the calculated results ofΔG_(TS).Finally,the kinetic characteristics in different solvents were further examined by kinetic modeling.It is found that in benzene and n-pentane,the polymerization rate exhibits first-order kinetics.While,slow initiation and fast propagation were observed in tetrahydrofuran(THF)due to the slow free ion formation rate,leading to a deviation from first-order kinetics.

SYNTHESIS OF BLOCK COPOLYMER BY INTEGRATED LIVING ANIONIC POLYMERIZATION-ATOM TRANSFER RADICAL POLYMERIZATION(ATRP)

Alpha-trichloroacetoxy terminated polystyrene oligomer (PS-CH2CH2OCOCCl3) and poly-(styrene-b-butadiene) oligomer [P(S-b-B)-CH2CH2OCOCCl3)] were synthesized by living anionic polymeri-zation using n-butyllithium as initiator. Then the PS-CH2CH2OCOCCl3 (PS-Cl-3) or P(S-b-B)-CH2CH2O-COCCl3 (PSB-Cl-3) was used as the macroinitiator in the polymerization of(meth)acrylates in the presence of CuX/bpy. AB diblock and ABC triblock copolymers were prepared by the integrated living anionic polymerization (LAP)-atom transfer radical polymerization (ATRP). The structures of the PSB-Cl-3 and the P(S-b-MMA) were identified by FTIR and H-1-NMR spectrum, respectively. A new way to design block copolymers (the combination of LAP and ATRP) was developed.

Synthesis of Star-like Polybutadienes by a Combination of Living Anionic Polymerization and “Click” Coupling Method

Three-arm and four-arm star-like polybutadienes （PBds） were synthesized via the combination of living anionic polymerization and the click coupling method. Kinetic study showed that the click reaction between the azido group terminated PBd-t-N3 and the alkyne-containing multifunctional linking reagent was fast and highly efficient. All coupling reactions were fully accomplished within 40 min at 50 ℃ in toluene in the presence of the reducing agent Cu（0）, proven by 1H-NMR, FTIR and GPC measurements. For the coupling reactions between the PBd-t-N3 polymer and dialkyne-containing compound, the final conversion of the coupled PBd-PBd polymer was ca. 97.0%. When a PBd-t-N3 polymer was reacted with trialkyne-containing or tetraalkyne-containing compound, the conversion of three-arm or four-arm PBd was around 95.5% or 87.0%, respectively. Several factors influencing the coupling efficiency were studied, including the molecular weight of the initial PBd-t-N3, arm numbers and the molar ratio of the azido group to the alkynyl group. The results indicated that the conversion of the target products would be promoted when the molecular weight of the PBd-t-N3 was low and the molar ratio of the azido to alkynyl groups was close to 1.

Synthesis of α-Bromine-Terminated Polystyrene Macroinitiator and Its Initiation of MMA Polymerization by ATRP

In the present paper the synthesis of block copolymers via the transformation from living anionic polymerization (LAP) to atom transfer radical polymerization (ATRP) was described. α-Bromine-terminated polystyrenes(PStBr) in the LAP step was prepared by using n-BuLi as initiator, tetrahydrofuran (THF) as the activator, α-methylstyrene (α-MeSt) as the capping group and liquid bromine (Br_2) as the bromating agent. The effects of reaction conditions such as the amounts of α-MeSt, THF, and Br_2 as well as molecular weight of polystyrene on the bromating efficiency (BE) and coupling extent (CE) were examined. The present results show that the yield of PStBr obtained was more than 93. 8% and the coupling reaction was substantially absent. PStBr was further used as the macroinitiator in the polymerization of methyl-methacrylate (MMA) in the presence of copper (I ) halogen and 2, 2' -bipyridine (bpy) complexes. It was found that the molecular weight of the resulted PSt-b-PMMA increased linearly with the increase of the conversion of MMA and the polydispersity was 1. 2-1.6. The structures of PStBr and P(St-b-MMA) were characterized by ~1H NMR spectra.

ANIONIC SYNTHESIS OF A “CLICKABLE” MIDDLE-CHAIN AZIDE-FUNCTIONALIZED POLYSTYRENE AND ITS APPLICATION IN SHAPE AMPHIPHILES

＂Click chemistry＂ is, by definition, a general functionalization methodology （GFM） and its marriage with living anionic polymerization is particularly powerful in precise macromolecular synthesis. This paper reports the synthesis of a ＂clickable＂ middle-chain azide-functionalized polystyrene （mPS-N3） by anionic polymerization and its application in the preparation of novel shape amphiphiles based on polyhedral oligomeric silsesquioxane （POSS）. The mPS-N3 was synthesized by coupling living poly（styryl）lithium chains （PSLi） with 3-chloropropylmethyldichlorosilane and subsequent nucleophilic substitution of the chloro group in the presence of sodium azide. Excess PSLi was end-capped with ethylene oxide to facilitate its removal by flash chromatography. The raPS-N3 was then derived into a giant lipid-like shape amphiphile in two steps following a sequential ＂click＂ strategy. The copper（I）-catalyzed azide-alkyne cycloaddition between mPS-N3 and alkyne-functionalized vinyl-substituted POSS derivative （VPOSS-alkyne） ensured quantitative ligation to give polystyrene with VPOSS tethered at the middle of the chain （mPS-VPOSS）. The thiol-ene reaction with 1-thioglycerol transforms the vinyl groups on the POSS periphery to hydroxyls, resulting in an amphiphilic shape amphiphile, mPS-DPOSS. This synthetic approach is highly efficient and modular. It demonstrates the ＂click＂ philosophy of facile complex molecule construction from a library of simple building blocks and also suggests that mPS-N3 can be used as a versatile ＂clickable＂ motif in polymer science for the precise synthesis of complex macromolecules.

Synthesis and characterization of amphiphilic centipede-like copolymer PS-PS-PMAA

A well-defined amphiphilic centipede-like copolymer of styrene and methacrylic acid (PS-PS-PMAA) was synthesized by the combination of living anionic polymerization and atom transfer radical polymerization (ATRP). The synthetic approach involves the first coupling reaction of polystyrene (PS) backbone bearing 1,1-diphenylethene (DPE) pendant groups with living polystyryllithium (PSLi), and sequential anionic polymerization of t-butyl methacrylate (tBMA) initiated by resulting 1,1-diphenylmethyl anion, and final hydrolysis of obtained PS-PS-PtBMA. The centipede-like copolymer PS-PS-PMAA was characterized by 1H NMR, IR, GPC, and SLS measurements. The critical micelle concentration (CMC) of PS-PS-PMAA in water was determined by fluorescence probe technique. The self-assembly behavior of PS-PS-PMAA in water-THF mixture was observed by TEM. The results showed that the micellar morphology can be varied, such as vesicle, sphere, and agglomerate, depending on the THF content. These phenomena are worthy of further research in polymer physics field.

Synthesis and characterization of well-defined comb-like branched polymers

Well-defined comb-like branched polymers having one branch in each repeating unit have been suc-cessfully synthesized by the coupling reaction of living polystyrene(PS)and polyisoprene(PI)anions with 1,1-diphenylethenyl(DPE)groups along PS backbone pre-pared via atom transfer radical polymerization(ATRP)of 4-vinylbenzyloxy benzophenone(Sc)followed by Wittig reaction.The resulting comb-like branched poly-mers were characterized by IR,1H-NMR,gel permeation chromatography(GPC)and static light scattering(SLS)in detail.The effect of living chains and DPE group molar ratio on grafting efficiency was discussed.The results show the coupling reaction of living chains and DPE groups was highly effective,and the coupling efficiency can be controlled via the feed molar ratios of living chains and DPE groups.Moreover,the effect of molecular weights of backbone(PSe)and PSLi or PILi on grafting efficiency was also discussed.The results show that when excess living polymers were used,the almost quantitative grafting efficiency could be achieved.

Synthesis and Properties of SEPS-g-PEO Copolymers with Varying Branch Lengths

Poly（ethylene oxide） （PEO） was controllably grafted from styrene-b-（ethylene-co-propylene）-b-styrene （SEPS） backbones by combining lithiation of styrenic units and living monomer-activated anionic ring-opening polymerization of ethylene oxide （EO） monomers with the aid of co-initiators triisobutyl aluminum. The as-synthesized SEPS-g-PEO copolymers were characterized by SEC, 1H-NMR, FTIR, SAXS, AFM and DSC. When the branch length is relatively small, increase of PEO fraction leads to the increase of the correlation length between neighboring hard domains, but the degree of correlation reduces. When the branch length is relatively large, the phase-separated structures become random both in terms of size and spatial correlation, and macro-phase separated structures appear. The crystallization behavior of the PEO branches can be effectively inhibited in SEPS-g-PEO, so no significant crystallization takes place until the fraction of PEO branches is 20.1 wt%, which greatly promotes the rapid delivery of hydrophilic drugs in the hot-melting pressure- sensitive adhesives （HMPSAs） based on SEPS-g-PEO. Their cumulative release amount of a model drug could achieve 80%, more than twice the value in the HMPSAs based on linear PEO-containing styrenic block copolymers.