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Abstract
In this study, six-arm star-shaped poly(N-vinylcaprolactam) (PNVCL) polymers prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization were subjected to aminolysis reaction using hexylamine. Chemically crosslinked gels or highly end-functionalized star polymers can be obtained depending mainly on the type of solvent used during the transformation of the RAFT functional group. An increase in the viscosity of the solution was observed when the aminolysis was carried out in THF. In contrast, when the reaction was conducted in dichloromethane, chain-end thiol (PNVCL)6 star polymers could be obtained. Moreover, when purified (PNVCL-SH)6 star polymers are in contact with THF, the gelation occurs in just a few minutes, with an obvious increase in viscosity, to form physical gels that become chemically crosslinked gels after 12 h. Interestingly, when purified (PNVCL-SH)6 star polymers were stirred in distilled water, even at high aqueous solution concentration (40 mg/mL), there was no increase in the viscosity or gelation, and no evident gels were observed. The analysis of the hydrodynamic diameter (Dh) by dynamic light scattering (DLS) did not detect quantifiable change even after 4 days of stirring in water. On the other hand, the thiol groups in the (PNVCL-SH)6 star polymers were easily transformed into trithiocarbonate groups by addition of CS2 followed by benzyl bromide as demonstrated by UV-Vis spectroscopical analysis and GPC. After the modification, the (PNVCL)6 star polymers exhibit an intense yellow color typical of the absorption band of trithiocarbonate group at 308 nm. To further demonstrate the highly effective new trithiocarbonate end-functionality, the PNVCL polymers were successfully chain extended with N-isopropylacrylamide (NIPAM) to form six-arm star-shaped PNIPAM-b-PNVCL block copolymers. Moreover, the terminal thiol end-functionality in the (PNVCL-SH)6 star polymers was linked via disulfide bond formation to l-cysteine to further demonstrate its reactivity. Zeta potential analysis shows the pH-responsive behavior of these star polymers due to l-cysteine end-functionalization. By this using methodology and properly selecting the solvent, various environment-sensitive star polymers with different end-groups could be easily accessible.
Highlights
Polymers prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization can be transformed just by eliminating the RAFT group [1,2,3,4,5]
According to a review of the literature, the removal of the thiocarbonylthio group is mostly carried out in THF as solvent. Based on these facts, this study began seeking the aminolysis of the xanthate group in the RAFT-made (PNVCL)6 star polymers
Thiol-terminated star-shaped poly(N-vinylcaprolactam) polymers were prepared by aminolysis reaction using hexylamine in dichloromethane as solvent. (PNVCL-SH)6 polymers are precursors of either cross-linked gels or highly effective postfunctionalized star polymers, depending on the selected solvent
Summary
Polymers prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization can be transformed just by eliminating the RAFT group [1,2,3,4,5]. The formation of disulfide groups can occur within the removal reaction, in the purification step or during postfunctionalization [3,4,5]. The ability of thiol end-capped polymers to form disulfide bonds strongly depends among many other factors on the R groups attached. Polymers 2021, 13, 3677 step or during postfunctionalization [3,4,5]. The ability of thiol end-capped polymers to form disulfide bonds strongly depends among many other factors on the R groups attached to it and on the structure of the monomer [6,7,8,9]. TohleecSuElaCr cohxriodmataiotong, raalmthooufgthhetrthisi(o2l-actaerdbopxoylyemtheyrl) sphhoowsspnhoineevhidyednrcoecholfomriadcero(TmCoElePc)uwlaar soxaidddaetidona,ftaelrthdoiuaglyhsitsri[s7(2].-cIanrbcoonxytreatshty, lW) pinhnoiskpheitnael. h[y9]drreopcholroterdidteh(eTaCmEiPn)owlyassisaodfdpeodlyaf(tNe-risdoiaplryospisyl[a7c].ryInlacmoindter)asptr,eWpainrendikbeyt RalA. [F9T] rpeoployrmteedrptidieniwtdonhnrzerxieeadarTatsishctusadHoepieemolannpsoFfntpci,eilitdeanyaorhrietenmfonoesosrlfmfotehyeroohamnorsifargesimccmlseαhlaempdo,tastrωomahaetfeilrimy-onsypootaeuthnloopibanTlic,lcedrosycHcriaeelem(nuamnFNretoracutnoo-fadhesirndutasseeuoonrtsrcpffpphotiioandoTrroeotreolgaeCmfy1hslpTwtymEeihhygCnttPalieeehuc.acEsrmepTncsmPaorudhpmobymaoreeerslsilorraaefiaaaaaumNyrrcubtvnraumt2sioeetthderdiacdoaeoonectu,sf)nrmcufswcsTeporioosncrChrofisdeogdipE1ftpcunheahPhTarshegewiirCainderepsuseaEngdeona.satPrlt.tdIbyot.hntrTyrehmbeTeirhtdbasRhheepetuNueAreruta.tcv2hGeFmiarTendeiaTuPhfdigtintotCemc,phoxaoawtootogilhtdrdylrisheyaioisspaosnicmnicutlhecstih.oslfoeeowfiu,Tnrrnifadsineahszto.seciaeshadtfmItptfieanteiooαermoarnprir,tlnbωmfhtlaoiohunlseil-arinyaothttmtehGtymloitdyoiheutPiosenhdnteliClr-o,es. tTheheptohlyioml efursncwtiaosndaelitteyrminintheedpinol9y5m%er(Es lwlmasandetetesrt)m. ined in 95% (Ellman test)
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