Synthesis Of Stereoblock Polymer Of Styrene And Functional Derivatives

In this doctoral dissertation,the theme focuses on the synthesis of stereoblock polymers based on styrene derivatives.The mechanisms behind their(co)polymerization activity,regio-and stereo-selectivity,chain propagation and chain transfer were researched.Additives triggered chain transfer to solvent in coordination polymerization was the first revealed new chain-transfer mechanism.The effects of substituents of polar divinyl styrenyl monomers on the polymerization activity and selectivity were studied.Finally,the syndiotactic-atactic stereodiblock and multiblock polymers were prepared by sequential addition on account of the living polymerization characteristics of the corresponding homopolymerizations.Details are listed as following:(1)Low-molecular-weight polystyrene(LMPS)is the precursor of a brominated styrene oligomer,which is a flame-retardant material and used for small,precise,thin electrical components.To date,LMPS is usually synthesized via anionic polymerization or radical suspension polymerization.As for the anionic polymerization,it needs a huge amount of anionic initiator,leaving metal residue in the products of difficult to be removed off.The molecular weight of the polymer offered by radical suspension polymerization is affected easily by stirring speed and temperature etc.Activated by[Ph3C][B(C6F5)4]and AliBu3(2,4,5,6-Me4-4H-cyclopenta[b]thiophenyl)Sc(CH2SiMe3)2THF)showed high activity to give syndiotactic polystyrene.With the addition of alloxybenzene(AOB)to the system,the syndioselectivity disappeared,and the molecular weights of the isolated products are much lower than the theoretic ones indicating the serious chain-transfer reaction.By the analysis of the MALDI-TOF and NMR of the isolated polymer,the unprecedented chain transfer to solvent by means of C-H activation mechanism in a coordination polymerization,was discovered.Meanwhile,the molecular weights of polymers were almost not affected by the polymerization temperature,catalyst structure and the monomer feed ratio.This work provides a highly efficient and productive method to prepare oligomers.(2)The chemoselective polymerization of divinyl monomers is a promising strategy to access functional materials via postpolymerization of polymers bearing pendant C=C groups,however,it encounters the cross-linking problem.Meanwhile,stereoregularity endow polymers with advanced properties than the atactic ones.Herein,chemo-and stereoselective polymerization of divinyl monomers has significance from academic and industrial aspects.A series of polar divinyl styrenyl monomers was designed and synthesized:1-(allyloxy)-4-vinylbenzene(AOS),1-(but-3-en-1-yloxy)-4-vinylbenzene(BOS),1-(pent-4-en-1-yloxy)-4-vinylbenzene(POS)and 1-(hex-5-en-1-yloxy)-4-vinylbenzene(HOS),which contain styrenyl C=C bond and olefinic C=C bond as well as an oxygen atom.The polymerization of polar divinyl styrenyl monomers by employing the half-sandwich thiophene-fused cyclopentadienyl scandium bis(alkyl)complexes(2,5-Me2-3-Ph-6H-cyclopenta[b]thiophenyl)Sc(CH2SiMe3)2THF(TMCp-Scl)and(2,4,5,6-Me4-4H-cyclopenta[b]thiophenyl)Sc(CH2SiMe3)2THF(TMCp-Sc2)as well as the constrained-geometry-configuration type pyridinyl methylene fluorenyl rare-earth metal complexes(Flu-CH2-Py)Sc(CH2SiMe3)2(CGC-Sc3)and(Flu-CH2-Py)Y(CH2SiMe3)THF(CGC-Y4)were studied.The results indicated that all of the rare-earth metal complexes showed high chemoselectivity across styrenyl C=C bond.However,TMCp-Scl and TMCp-Sc2 gaved atactic products;CGC-Sc3 and CGC-Y4 showed syndioselectivity for BOS,POS and HOS polymerizations,but nonstereoselectivity for AOS polymeization.Herein,the stereoselectivity could be controlled by the spacer length between the olefinic C=C bond and oxygen atom in the substituent group.The copolymerization of the polar divinyl styrenyl monomers with ethylene(E)mediated by CGC-Sc3 exhibited chemoselectivity to afford POS/E and HOS/E copolymers but homopolymer poly(AOS).The results indicated spacer length also showing great affects on copolymerization.(3)DFT calculations were carried out to investigate the mechanisms of the stereoselective polymerization of polar divinyl styrenyl monomers.Active species(2,4,5,6-Me4-4H-cyclopenta[b]thiophenyl)Sc(CH2SiMe3)+(TMCp-Sc2+)prefers to adopt two monomers,due to its opening coordination sphere,and there is no difference between the two coordinating monomers.For the chain propagation,the monomer inserted in re or si mode by overcoming the similar activation energies,providing atactic products.The sterically crowded active species(Flu-CH2-Py)Y(CH2SiMe3)+(CGC-Y4+)adopts only one monomer because of the pyridine side arm that coordinates to the central metal.For the polymerization of AOS containing a short spacer,there exists the concurrent Y-?1-O:?2-C=C coordination mode,which arouses site epimerization(or back-skipping)and the similar energy gaps for re-and si-insertion,offering atactic PAOS.For the monomer like POS containing a long spacer,site epimerization is frustrated and the styrenyl C=C bond alternating re-and si-insertion is more favored to give syndiotactic product.For AOS/E copolymerization mediated by CGC-Sc3+,the concurrent coordination inhibits E coordination/insertion to offer PAOS homopolymer.But the concurrent coordination is frustrated when POS is used,leaving chance for E coordination and insertion to afford copolymers.(4)Monomer 2,6-dimethoxy-styrene(DMOS)with two methoxy groups was synthesized.The polymerization of DMOS with the catalytic system(Flu-CH2-Py)Y(CH2SiMe3)THF/[Ph3C][B(C6F5)4]/AliBu3 showed the livingness mode although without stereoselectivity.DFT calculations reveal that the interaction between Y3+and one of the two methoxy groups in the last inserted monomer results in site epimerization,and the coming monomer insertion in re or si mode is similar.Thus,the atactic polymers were obtained.Through the sequential addition,the stereoblock copolymers consisting of amorphous atactic poly(DMOS)and crystalline syndiotactic polystyrene or poly(ortho-methoxystyrene)were obtained successfully.