Polymorphic And Stereocomplex Crystallizations Of Supramolecular Polyesters Bonded By Multiple Hydrogen Bonds

Supramolecular polymers(SMPs)represent a unique class of materials whose particular functions are derived from the dynamic noncovalent interactions.Multiple H-bonds have been extensively used as the building blocks to create SMPs,due to its high selectivity,directionality and high sensitivity to external stimuli.2-Ureido-4[1H]-pyrimidinone(UPy)can form the self-complementary quadruple H-bonds by dimerization.UPy has a high dimerization constant and is easy to synthesize and functionalize.Thus,when the UPy groups are incorporated to the monomers and the terminals or side chains of oligomers,SMPs can be formed via the H-bonds interactions between UPy dimers.SMPs have quite different crystallization behavior from the conventional polymers,because of the confinement effects of supramolecular units.Crystallization of SMPs undergoes in a "confined" and "dynamic" manner.Stereocomplex(SC)crystallization is a unique cocrystallization manner of the polymers with opposite configurations or chiralities.The SC materials usually have higher melting temperature,better heat resistance,larger strength and modulus,and better chemical and hydrolytic resistance than the homocrystalline(HC)materials with the similar chemical structure.The intermolecular interaction between complementary polymer enantiomers has been a key factor influencing their SC crystallization behavior.Therefore,the incorporation of UPy groups containing self-complementary H-bonds between complementary enantiomers would facilitate the SC crystallites.First,we selected the UPy-bonded PLLA as a model SMP and investigated the crystallization kinetics,polymorphic crystalline structure and phase transition of supramolecular PLLAs(SM-PLLAs).A series of hydroxyl-terminated 2 and 3-arm PLLA precursors with different molecular weight were first synthesized by the ring-opening polymerization of L-lactide using 1,6-hexanediol and trimethylolpropane as the initiator.The SM-PLLAs were prepared via incorporating UPy groups to the terminals of PLLA precursors.Temperature-variable fourier transform infrared spectroscopy(FTIR)spectra of UPy-terminated PLLA confirmed that UPy-UPy H-bonds are partially destroyed and UPy dimers are partly melted with heating to～90 ℃.Crystallization rate and crystallinity of SM-PLLAs are strongly depressed in both nonisothermal and isothermal crystallizations,as compared to the non-functionalized PLLA precursors.Crystalline structure of SM-PLLAs is sensitive to the crystallization temperature(Tc).A low Tc(75～95℃)facilitates the formation of metastable β crystals of PLLA in SM-PLLAs;while a high Tc(100～130 ℃)favors the generation of a(or α’)crystals.The β crystals formed in SM-PLLAs transform into the more stable a crystals in the following heating process.We propose that the preferential formation of β crystals is ascribed to the "strong"confinements of UPy motifs,which might exert an orientation and stretching effects to the linked PLLA chains during crystallization.This study has paved a way to obtain the PLLAβ-form from SM-PLLA under the crystallization conditions free of pressure,stretching,and shearing.Second,we investigated the effect of supramolecular interactions on the crystallization kinetics,polymorphic crystalline structure and phase transition of poly(butylene adipate)(PB A).PB As with a broad range of UPy substitution degrees ranging from 14%to 91%were prepared by end-functionalization.When the content of UPy is low,the influence of supramolecular units on the crystallization rate is negligible;when the content is high,the crystallization is hindered with the incorporation of UPy groups.Compared to the non-functionalized PBA,a-crystals are favored in the UPy-functionalized PBAs.With the increase of UPy content,this trend becomes more pronounced.The possible mechanism of the promoted a-form crystals may be ascribed to the decreased equilibrium melting temperature.Furthermore,the crystallization kinetics and polymorphic crystalline structure of PBA are influenced by the fusion temperature(Tf).After the non-functionalized PBA is melted at different Tf,the crystalline structure is basically unchanged.For UPy-functionalized PBA,higher Tf tends to facilitate the formation of a-crystals.The β-to-α phase transition behavior of PBA is significantly altered in terms of the depressed onset temperature when UPy unit exists.During the heating process,part of PBA β-crystals melt directily and the rest transforms into a phase;while the molten β-crystals of UPy-functionalized PBA is fully transformed to a crystals without the formation of amorphous chains.Due to the presence of supramolecular units,the LPs of UPy functionalized PBA is different from non-functioanlized PBA depending on the UPy contents.Last,we investigated the influence of supramolecular interaction on the crystallization behavior of mono-and multi-functionalized PLLAPDLA SMPs.Solvent-free ring-opening polymerization(ROP)of lactide using the self-complementary quadruple hydrogen bonding UPy-functionalized alcohol as the initiator was achieved to attain the UPy mono-functionalized PLLA and PDLA capable of undergoing the supramolecular self-assembly.This ROP polymerization exhibits good controllability and the synthesized polymers have the controlled molecular weights and well-defined terminal structure.Telechelic and 3-arm star-shaped PLLAs and PDLAs end-functionalized by the UPy self-complementary quadruple hydrogen-bonding motifs were synthesized by terminally modifying the corresponding hydroxyl-terminated polymers.The PLLA/PDLA stereoblock supramolecular polymers(sb-SMPs)with linear and branched architectures were further prepared by mixing the UPy end-functionalized telechelic,3-arm star-shaped PLLA and PDLA through the dimerization of UPy groups.Specific viscosities of mono-and multi-UPy-functionalized PLLA in dilute solution show the strong concentration dependence,demonstrating the formation of supramolecular structure by UPy dimerization.Crystallization kinetics,polymorphic crystalline structure,and crystalline structural organization of UPy-functionalized PLLA/PDLA blends were investigated and compared to the corresponding non-functionalized blends.The UPy end functionalization not only accelerates the crystallization but also facilitates the formation of high-melting-point SCs in the PLLA/PDLA blends.The stereocomplexation ability of UPy-functionalized PLLA/PDLA blends further enhances with decreasing the molecular weights of PLLA,PDLA and increasing the content of UPy end functionality.The incorporation of UPy end functionality also promotes the melt recrystallization of HCs to SCs upon heating.It is proposed that the promoted SC formation of UPy-functionalized PLLA/PDLA blend is originated from the enhanced interactions between enantiomeric chains.