Ring-Opening Polymerization Of 1, 4-Dioxan-2-One Initiated By Tri (2, 6-Di-Tert-Butyl-4-Methylphenolate)s Rare Earth Complex As Novel Single Component Initiator

Aliphatic polyesters and polycarbonates, as the new functional polymer materialswith outstanding biodegradability, bioabsorbability and biocompatibility, haveattracted great interest from researchers in recent years. Poly(1, 4-dioxan-2-one)(PPDO) as a well-known biodegradable aliphatic polyester material, has its ownspecial characters. Except of its ultimate biodegradability due to the existence of etherbonds in polymer chains, the unique also endue it with good flexibility. The highmolecular weight of PPDO has been used as suture material in gynecology. The moreapplications in biomedical fields, such as bone or tissue fixation device and drugdelivery system, have also been reported frequently in recent years. The ring-openingpolymerization has advantages such as mild polymerization condition, predictablemolecular weight or structures and molecular weight distribution compared withcondensation reaction. However, only a few catalysts (such as Sn(Oct)₂) have beenused to synthesize PPDO with debating mechanisms, low reaction rate and highpolymerization temperature. In this dissertation, a new kind of initiator-rare earthtri(2, 6-di-tert-butyl-4-methylphenolate) as single component has been developed toring-opening homopolymerize 1, 4-dioxan-2-one (PDO) and copolymerize withε-caprolactone (ε-CL) at mild conditions. Both the polymerization features andmechanisms are discussed in details.Rare earth tris(2, 6-di-tert-butyl-4-methylphenolate) [Ln(OAr)₃, Ln=La, Nd, Y, Dy,Sm] was firstly studied on the polymerizations of PDO. It was found active ininitiating polymerizations of PDO, and La(OAr)₃ is the best one colligatingconversion and viscosity average molecular weight of PPDO. PPDO with highmonomer conversion (86%) and high viscosity average molecular weight (360,000)can be prepared in mild conditions (polymerization temperature is 40℃,polymerization time is 8h) through ring-opening polymerization of PDO initiated byLa(OAr)₃. Three characteristic peaks of PPDO have been found in the ¹H-NMRspectrum:δ=4.16ppm, (—OCH₂COO—, s),δ=3.80ppm (—OCH₂CH₂OCO—, t),δ=4.36ppm (—OCH₂CH₂OCO—, t). The most conscipicious difference between PPDO and PDO is the band at about 1431cm^-1 in the IR spectra, which appears in thespectrum of the polymer, but not in the spectrum of the monomer. The T_g of PPDO(M_v=163, 000) is -13.8℃, the Tm is 107℃, and the T_c is 28.5℃measured by DSC.The thermal degradation of PPDO (M_v=163, 000) is detectable above 150℃andreaches a loss of weight of 17-18% at 200℃measured by TGA. The XRD of PPDOshows that the diffraction peaks (20) are at 20-30°.The polymerization mechanisms of PDO initiated by La(OAr)₃ were fully studiedfor the first time, and proved to be a "coordintion insertion anionic mechanism".Monomer coordinated to rare earth metal on the carbonyl group, and opened ring viaacyl-oxygen bond cleavage, forming a growing chain. The following monomerrepeated these steps to insert into the La—O bond in propagation process. The arylend group of growing chain was replaced by alcohol in termination or precipitationresulting alkyl carbonate end of polymers.The copolymerizations of PDO withε-CL was carried out by La(OAr)₃ as singlecomponent initiator for the first time, and the influences of f_CL, polymerization timeand the molar ratio of monomer to initiator on the monomer conversion and theintrinsic viscosity of the copolymer were explored. P(PDO-co-CL) copolymers withmonomer conversion of 76% and the intrinsic viscosity (η) of 0.77 can be prepared inmild polymerization conditions (polymerization temperature is 40℃, polymerizationtime is 7h). Both the characteristic peaks of PCL chain and PPDO chain were givenin the ¹H-NMR spectrum:δ=2.36ppm,δ=1.70ppm,δ=1.42ppm andδ=4.08ppmof PCL,δ=4.18ppm,δ=3.78ppm andδ=4.37ppm of PPDO. The carbonyl groupof the copolymer can be found a single peak at 1750cm^-1 in the IR spectrum, a clovendouble peak at 1250cm^-1 for the C—O group of the copolymer. The T_g of copolymeris -58.4℃, the Tm is 35.6℃, and the T_c is -2.4℃measured by DSC. TGA curves ofcopolymer is more smooth compared with homopolymer(PPDO), the thermaldegradation through only one step of copolymer is detectable at 50℃and reaches aloss of weight nearly 100% at 400℃. The results of characterization show thatP(PDO-co-CL) obtained using La(OAr)₃ as single component initiator is randomcopolymer. Both 2, 6-di-tert-butyl-4-methylphenol and rare earth metals are low toxicities.Ln(OAr)₃ which is stable, active, easy to synthesize and purify can be broadly used inring-opening polymerization to synthesize biodegradable materials likepolycarbonates and polyesters for medical use.