BAIP-Ti（IV）配合物催化D，L-丙交酯开环聚合及PDLLA的静电纺丝研究
摘要
聚乳酸(polylactide,PLA)是一种具有优良生物相容性和生物降解性的热塑性聚酯材料。近年来许多研究都集中于聚乳酸的合成及应用上，其中尤以丙交酯(lactide,LA)的开环聚合以及在医药和组织工程领域的应用为国内外研究的热点。目前辛酸亚锡(Sn(Oct)_2)用于催化丙交酯(LA)开环聚合合成聚乳酸是最为广泛的催化剂，但辛酸亚锡有其自身的缺点，如在高于100℃时易分解或在微量水的影响下易发生水解反应，所产生的副产物在丙交酯的开环聚合过程中不可避免地发生副反应。此外，辛酸亚锡作为催化剂易于失活而影响催化效率，同时也可使得聚乳酸的分散度增加，影响聚乳酸的力学性能。所以获得热稳定、对水惰性的高活性金属催化剂催化丙交酯开环聚合是近年来研究的热点。
本研究合成了一种新颖的热稳定、高活性的双(烷氧-亚胺芳氧)基钛(Ⅳ)[BAIP-Ti(Ⅳ)]配合物，用于催化D,L-丙交酯的开环聚合。采用各种结构分析手段表征了钛(Ⅳ)配合物的结构，探讨了用该配合物催化D,L-丙交酯(D,L-LA)开环聚合的动力学及机理，同时对获得的聚(D,L-丙交酯)(PDLLA)进行了力学测试及细胞生物相容性的初步评价。进而对获得的低分散度的PDLLA进行了纺丝研究，考察了PDLLA的样品浓度、流量以及真空冷冻干燥条件对纺丝直径分布及形貌的影响。最后以小鼠胚胎成骨细胞株(MC3T3-E1细胞)为模型，用扫描电镜(SEM)观察在PDLLA纤维材料表面的生长情况，探讨了成骨细胞在PDLLA微纳米丝支架材料上的生长情况。本研究的内容和结论如下：
1.以3,5-二叔丁基水杨醛，乙醇胺以及钛酸四乙酯为原料，合成了BAIP-Ti(Ⅳ)配合物，表征了它的结构。
①配合物的合成：以3,5-二叔丁基水杨醛和乙醇胺为原料，经过氨醛缩合反应得到希夫碱，再与钛酸四乙酯进行醇交换反应，经重结晶后得到BAIP-Ti(Ⅳ)配合物。
②采用元素分析(EA)、傅立叶变换红外光谱(FTIR)、核磁共振波谱(NMR)、多晶粉末衍射(XRD)、X射线荧光衍射(XRF)、热重/差热分析(TG/DSC)以及单晶衍射等方法对该配合物的结构进行了表征，结果表明Ti原子与两个烷氧-亚胺芳氧基以四个O-Ti共价键和两个N→Ti配位键成键,显示该配合物是以Ti原子为中心的双配基配合物。
2.以BAIP-Ti(Ⅳ)配合物为催化剂催化D,L-丙交酯开环聚合，成功制备了PDLLA。研究了不同的聚合条件对PDLLA的分子量和分散度的影响，并考察了D,L-丙交酯开环聚合的动力学及机理。
①考察了单体与催化剂的浓度比、聚合温度与时间对D,L-丙交酯单体开环聚合的影响，得到的最佳反应条件是：[M]/[Ti]=2000，聚合温度160℃，聚合时间16h，获得分子量分布(M_w/M_n)为1.17，数均分子量(Mn)为8.8×10~4g/mol的聚乳酸。
②动力学表明该配合物在130℃以上能有效地催化D,L-丙交酯开环聚合，该聚合反应对D,L-丙交酯单体浓度和催化剂浓度的反应级数分别为2级和1级,表观活化能(ΔEa)和频率因子(A)分别为76.63kJ/mol和2.91×10~(11)。其表观活化能与辛酸亚锡[Sn(Oct)_2]催化L-丙交酯开环聚合的活化能70.90kJ/mol接近。表明BAIP-Ti(Ⅳ)配合物催化D,L-丙交酯开环聚合反应是较易进行的聚合反应，且BAIP-Ti(Ⅳ)配合物与Sn(Oct)_2一样对D,L-丙交酯开环聚合有着较高的催化活性。
③通过苄醇封端得到的活化钛(Ⅳ)醇聚合物的~1HNMR图分析,表明D,L-丙交酯单体的开环机理为配位-插入机理,金属烷氧基Ti-O键是引发D,L-丙交酯单体的酰氧键断裂开环而非烷氧基断裂开环聚合的活性中心，从而实现链的增长。
④在各聚合温度下，单体转化率与数均分子量(M_n)呈线性关系，窄分子量分布(M_w/M_n=1.10-1.25)的聚乳酸，BAIP-Ti(Ⅳ)配合物显示出具有“活性”可控聚合的倾向。
3.采用FTIR,~1HNMR,~(13)C NMR以及TG/DSC对BAIP-Ti(Ⅳ)配合物合成的聚乳酸(Ti-P)进行了结构表征、热稳定性和力学测试的研究，并对其拉伸断裂面的微观形貌进行了SEM观察。
①~1HNMR和~(13) CNMR研究表明，该Ti-P聚乳酸在本质上是属于无规的聚合物，在~(13)C NMR中δ=69.4ppm处无明显iss序列结构吸收峰的存在，表明聚合过程中无明显的分子内酯交换反应的发生。同时，由isi(δ=5.16ppm)和sis(δ=5.23ppm)与其他几组信号(δ=5.17,5.20和5.21ppm)的强度比值(Pr)，即选择性系数Pr为0.64，表明该配合物催化D,L-丙交酯开环聚合显示出杂同立构选择性开环聚合的特征，并显示出可控聚合倾向的行为。
②通过~(13)C NMR图中四元立构序列的次甲基碳和六元立构序列羰基碳信号强度的研究，发现D,L-丙交酯在聚合过程中等规加成的概率为Pi=0.58，表明BAIP-Ti(Ⅳ)基钛(Ⅳ)配合物催化D,L-丙交酯开环聚合存在中等程度的等规加成立构选择性。
③以辛酸亚锡合成的聚乳酸(Sn-P)和BAIP-Ti(Ⅳ)配合物合成的聚乳酸(Ti-P)做对比，TG/DSC分析表明分子量接近分散度不同的Ti-P比Sn-P有着更稳定的热性能和较高的玻璃化转变温度(Tg)，这与Ti-P中几乎不存在支化的聚乳酸或对热不稳定基团的因素有关。
④拉伸和压缩试验表明Ti-P比Sn-P具有更好抗压和抗拉的力学性能。Ti-P材料拉伸断裂面的SEM显示，其断裂面清晰光滑，无明显的凹槽或突起，也无明显的丝状断裂层面，显示出明显脆性断裂的特征。
4.以Sn-P为对比，考察了Ti-P材料和Sn-P的亲/疏水性能和降解性能。其中，用静态水接触角和吸水率评价材料的亲/疏水性，体外实时降解试验评价材料的降解性能，评价指标为失重率和pH值的变化。
①Ti-P材料比Sn-P材料有较高的接触角和较低的吸水率，可能是分散度小的Ti-P材料表面的-COOH和-OH等活性基团比Sn-P材料少，使其亲水性降低。
②体外降解实验表明，Ti-P材料比Sn-P材料的失重率进入快速增长区和降解液pH值进入陡降区的时间比Sn-P延迟，两种材料都明显呈现酸致自催化降解现象。这是由于Ti-P材料的分散度比Sn-P材料低，Ti-P材料中亲水性的酸性羧基基团的浓度也较低，导致Ti-P材料的酸致自催化明显好于Sn-P材料。
5.从MC3T3-E1细胞在玻璃空白、Ti-P和Sn-P材料表面的细胞形态，F-肌动蛋白的分布情况、增殖活力以及增殖速率等指标的测定进行了生物相容性的初步评价，结果表明:
①MC3T3-E1细胞在Ti-P、Sn-P以及玻片材料上培养48h后，MC3T3-E1细胞在三种材料上都能良好的铺展开。
②通过激光共聚焦显微镜观察MC3T3-E1细胞在Ti-P、Sn-P以及盖玻片材料上F-肌动蛋白的分布情况表明，培养48h的MC3T3-E1细胞在三种材料上均显示出较好的细胞黏附和铺展状态。
③MC3T3-E1细胞在Ti-P、Sn-P以及玻片上增殖活力以及增殖速率的测定，结果表明细胞在Ti-P材料上的增殖趋势与在Sn-P材料上相似，但显著高于玻片对照组细胞的增殖活力，而Sn-P材料和Ti-P材料之间细胞的增殖活力间不存在显著性差异。
6.用制备的数均分子量为8.8×10~4g/mol，分子量分布(M_w/M_n)为1.17的Ti-P进行静电纺丝研究，考察了纺丝条件对纤维直径及形貌的影响。并以MC3T3-E1细胞为细胞模型，初步考察了以Ti-P作为纤维支架材料的细胞生物相容性的研究，结果表明：
①在0.8kv/cm电场强度下，Ti-P电纺浓度为0.20g/ml、流速为0.8ml/h时，可得到大多数纤维直径分布在1.1-1.7μm之间的微米纤维丝。
②经过真空冷冻干燥8h后，可以得到直径主要分布在500-900nm之间的纳米纤维丝，占整个纳米丝直径分布的74%。
③通过MC3T3-E1细胞在Ti-P纤维丝上的培养研究，发现细胞能很好地黏附于Ti-P纤维丝上并正常生长，培养5天后部分细胞产生明显的细胞基质分泌物。结果证明Ti-P经过真空冷冻干燥后的纳米纤维丝对MC3T3-E1细胞显示出良好的生物相容性，并显示出重要的生物学特征。
综上所述，本文以合成的BAIP-Ti(Ⅳ)配合物为催化剂对D,L-丙交酯的开环聚合显示出可控聚合的行为，合成的Ti-P比Sn-P显示出较好的理化性能，并对成骨细胞显示出良好的生物相容性。
Polylactide (PLA) is one kind of thermoplastic polyester materials with good biocompatibility and biodegradability and widely used in the fields of biologic medicine and tissue engineering. Recently, people play more attentions to the synthesis and applications of PLA. Thereofore, it has drawn more attentions for ring-opening polymerization (ROP) of the lactide and its applications in medicine and tissue engineering. The tin(Ⅱ) bis(2-ethylhexanoate)(Sn(Oct)2) was widely employed as the initiator for ROP of lactide to produce PLA. However, it is important to note that Sn(Oct)2could be decomposed over100℃and hydrolyzed in presence of a little water. Subsequently, some side reactions would be take place during ROP of lactide. The results were that the (Sn(Oct)2) as initiator was inactivation for ROP of lactide and its catalytical activation was decreased. Therefore, it made molecular weight distributions of PLA broaden and the mechanics strength of PLA material was also decreased. So it was needed to develop new thermal stabilitable metal catalyst with insensitive property in presence of water to produce PLA by the method of ROP of lactide.
In the thesis, a thernmal stabilitable and higher reactive catalyst, bis(alkoxy-imine-phenoxy) titanium (Ⅳ)[BAIP-Ti(Ⅳ)] complex, was synthesed and characterizaed by all kinds of methods to ROP for D,L-lactide. Experimental results revealed that the BAIP-Ti(Ⅳ) complex could efficiently catalyze the ROP of D,L-lactide, and the kinetic and mechanism of ROP reactions were studied. The mechanical test and biocompatibility of the obtained poly(D,L-lactide)(PDLLA) were assyed and preliminary estimated, repectively. In the electraspun experiments of the PDLLA, we studied the influencing factors of variations in diameter distribution and morphology of the PDLLA fibers, such as concentration and feed rate of PDLLA solution, and the duration of the vacuum freeze-drying. As well, the cultivation and growth of the MC3T3-E1cells on the of macro-and nanao-structure PDLLA fiber as scaffold materials was studied. The main works and results were included as follows:
1. BAIP-Ti(Ⅳ) complex was synthesized with the3,5-di-tert-butylsalicylaldehyde, ethanolamine and tetraethyl titanate as react reagents, and its structure was characterizaed.
①Preparation of BAIP-Ti(IV) complex:the Schiff base was synthezed through the condensation reaction between3,5-di-tert-butylsalicylaldehyde and ethanolamine. Then the Schiff base reacted with tetraethyl titanate following the alcohol exchange protocol, and the BAIP-Ti(IV) complex was obtained by the method of recrystallization.
②The BAIP-Ti(IV) complex was structurally characterized by element analysis(EA), fourier transform infrared spectrometry (FTIR), nuclear magnetic resonance spectrometer(NMR), X-ray fluorescence spectroscopy(XRF) and thermal gravity analysis/differential scanning calorimeter (TG/DSC) methods. The X-ray diffraction results showed that the titanium ion was bonded with two3,5-di-tert-butylsalicylaldehyde-co-ethanolamine ligands in O,N,O-tridentate mode, and there were four O-Ti covalent bonds and two N→Ti coordiantion bonds. It indicated that the center of titanium ion in the complex was closed by double ligands.
2. The poly(D,L-lactide)(PDLLA) was produced by ROP of D,L-lactide in bulk phase with the BAIP-Ti(IV) complex as catalyst. We studied the molecular weights and polydispersity index (PDI) under different experimental factors, and the kinetic and mechanisim of ROP for D,L-lactide were also studied.
①The ROP of D,L-lactide could affected by the molar ratio of D,L-lactide to complex ([M]0/[Ti]0, M=D,L-lactide monomer,[Ti]=titanium (IV) complex), polymerization temperature and duration time. In our study, we had gained the polylactide with maximum number average molecular weight (Mn=8.80×104g/mol) at [M]0/[Ti]0molar ratio of2000/1for a16h duration at160℃
②The kinetic studies showed that the ROP of D,L-lactide could be performed over130℃and the polymerization reaction was the second-order on D,L-lactide monomer concentration and the first-order on catalyst concentration, and the values of apparent activation energy (△Ea=76.63kJ/mol) and frequency factor (A=2.9×1011) were obtained according to the Arrhenius equation. And the values of apparent activation energy (△Ea=76.63kJ/mol) was closed to the that (△Ea=70.9kJ/mol) of ROP of L-LA with Sn(Oct)2as catalyst. The results showed that the the ROP of D,L-lactide catalyzed by the BAIP-Ti(IV) complex was a facile reaction and the BAIP-Ti(IV) complex had the higher catalytical activation with the Sn(Oct)2as catalyst.
③The mechanism study by1H NMR spectrum of poly(D,L-lactide) with terminal benzyl ester group and the BAIP-Ti(IV) complex revealed that the polymerization proceeded through the traditional activated monomer mechanism and the acyl-oxygen bond cleavage mode of monomer.
④The D,L-lactide polymerization results at different temperature showed that a linear relationship between Mn and rac-lactide monomer conversion and the narrow molecular distributionlow (MW/Mn=1.10-1.25) of the PDLLA at160℃implied the highly controlled and "living" character of the polymerization process.
3. The PDLLA was chanracterizaed by fourier transform infrared spectrometry (FTIR), nuclear magnetic resonance spectrometer (1H NMR and13C NMR), and thermal gravity analysis/differential scanning calorimeter (TG/DSC) methods. The mechanical property of PDLLA was perfomed from the tension and compression experiments, and the tension fracture face morphology of PDLLA was chanracterizaed by the scanning electron microscopy (SEM).
①The results from1H NMR and13C NMR of PDLLA showed that that the PDLLA (Ti-P) was essential atactic polymer. There was the methine protons resonance absorption peak in iss stereosequence at69.4ppm, and it indicatd that no intermolecular transesterification took place in ROP of lactide with BAIP-Ti(IV) complex as catalyst. And the proportionality (Pr) of intensities of isi (δ=5.16ppm) and sis (δ=5.23ppm) steresquences to the intensities of other steresquences was0.64, the results showed the complex showed a hetereotactic and well-controlled selectional manner for ROP of D,L-lactide.
②From the study of the intensities of methenyl carbon with tetrad stereosequences and carbonyl carbon with hexad stereosequences in the13C NMR of PDLLA and the probability(P1=0.58) of isotactic addition reaction, it revealed that there was moderate stereoselectivity of isotactic addition reactions with BAIP-Ti(IV) complex as catalyst for ROP of D,L-lactide.
③In contrast to the Sn-P (the PDLLA with Sn(Oct)2as catalyst), the Ti-P (the PDLLA with BAIP-Ti(IV) complex as catalyst), in spite of similar Ma and different PDI and the TG/DSC analysis showed that the Ti-P polymer was more thermal stable and higer glass transition temperature (Tg) than Sn-P polymer, which indicated that there was few branched polylactide and thermally unstable groups.
④The results of tension and compression experiments showed that the properties of tension and compression of Ti-P polymer was stronger than that of Sn-P polymer.and the SEM of Ti-P fracture face morphology showed that fracture face was smooth, and no apparente gab and claw could be observed. As well, there was no the distinct silkiness and fracture deck in the SEM. Therefore, it revealed that the Ti-P material showed the chanracter of brittleness fracture.
4. In contrast to the Sn-P material, the surface hydrophilicity/hydrophobicity and degradation of Ti-P materoal were investigated The evaluating indicators of surface wettability were mainly static water contact angle and water absorption ratio, while the evaluating indicators of degradation behavior were pH value changes and the weight loss ratio.
①The results showed that static water contact angle of Ti-P materoal was higher than that of Sn-P material, while the water absorption ratio was smaller than that of Sn-P material. So hydrophilicity of Ti-P material was lower than that of Sn-P materal due to the lower number of carboxyl and hydroxyl on the surface of Ti-P materoal.
②The dgradable experimental results of Ti-P and Sn-P materials in vitro revealed that the interval time of quickly grew weight loss and sudden depressed degradable pH value of Ti-P was longer than that of Sn-P, and there was obvious acid catalyzed auto-accelerating degradation. The PDI value of Ti-P material was lower than that of Sn-P material and the fewer number of hydrophilicity group on the surface of Ti-P material decrease the concentration of carboxyls, which made the lower auto-accelerating degradation of Ti-P material than that of Sn-P material.
5. The biocompatibility of MC3T3-E1cell on the surface of glass, Ti-Pmaterial and Sn-P material were investigated, respectively. The MC3T3-E1cell was estimated by means of morphology, the distribution of F-actin, proliferation. The results revealed:
①All MC3T3-E1cells had spread on three materials, such as glass, Ti-Pand Sn-P material after48h cultivation.
②The F-actin morphology of MC3T3-E1cells spread on glass, Ti-Pand Sn-P material was observed by the method of laser confocal microscopy. The results showed that MC3T3-E1cells had adhered and apread the three materials well after48h cultivation.
③The proliferation vigor and speed of MC3T3-E1cells on the surface of glass, Ti-P and Sn-P material were assayed. The result proved that proliferation tendency of MC3T3-E1cells on Ti-P material was similar to that of Sn-P material, however, the proliferation vigor of MC3T3-E1cells on the two materials was notable higer than that of glass, and there wasn't significant difference between Ti-P and Sn-P material.
6. The electraspun of PDLLA (Ti-P) with8.8×104g/mol molecular weight and low PDI (1.17) was studied. In the study, we investigated that the experimental factors could affect on fiber diameter distribution and morphology. As the studied model, the biocompatibility of MC3T3-E1cell on the Ti-P fibers was investigated. The results revealed:
①The miacrofibers could obtained at0.8kv/cm electric field strength,0.20g/ml concentration of Ti-P solution and0.8ml/h feed rate, the diameter distribution of these fibers was range from1.1to1.7μm
②The nanofibers could obtained under cryodesiccate at vaccum for8h, and their diameter distribution was range from500to900nm and these fiber was about74%in total diameter distribution.
③The MC3T3-E1cells were found that they could adhere to the surface of the Ti-P fibers when MC3T3-E1cells were cultured on the surface of the Ti-P fibers. However, there was visible extracellular matrix (ECM) on the surface of MC3T3-E1cell after5days cultivation. It revealed that the Ti-P fibers after cryodesiccate at vaccum were good biocompatibility to MC3T3-E1cells and these MC3T3-E1cells had showed important biologic feature.
In conclusion, the BAIP-Ti(IV) complex had showed that controlled manner for ROP of D,L-Lactide. The physical and chemical properties of Ti-P polymer were gooder than that of Sn-P polymer and the Ti-P polymer was biocompatibility for the adhesion and proliferation of osteoblast.
本研究合成了一种新颖的热稳定、高活性的双(烷氧-亚胺芳氧)基钛(Ⅳ)[BAIP-Ti(Ⅳ)]配合物，用于催化D,L-丙交酯的开环聚合。采用各种结构分析手段表征了钛(Ⅳ)配合物的结构，探讨了用该配合物催化D,L-丙交酯(D,L-LA)开环聚合的动力学及机理，同时对获得的聚(D,L-丙交酯)(PDLLA)进行了力学测试及细胞生物相容性的初步评价。进而对获得的低分散度的PDLLA进行了纺丝研究，考察了PDLLA的样品浓度、流量以及真空冷冻干燥条件对纺丝直径分布及形貌的影响。最后以小鼠胚胎成骨细胞株(MC3T3-E1细胞)为模型，用扫描电镜(SEM)观察在PDLLA纤维材料表面的生长情况，探讨了成骨细胞在PDLLA微纳米丝支架材料上的生长情况。本研究的内容和结论如下：
1.以3,5-二叔丁基水杨醛，乙醇胺以及钛酸四乙酯为原料，合成了BAIP-Ti(Ⅳ)配合物，表征了它的结构。
①配合物的合成：以3,5-二叔丁基水杨醛和乙醇胺为原料，经过氨醛缩合反应得到希夫碱，再与钛酸四乙酯进行醇交换反应，经重结晶后得到BAIP-Ti(Ⅳ)配合物。
②采用元素分析(EA)、傅立叶变换红外光谱(FTIR)、核磁共振波谱(NMR)、多晶粉末衍射(XRD)、X射线荧光衍射(XRF)、热重/差热分析(TG/DSC)以及单晶衍射等方法对该配合物的结构进行了表征，结果表明Ti原子与两个烷氧-亚胺芳氧基以四个O-Ti共价键和两个N→Ti配位键成键,显示该配合物是以Ti原子为中心的双配基配合物。
2.以BAIP-Ti(Ⅳ)配合物为催化剂催化D,L-丙交酯开环聚合，成功制备了PDLLA。研究了不同的聚合条件对PDLLA的分子量和分散度的影响，并考察了D,L-丙交酯开环聚合的动力学及机理。
①考察了单体与催化剂的浓度比、聚合温度与时间对D,L-丙交酯单体开环聚合的影响，得到的最佳反应条件是：[M]/[Ti]=2000，聚合温度160℃，聚合时间16h，获得分子量分布(M_w/M_n)为1.17，数均分子量(Mn)为8.8×10~4g/mol的聚乳酸。
②动力学表明该配合物在130℃以上能有效地催化D,L-丙交酯开环聚合，该聚合反应对D,L-丙交酯单体浓度和催化剂浓度的反应级数分别为2级和1级,表观活化能(ΔEa)和频率因子(A)分别为76.63kJ/mol和2.91×10~(11)。其表观活化能与辛酸亚锡[Sn(Oct)_2]催化L-丙交酯开环聚合的活化能70.90kJ/mol接近。表明BAIP-Ti(Ⅳ)配合物催化D,L-丙交酯开环聚合反应是较易进行的聚合反应，且BAIP-Ti(Ⅳ)配合物与Sn(Oct)_2一样对D,L-丙交酯开环聚合有着较高的催化活性。
③通过苄醇封端得到的活化钛(Ⅳ)醇聚合物的~1HNMR图分析,表明D,L-丙交酯单体的开环机理为配位-插入机理,金属烷氧基Ti-O键是引发D,L-丙交酯单体的酰氧键断裂开环而非烷氧基断裂开环聚合的活性中心，从而实现链的增长。
④在各聚合温度下，单体转化率与数均分子量(M_n)呈线性关系，窄分子量分布(M_w/M_n=1.10-1.25)的聚乳酸，BAIP-Ti(Ⅳ)配合物显示出具有“活性”可控聚合的倾向。
3.采用FTIR,~1HNMR,~(13)C NMR以及TG/DSC对BAIP-Ti(Ⅳ)配合物合成的聚乳酸(Ti-P)进行了结构表征、热稳定性和力学测试的研究，并对其拉伸断裂面的微观形貌进行了SEM观察。
①~1HNMR和~(13) CNMR研究表明，该Ti-P聚乳酸在本质上是属于无规的聚合物，在~(13)C NMR中δ=69.4ppm处无明显iss序列结构吸收峰的存在，表明聚合过程中无明显的分子内酯交换反应的发生。同时，由isi(δ=5.16ppm)和sis(δ=5.23ppm)与其他几组信号(δ=5.17,5.20和5.21ppm)的强度比值(Pr)，即选择性系数Pr为0.64，表明该配合物催化D,L-丙交酯开环聚合显示出杂同立构选择性开环聚合的特征，并显示出可控聚合倾向的行为。
②通过~(13)C NMR图中四元立构序列的次甲基碳和六元立构序列羰基碳信号强度的研究，发现D,L-丙交酯在聚合过程中等规加成的概率为Pi=0.58，表明BAIP-Ti(Ⅳ)基钛(Ⅳ)配合物催化D,L-丙交酯开环聚合存在中等程度的等规加成立构选择性。
③以辛酸亚锡合成的聚乳酸(Sn-P)和BAIP-Ti(Ⅳ)配合物合成的聚乳酸(Ti-P)做对比，TG/DSC分析表明分子量接近分散度不同的Ti-P比Sn-P有着更稳定的热性能和较高的玻璃化转变温度(Tg)，这与Ti-P中几乎不存在支化的聚乳酸或对热不稳定基团的因素有关。
④拉伸和压缩试验表明Ti-P比Sn-P具有更好抗压和抗拉的力学性能。Ti-P材料拉伸断裂面的SEM显示，其断裂面清晰光滑，无明显的凹槽或突起，也无明显的丝状断裂层面，显示出明显脆性断裂的特征。
4.以Sn-P为对比，考察了Ti-P材料和Sn-P的亲/疏水性能和降解性能。其中，用静态水接触角和吸水率评价材料的亲/疏水性，体外实时降解试验评价材料的降解性能，评价指标为失重率和pH值的变化。
①Ti-P材料比Sn-P材料有较高的接触角和较低的吸水率，可能是分散度小的Ti-P材料表面的-COOH和-OH等活性基团比Sn-P材料少，使其亲水性降低。
②体外降解实验表明，Ti-P材料比Sn-P材料的失重率进入快速增长区和降解液pH值进入陡降区的时间比Sn-P延迟，两种材料都明显呈现酸致自催化降解现象。这是由于Ti-P材料的分散度比Sn-P材料低，Ti-P材料中亲水性的酸性羧基基团的浓度也较低，导致Ti-P材料的酸致自催化明显好于Sn-P材料。
5.从MC3T3-E1细胞在玻璃空白、Ti-P和Sn-P材料表面的细胞形态，F-肌动蛋白的分布情况、增殖活力以及增殖速率等指标的测定进行了生物相容性的初步评价，结果表明:
①MC3T3-E1细胞在Ti-P、Sn-P以及玻片材料上培养48h后，MC3T3-E1细胞在三种材料上都能良好的铺展开。
②通过激光共聚焦显微镜观察MC3T3-E1细胞在Ti-P、Sn-P以及盖玻片材料上F-肌动蛋白的分布情况表明，培养48h的MC3T3-E1细胞在三种材料上均显示出较好的细胞黏附和铺展状态。
③MC3T3-E1细胞在Ti-P、Sn-P以及玻片上增殖活力以及增殖速率的测定，结果表明细胞在Ti-P材料上的增殖趋势与在Sn-P材料上相似，但显著高于玻片对照组细胞的增殖活力，而Sn-P材料和Ti-P材料之间细胞的增殖活力间不存在显著性差异。
6.用制备的数均分子量为8.8×10~4g/mol，分子量分布(M_w/M_n)为1.17的Ti-P进行静电纺丝研究，考察了纺丝条件对纤维直径及形貌的影响。并以MC3T3-E1细胞为细胞模型，初步考察了以Ti-P作为纤维支架材料的细胞生物相容性的研究，结果表明：
①在0.8kv/cm电场强度下，Ti-P电纺浓度为0.20g/ml、流速为0.8ml/h时，可得到大多数纤维直径分布在1.1-1.7μm之间的微米纤维丝。
②经过真空冷冻干燥8h后，可以得到直径主要分布在500-900nm之间的纳米纤维丝，占整个纳米丝直径分布的74%。
③通过MC3T3-E1细胞在Ti-P纤维丝上的培养研究，发现细胞能很好地黏附于Ti-P纤维丝上并正常生长，培养5天后部分细胞产生明显的细胞基质分泌物。结果证明Ti-P经过真空冷冻干燥后的纳米纤维丝对MC3T3-E1细胞显示出良好的生物相容性，并显示出重要的生物学特征。
综上所述，本文以合成的BAIP-Ti(Ⅳ)配合物为催化剂对D,L-丙交酯的开环聚合显示出可控聚合的行为，合成的Ti-P比Sn-P显示出较好的理化性能，并对成骨细胞显示出良好的生物相容性。
Polylactide (PLA) is one kind of thermoplastic polyester materials with good biocompatibility and biodegradability and widely used in the fields of biologic medicine and tissue engineering. Recently, people play more attentions to the synthesis and applications of PLA. Thereofore, it has drawn more attentions for ring-opening polymerization (ROP) of the lactide and its applications in medicine and tissue engineering. The tin(Ⅱ) bis(2-ethylhexanoate)(Sn(Oct)2) was widely employed as the initiator for ROP of lactide to produce PLA. However, it is important to note that Sn(Oct)2could be decomposed over100℃and hydrolyzed in presence of a little water. Subsequently, some side reactions would be take place during ROP of lactide. The results were that the (Sn(Oct)2) as initiator was inactivation for ROP of lactide and its catalytical activation was decreased. Therefore, it made molecular weight distributions of PLA broaden and the mechanics strength of PLA material was also decreased. So it was needed to develop new thermal stabilitable metal catalyst with insensitive property in presence of water to produce PLA by the method of ROP of lactide.
In the thesis, a thernmal stabilitable and higher reactive catalyst, bis(alkoxy-imine-phenoxy) titanium (Ⅳ)[BAIP-Ti(Ⅳ)] complex, was synthesed and characterizaed by all kinds of methods to ROP for D,L-lactide. Experimental results revealed that the BAIP-Ti(Ⅳ) complex could efficiently catalyze the ROP of D,L-lactide, and the kinetic and mechanism of ROP reactions were studied. The mechanical test and biocompatibility of the obtained poly(D,L-lactide)(PDLLA) were assyed and preliminary estimated, repectively. In the electraspun experiments of the PDLLA, we studied the influencing factors of variations in diameter distribution and morphology of the PDLLA fibers, such as concentration and feed rate of PDLLA solution, and the duration of the vacuum freeze-drying. As well, the cultivation and growth of the MC3T3-E1cells on the of macro-and nanao-structure PDLLA fiber as scaffold materials was studied. The main works and results were included as follows:
1. BAIP-Ti(Ⅳ) complex was synthesized with the3,5-di-tert-butylsalicylaldehyde, ethanolamine and tetraethyl titanate as react reagents, and its structure was characterizaed.
①Preparation of BAIP-Ti(IV) complex:the Schiff base was synthezed through the condensation reaction between3,5-di-tert-butylsalicylaldehyde and ethanolamine. Then the Schiff base reacted with tetraethyl titanate following the alcohol exchange protocol, and the BAIP-Ti(IV) complex was obtained by the method of recrystallization.
②The BAIP-Ti(IV) complex was structurally characterized by element analysis(EA), fourier transform infrared spectrometry (FTIR), nuclear magnetic resonance spectrometer(NMR), X-ray fluorescence spectroscopy(XRF) and thermal gravity analysis/differential scanning calorimeter (TG/DSC) methods. The X-ray diffraction results showed that the titanium ion was bonded with two3,5-di-tert-butylsalicylaldehyde-co-ethanolamine ligands in O,N,O-tridentate mode, and there were four O-Ti covalent bonds and two N→Ti coordiantion bonds. It indicated that the center of titanium ion in the complex was closed by double ligands.
2. The poly(D,L-lactide)(PDLLA) was produced by ROP of D,L-lactide in bulk phase with the BAIP-Ti(IV) complex as catalyst. We studied the molecular weights and polydispersity index (PDI) under different experimental factors, and the kinetic and mechanisim of ROP for D,L-lactide were also studied.
①The ROP of D,L-lactide could affected by the molar ratio of D,L-lactide to complex ([M]0/[Ti]0, M=D,L-lactide monomer,[Ti]=titanium (IV) complex), polymerization temperature and duration time. In our study, we had gained the polylactide with maximum number average molecular weight (Mn=8.80×104g/mol) at [M]0/[Ti]0molar ratio of2000/1for a16h duration at160℃
②The kinetic studies showed that the ROP of D,L-lactide could be performed over130℃and the polymerization reaction was the second-order on D,L-lactide monomer concentration and the first-order on catalyst concentration, and the values of apparent activation energy (△Ea=76.63kJ/mol) and frequency factor (A=2.9×1011) were obtained according to the Arrhenius equation. And the values of apparent activation energy (△Ea=76.63kJ/mol) was closed to the that (△Ea=70.9kJ/mol) of ROP of L-LA with Sn(Oct)2as catalyst. The results showed that the the ROP of D,L-lactide catalyzed by the BAIP-Ti(IV) complex was a facile reaction and the BAIP-Ti(IV) complex had the higher catalytical activation with the Sn(Oct)2as catalyst.
③The mechanism study by1H NMR spectrum of poly(D,L-lactide) with terminal benzyl ester group and the BAIP-Ti(IV) complex revealed that the polymerization proceeded through the traditional activated monomer mechanism and the acyl-oxygen bond cleavage mode of monomer.
④The D,L-lactide polymerization results at different temperature showed that a linear relationship between Mn and rac-lactide monomer conversion and the narrow molecular distributionlow (MW/Mn=1.10-1.25) of the PDLLA at160℃implied the highly controlled and "living" character of the polymerization process.
3. The PDLLA was chanracterizaed by fourier transform infrared spectrometry (FTIR), nuclear magnetic resonance spectrometer (1H NMR and13C NMR), and thermal gravity analysis/differential scanning calorimeter (TG/DSC) methods. The mechanical property of PDLLA was perfomed from the tension and compression experiments, and the tension fracture face morphology of PDLLA was chanracterizaed by the scanning electron microscopy (SEM).
①The results from1H NMR and13C NMR of PDLLA showed that that the PDLLA (Ti-P) was essential atactic polymer. There was the methine protons resonance absorption peak in iss stereosequence at69.4ppm, and it indicatd that no intermolecular transesterification took place in ROP of lactide with BAIP-Ti(IV) complex as catalyst. And the proportionality (Pr) of intensities of isi (δ=5.16ppm) and sis (δ=5.23ppm) steresquences to the intensities of other steresquences was0.64, the results showed the complex showed a hetereotactic and well-controlled selectional manner for ROP of D,L-lactide.
②From the study of the intensities of methenyl carbon with tetrad stereosequences and carbonyl carbon with hexad stereosequences in the13C NMR of PDLLA and the probability(P1=0.58) of isotactic addition reaction, it revealed that there was moderate stereoselectivity of isotactic addition reactions with BAIP-Ti(IV) complex as catalyst for ROP of D,L-lactide.
③In contrast to the Sn-P (the PDLLA with Sn(Oct)2as catalyst), the Ti-P (the PDLLA with BAIP-Ti(IV) complex as catalyst), in spite of similar Ma and different PDI and the TG/DSC analysis showed that the Ti-P polymer was more thermal stable and higer glass transition temperature (Tg) than Sn-P polymer, which indicated that there was few branched polylactide and thermally unstable groups.
④The results of tension and compression experiments showed that the properties of tension and compression of Ti-P polymer was stronger than that of Sn-P polymer.and the SEM of Ti-P fracture face morphology showed that fracture face was smooth, and no apparente gab and claw could be observed. As well, there was no the distinct silkiness and fracture deck in the SEM. Therefore, it revealed that the Ti-P material showed the chanracter of brittleness fracture.
4. In contrast to the Sn-P material, the surface hydrophilicity/hydrophobicity and degradation of Ti-P materoal were investigated The evaluating indicators of surface wettability were mainly static water contact angle and water absorption ratio, while the evaluating indicators of degradation behavior were pH value changes and the weight loss ratio.
①The results showed that static water contact angle of Ti-P materoal was higher than that of Sn-P material, while the water absorption ratio was smaller than that of Sn-P material. So hydrophilicity of Ti-P material was lower than that of Sn-P materal due to the lower number of carboxyl and hydroxyl on the surface of Ti-P materoal.
②The dgradable experimental results of Ti-P and Sn-P materials in vitro revealed that the interval time of quickly grew weight loss and sudden depressed degradable pH value of Ti-P was longer than that of Sn-P, and there was obvious acid catalyzed auto-accelerating degradation. The PDI value of Ti-P material was lower than that of Sn-P material and the fewer number of hydrophilicity group on the surface of Ti-P material decrease the concentration of carboxyls, which made the lower auto-accelerating degradation of Ti-P material than that of Sn-P material.
5. The biocompatibility of MC3T3-E1cell on the surface of glass, Ti-Pmaterial and Sn-P material were investigated, respectively. The MC3T3-E1cell was estimated by means of morphology, the distribution of F-actin, proliferation. The results revealed:
①All MC3T3-E1cells had spread on three materials, such as glass, Ti-Pand Sn-P material after48h cultivation.
②The F-actin morphology of MC3T3-E1cells spread on glass, Ti-Pand Sn-P material was observed by the method of laser confocal microscopy. The results showed that MC3T3-E1cells had adhered and apread the three materials well after48h cultivation.
③The proliferation vigor and speed of MC3T3-E1cells on the surface of glass, Ti-P and Sn-P material were assayed. The result proved that proliferation tendency of MC3T3-E1cells on Ti-P material was similar to that of Sn-P material, however, the proliferation vigor of MC3T3-E1cells on the two materials was notable higer than that of glass, and there wasn't significant difference between Ti-P and Sn-P material.
6. The electraspun of PDLLA (Ti-P) with8.8×104g/mol molecular weight and low PDI (1.17) was studied. In the study, we investigated that the experimental factors could affect on fiber diameter distribution and morphology. As the studied model, the biocompatibility of MC3T3-E1cell on the Ti-P fibers was investigated. The results revealed:
①The miacrofibers could obtained at0.8kv/cm electric field strength,0.20g/ml concentration of Ti-P solution and0.8ml/h feed rate, the diameter distribution of these fibers was range from1.1to1.7μm
②The nanofibers could obtained under cryodesiccate at vaccum for8h, and their diameter distribution was range from500to900nm and these fiber was about74%in total diameter distribution.
③The MC3T3-E1cells were found that they could adhere to the surface of the Ti-P fibers when MC3T3-E1cells were cultured on the surface of the Ti-P fibers. However, there was visible extracellular matrix (ECM) on the surface of MC3T3-E1cell after5days cultivation. It revealed that the Ti-P fibers after cryodesiccate at vaccum were good biocompatibility to MC3T3-E1cells and these MC3T3-E1cells had showed important biologic feature.
In conclusion, the BAIP-Ti(IV) complex had showed that controlled manner for ROP of D,L-Lactide. The physical and chemical properties of Ti-P polymer were gooder than that of Sn-P polymer and the Ti-P polymer was biocompatibility for the adhesion and proliferation of osteoblast.
引文
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