Study on Macromolecular Chain Structure Regulation and Performance of Poly(propylene carbonate)

二氧化碳是主要的温室气体，同时也是一种廉价的、取之不尽用之不竭的碳氧资源。以二氧化碳为原料合成可生物降解的脂肪族聚碳酸酯是二氧化碳固定和利用领域的重要课题，其中最受关注的是由二氧化碳和环氧丙烷共聚合所制备的聚碳酸丙烯酯（PPC）。高分子量PPC可用于制备连续薄膜，这类薄膜除了保持可生物降解性能之外，还具有较好的透明性，而且其室温下对氧气的阻隔性能优良，在食品保鲜和药品包装等方面有很大的应用前景。但是， PPC在高温（40℃以上）强度迅速下降、低温（15℃以下）脆性加剧，为使PPC象普通塑料如聚乙烯那样实现大规模化应用，必须解决其高温增强和低温增韧的难题。 从高分子链结构来看，PPC的分子链主要由弱极性或非极性基团构成，且普通PPC的区域和立体规整性较低，导致PPC中分子链间作用力较弱，而且是无定型结构，降低了PPC耐温性能和机械力学性能。本博士论文的主要的思路就是通过调控PPC的链结构来提高其分子间作用力，从而改善PPC的耐温性能和力学性能。本论文围绕PPC的近程结构和远程结构控制，提出了改进PPC 链结构的五个方法，即提高PPC的分子量、制备交联型PPC、合成区域规整结构PPC、合成等规结构PPC、合成等规-无规立体嵌段PPC。通过研究链结构变化对PPC 热性能和机械性能的影响，证明通过共聚物链结构的设计和调控，可以大幅度增强PPC 的分子间作用力，拓展PPC 的使用温度范围。本文的主要内容包括： 1．通过在Y(Cl3COO)3-ZnEt2-glycerine三元催化体系或通过在环氧丙烷和二氧化碳聚合反应体系中引入少量双环氧单体如乙二醇二缩水甘油醚、丁二醇二缩水甘油醚或新戊二醇二缩水甘油醚，合成了数均分子量超过20万的PPC。与数均分子量为10万左右的PPC相比，分子量在20万左右的PPC的起始热分解温度增加了37℃，一定程度上改善了PPC的加工性能和使用性能。 2．将提高聚合物分子量改善其耐温和力学性能的思路进一步延伸，提出了分子量“无限大”的交联聚合物思路。通过在二氧化碳与环氧丙烷共聚反应中引入少量含有双键的环氧化合物(烯丙基缩水甘油醚)，可以在基本上不影响聚合反应收率的前提下，得到侧链带双键的可交联的PPC。通过侧链双键的自由基反应，获得交联的PPC，其耐热性能、力学性能与未交联的PPC相比均有显著提高，大幅度提高了共聚物的尺寸稳定性。在65℃下未交联PPC的永久形变为17.2%，根据PPC交联程度的不同，交联PPC的永久形变在0-15%间可控，而且交联后的PPC仍然具有很好的生物降解性能。 3．通过在稀土三元催化体系中加入适量的路易斯碱，既保持了稀土三元催化剂可以得到高分子量PPC的优点，另外路易斯碱与活性中心配位，可提高活性中心的亲核性和环氧丙烷开环选择性，得到高分子量规整区域结构的PPC。聚合物的头尾连接最高达84.6%，数均分子量为136kg/mol。XPS研究证实路易斯碱与中心锌金属发生配位，提高了金属碳酸酯的亲核性，进而提高了活性中心进攻环氧丙烷单体的选择性，从而改善了PPC区域规整性。进一步研究了非均相稀土三元催化剂中多质子化合物配体以及手性多质子化合物配体对PPC区域结构和头尾连接的影响，结果表明电子效应是影响PPC区域结构的主要因素，为以后催化剂的设计和制备提供了依据。与高头尾结构的低分子量PPC不同，高头尾结构的高分子量PPC性能有了很大的改善。当头尾连接结构含量从69.7%增加到83.2%时，PPC的玻璃化温度增加8℃，起始热分解温度提高45℃，表明提高PPC的区域规整性确实可以改善其耐温性能。 4．利用二氧化碳和环氧丙烷(PO)共聚时，单体PO的开环方式决定聚合产物立体结构的特点，在Y(Cl3COO)3-ZnEt2-glycerin/PHEN催化体系下，通过S-PO与 CO2的共聚，制得数均分子量为97kg/mol的等规结构PPC。13C-NMR谱图证实聚合物为等规结构。 5. 通过简单的顺序加料方式，在Y(Cl3COO)3-ZnEt2-glycerin/PHEN催化体系下，通过S-PO/CO2和rac-PO/CO2嵌段共聚，制得数均分子量为139kg/mol的等规-无规立体嵌段PPC。13C-NMR谱图及分子量的数据证实聚合物为等规-无规立体嵌段结构。Fixation of carbon dioxide into biodegradable aliphatic polycarbonates is one of the most challenging works in carbon dioxide fixation field, among which, Poly(propylene carbonate) (PPC) from alternating copolymerization of carbon dioxide and propylene oxide (PO), is most attractive. Though transparent film having low oxygen permeability can be made from PPC, its thermal and mechanical properties are relatively poor. One reason lies in that PPC is a weak polar polymer with weak molecular chain interaction, and the existence of many weak polar, flexible C-O-C blonds in the backbone leads to its amorphous state. Therefore, enhancing molecular chain interaction of PPC is badly needed, and one possible way is to control its macromolecular chain structure. This dissertation focused on improving thermal and mechanical performances of PPC by controlling its chain structures, disclosing five measures including rasing molecular weight of copolymer, preparing crosslinked PPC, synthesizing regio-regular structure PPC, preparing isotactic PPC, and synhesizing isotactic-block-atactic PPC. The results of this dissertation are summarized below: 1. PPC with number average molecular weight (Mn) higher than 200 kg/mol was prepared via the terpolymerization of CO2, PO and diepoxide under Y(CCl3OO)3–ZnEt2–glycerine coordination catalyst. The molecular weight of PPC has dramatic influence on its thermal and mechanical performance, a 37℃ increase of the onset decomposition temperature was observed when Mn increased from 109 kg/mol to 227 kg/mol. 2. A kind of UV-crosslinkable PPC was prepared by terpolymerization of CO2, PO and allyl glycidyl ether (AGE) under Y(CCl3OO)3–ZnEt2–glycerine coordination catalyst. Low level of AGE in the comonomers did not affect copolymerization yield significantly, and low level of AGE unit in PPC was sufficient for crosslinking. In comparison with common PPC, the crosslinked PPC showed much improvement in the thermal and mechanical properties, including Td, modulus, hot set elongation and permanent deformation, etc., especially at higher temperature like 65℃, which greatly extended the suitable application temperature range of PPC. 3. Lewis base modification strategy on rare earth ternary catalyst was disclosed to enhance nucleophilic ability of active center during copolymerization of CO2 and PO, PPC with head-to-tail(H-T) linkages over 83% and Mn up to 100 kg/mol, was synthesized using Y(CCl3OO)3–ZnEt2–glycerine as catalyst and 1,10-Phenanthroline (PHEN) as cocatalyst. Coordination of PHEN with active Zn-O active center enhanced the nucleophilic ability of the metal carbonate, which became more regio-specific in attacking carbon in PO, leading to PPC with improved H-T linkages. Unlike the highly regio-regular structure PPC but with relatively low Mn recently reported in the literature, the high molecular weight regio-regular PPC did show significant improvement in thermal and mechanical performances. PPC with H-T linkages up to 83.2% showed Tg of 43.3℃, while Tg of PPC with H-T linkages of 69.7% was only 36.1℃. 4. Isotactic- PPC with Mn up to 97 kg/mol was prepared via the copolymerization of carbon dioxide and S-PO under Y(CCl3OO)3–ZnEt2–glycerine/PHEN catalyst. 5. Isotactic-block-atactic PPC was prepared first on copolymerization of carbon dioxide and S-propylene oxide under Y(CCl3OO)3–ZnEt2–glycerine/PHEN catalyst, in which the isotactic segments were formed, followed by the addition of rac-PO and carbon dioxide that subsequently induced the atactic chain growth to give the isotactic-block-atactic PPC with Mn of 139kg/mol