CO_2与环氧丙烷交替共聚过程中的区域和立体化学控制
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摘要
聚合物的物理性质在很大程度上是由聚合物链相邻位置的相对立体化学(即聚合物单元中的原子或基团的空间位置)所决定的。因此,调控聚合物的微观结构就成为立体选择性催化聚合领域中非常重要的课题之一。1969年由Inoue首次报道了CO_2与环氧丙烷(PO)的共聚反应,此后这一领域的研究受到了越来越多的关注。本课题组曾报道过由手性四齿希夫碱钴配合物(SalenCo(Ⅲ)Ⅹ)作为亲电试剂,季铵盐/有机强碱作为亲核试剂,组成双功能协同催化体系,在温和条件下成功催化CO_2和环氧丙烷的不对称交替共聚反应。该催化体系表现出非常高的催化活性和聚合物选择性(聚合物含量＞99%),得到了具有完全交替结构(碳酸酯单元含量＞99%)、高区域规整度(头尾连接单元HT%=96%)和中等光学活性(对映体过量值ee%≈70%)的聚碳酸丙烯酯(PPC)。而Salen型铬配合物虽然也可以有效的催化CO_2和环氧丙烷的共聚反应,但得到的却是区域无规的聚碳酸丙烯酯,同时催化活性和立体选择性也较差。
在本论文的研究中,采用Salen型铬配合物与位阻型的有机强碱组成亲电-亲核双组分催化体系,在温和条件下催化CO_2和环氧丙烷的交替共聚反应,得到了具有不同头尾连接单元的聚碳酸酯(64-93%)。研究发现,改变铬配合物的轴向负离子和Salen配体的二胺骨架以及添加不同的助催化剂都会严重影响所得聚合物的区域和立体化学。由具有弱亲核性的轴向负离子和共轭环状二胺骨架的Salen型铬配合物作为亲电试剂,和具有弱配位能力的大位阻型有机强碱作为亲核试剂,是调控CO_2和环氧丙烷交替共聚反应立体化学的一种理想催化体系,可以得到＞99%碳酸酯连接单元和较高头尾连接单元的聚碳酸酯。利用飞行时间质谱对聚合物链端进行了跟踪检测,直接观察到了聚合反应的引发及链增长过程,证明大位阻有机碱在共聚反应过程中对聚合物的链增长起着重要的引发作用。双功能催化体系中,不但Salen铬配合物的配体结构影响着聚合产物的立体化学,聚合物的链末端控制效应同样对聚碳酸酯的立体化学产生重要的影响。
本论文在Salen构型配体的基础上,设计合成了一系列手性Salan型铬配合物,并将其与季铵盐/有机强碱组成亲电-亲核双组分催化体系,在室温下可以有效的催化CO_2和环氧丙烷的不对称交替共聚反应,实现了CO_2和环氧丙烷的不对称交替共聚反应的区域和立体化学控制,得到95%的头尾连接单元和中等光学纯度(对映体过量值ee%≈70%)的全同结构的聚碳酸酯。研究发现除了手性二胺骨架的不对称诱导作用,苯环上的大位阻取代基、N,N'-二取代基的位阻作用以及N原子上sp~3杂化的供电子效应都对调控聚合物的区域和对映体选择性有很大作用。通过研究聚碳酸酯的立体化学发现聚合物的对映体选择性与环氧丙烷区域选择性开环存在一定的影响作用。
实验表明,SalanCr(Ⅲ)Ⅹ配合物的催化体系在催化活性、产物选择性、聚合物区域选择性和对映体选择性方面都要优于以往广泛研究的SalenCr(Ⅲ)Ⅹ配合物的催化体系。利用飞行时间质谱对聚合物链端进行了跟踪检测,直接观察到了聚合反应的引发及链增长过程,证明有机碱DMAP在共聚反应过程中对聚合物的链增长起着重要的引发作用。同时通过飞行时间质谱和液体红外吸收光谱等手段检测到两种铬配合物与DMAP的配位方式,以及两种反应体系催化CO_2与PO的诱导期和链增长速度上都存在着显著的差异,在此基础上进一步提出了SalanCr(Ⅲ)X/DMAP作为催化体系时的聚合反应机理。
The physical properties of a polymer are determined mainly by the relative stereochemistry(the spatial arrangement of atoms or groups in a polymeric unit) of adjacent locations in the polymeric chains.As a result,the control of polymer microstructure is one of the most important goals in the area of stereoselective polymerization catalysis.There are much interested in CO_2/aliphatic epoxides(such as propylene oxide) copolymerization,which was first reported by Inoue in 1969.Recently,our group reported that the binary catalyst systems composed of the chiral tetradentate Schiff base cobalt complex(SalenCo(Ⅲ)X) as the electrophlie and ionic ammonium salt or sterically hindered strong base as the nucleophile can effectively promote the asymmetric copolymerization of CO_2 and racemic propylene oxide(PO) to produce poly(propylene carbonate)(PPC) with excellent activity,relative high enantioselectivity(ee%～70%),＞95%head-to-tail connectivity,and＞99%carbonate linkages. In sharp contrast to the regio-and stereoeslective polymerization catalyzed by chiral SalenCo(Ⅲ)X complexes,although chromium Salen derivatives were also proved to be an effective catalyst for the coupling of PO and CO_2 to afford linear PPC,the polycarbonate produced is regioirregular,and the catalyst activity and polymer enantioselectivity is also very poor.
In this dissertation,it is found that various poly(propylene carbonate)s(PPC)s with different head-to-tail linkages(from～60%to 93%) could be synthesized by the copolymerization of PO and CO_2 with chromium Salen complexes as catalyst.Altering the axial X group and Salen ligand framework of the chromium complexes or/and the addition of cocatalyst significantly affect stereochemistry of the resulting polymers.A SalenCrX complex with a low nucleophilic axial X anion and a conjugated cyclic diamine backbone in the Salen ligand,combined with a sterically hindered strong organic base,is an ideal binary catalyst system for the copolymerization of CO_2 and PO to produce polycarbonate with relative high head-to-tail connectivity and＞99%carbonate linkages.Continuous determination of a polymer chain end group(initiating and chain growth species) at various times by electrospray ionization mass spectrometry(ESI-MS) indicates that the sterically hindered strong organic base predominately plays an initiator of polymer-chain growth in the copolymerization process.Apart from the effect of the ligand set of the chromium Salen complexes on polymer stereochemistry,a "polymer chain-end control" mechanism also significantly affects the stereochemistry of the resulting polycarbonates.
Chiral SalanCr(Ⅲ)X complexes(a saturated version of SalenCr(Ⅲ)X) are designed,and in conjunction with an ionic quaternary ammonium salt or organic strong base can efficiently catalyze the asymmetric,regio-and stereoselective alternating copolymerization of CO_2 with rac-PO at mild conditions to afford isotactic-enriched polycarbonates with～95%head-to-tail linkages and moderate enantioselectivity.The facts indicate that apart from the asymmetric induction of chiral diamine backbone,sterically hindered substitute groups on the aromatic rings,and the presence of sp~3-hydridized amino donors and its N,N'-disubstituted groups in chiral SalanCr(Ⅲ)X complexes all play important roles in controlling polymer stereochemistry and enantioselectivity.The stereochemical studies of PPCs derived from the copolymerization of CO_2 with chiral PO at various conditions reveal that the polymer enantioselectivity is proportional to its head-to-tail linkages with regard to regioselective ring-opening of the epoxide.
These new catalyst systems predominantly exceed the previously much-studied SalenCr(Ⅲ)X systems in catalytic activity,polymer enantioselectivity and stereochemistry control.Continuous determination of a polymer chain end group(initiating and chain growth species) at various times by electrospray ionization mass spectrometry(ESI-MS) indicates that DMAP predominately plays an initiator of polymer-chain growth in the copolymerization process.Electrospray ionization mass spectrometry(ESI-MS) in combination with FT-IR spectra study suggest that the coordination modes of these two chromium complexes with DMAP and the induction periods of CO_2/PO copolymerization and the rate of polymer-chain growth with these two catalyst systems are very different.Furthermore,a straightforward approach to insight into the details of copolymerization of CO_2 and epoxide by SalanCr(Ⅲ)X/DMAP catalyst system is provided.
在本论文的研究中,采用Salen型铬配合物与位阻型的有机强碱组成亲电-亲核双组分催化体系,在温和条件下催化CO_2和环氧丙烷的交替共聚反应,得到了具有不同头尾连接单元的聚碳酸酯(64-93%)。研究发现,改变铬配合物的轴向负离子和Salen配体的二胺骨架以及添加不同的助催化剂都会严重影响所得聚合物的区域和立体化学。由具有弱亲核性的轴向负离子和共轭环状二胺骨架的Salen型铬配合物作为亲电试剂,和具有弱配位能力的大位阻型有机强碱作为亲核试剂,是调控CO_2和环氧丙烷交替共聚反应立体化学的一种理想催化体系,可以得到＞99%碳酸酯连接单元和较高头尾连接单元的聚碳酸酯。利用飞行时间质谱对聚合物链端进行了跟踪检测,直接观察到了聚合反应的引发及链增长过程,证明大位阻有机碱在共聚反应过程中对聚合物的链增长起着重要的引发作用。双功能催化体系中,不但Salen铬配合物的配体结构影响着聚合产物的立体化学,聚合物的链末端控制效应同样对聚碳酸酯的立体化学产生重要的影响。
本论文在Salen构型配体的基础上,设计合成了一系列手性Salan型铬配合物,并将其与季铵盐/有机强碱组成亲电-亲核双组分催化体系,在室温下可以有效的催化CO_2和环氧丙烷的不对称交替共聚反应,实现了CO_2和环氧丙烷的不对称交替共聚反应的区域和立体化学控制,得到95%的头尾连接单元和中等光学纯度(对映体过量值ee%≈70%)的全同结构的聚碳酸酯。研究发现除了手性二胺骨架的不对称诱导作用,苯环上的大位阻取代基、N,N'-二取代基的位阻作用以及N原子上sp~3杂化的供电子效应都对调控聚合物的区域和对映体选择性有很大作用。通过研究聚碳酸酯的立体化学发现聚合物的对映体选择性与环氧丙烷区域选择性开环存在一定的影响作用。
实验表明,SalanCr(Ⅲ)Ⅹ配合物的催化体系在催化活性、产物选择性、聚合物区域选择性和对映体选择性方面都要优于以往广泛研究的SalenCr(Ⅲ)Ⅹ配合物的催化体系。利用飞行时间质谱对聚合物链端进行了跟踪检测,直接观察到了聚合反应的引发及链增长过程,证明有机碱DMAP在共聚反应过程中对聚合物的链增长起着重要的引发作用。同时通过飞行时间质谱和液体红外吸收光谱等手段检测到两种铬配合物与DMAP的配位方式,以及两种反应体系催化CO_2与PO的诱导期和链增长速度上都存在着显著的差异,在此基础上进一步提出了SalanCr(Ⅲ)X/DMAP作为催化体系时的聚合反应机理。
The physical properties of a polymer are determined mainly by the relative stereochemistry(the spatial arrangement of atoms or groups in a polymeric unit) of adjacent locations in the polymeric chains.As a result,the control of polymer microstructure is one of the most important goals in the area of stereoselective polymerization catalysis.There are much interested in CO_2/aliphatic epoxides(such as propylene oxide) copolymerization,which was first reported by Inoue in 1969.Recently,our group reported that the binary catalyst systems composed of the chiral tetradentate Schiff base cobalt complex(SalenCo(Ⅲ)X) as the electrophlie and ionic ammonium salt or sterically hindered strong base as the nucleophile can effectively promote the asymmetric copolymerization of CO_2 and racemic propylene oxide(PO) to produce poly(propylene carbonate)(PPC) with excellent activity,relative high enantioselectivity(ee%～70%),＞95%head-to-tail connectivity,and＞99%carbonate linkages. In sharp contrast to the regio-and stereoeslective polymerization catalyzed by chiral SalenCo(Ⅲ)X complexes,although chromium Salen derivatives were also proved to be an effective catalyst for the coupling of PO and CO_2 to afford linear PPC,the polycarbonate produced is regioirregular,and the catalyst activity and polymer enantioselectivity is also very poor.
In this dissertation,it is found that various poly(propylene carbonate)s(PPC)s with different head-to-tail linkages(from～60%to 93%) could be synthesized by the copolymerization of PO and CO_2 with chromium Salen complexes as catalyst.Altering the axial X group and Salen ligand framework of the chromium complexes or/and the addition of cocatalyst significantly affect stereochemistry of the resulting polymers.A SalenCrX complex with a low nucleophilic axial X anion and a conjugated cyclic diamine backbone in the Salen ligand,combined with a sterically hindered strong organic base,is an ideal binary catalyst system for the copolymerization of CO_2 and PO to produce polycarbonate with relative high head-to-tail connectivity and＞99%carbonate linkages.Continuous determination of a polymer chain end group(initiating and chain growth species) at various times by electrospray ionization mass spectrometry(ESI-MS) indicates that the sterically hindered strong organic base predominately plays an initiator of polymer-chain growth in the copolymerization process.Apart from the effect of the ligand set of the chromium Salen complexes on polymer stereochemistry,a "polymer chain-end control" mechanism also significantly affects the stereochemistry of the resulting polycarbonates.
Chiral SalanCr(Ⅲ)X complexes(a saturated version of SalenCr(Ⅲ)X) are designed,and in conjunction with an ionic quaternary ammonium salt or organic strong base can efficiently catalyze the asymmetric,regio-and stereoselective alternating copolymerization of CO_2 with rac-PO at mild conditions to afford isotactic-enriched polycarbonates with～95%head-to-tail linkages and moderate enantioselectivity.The facts indicate that apart from the asymmetric induction of chiral diamine backbone,sterically hindered substitute groups on the aromatic rings,and the presence of sp~3-hydridized amino donors and its N,N'-disubstituted groups in chiral SalanCr(Ⅲ)X complexes all play important roles in controlling polymer stereochemistry and enantioselectivity.The stereochemical studies of PPCs derived from the copolymerization of CO_2 with chiral PO at various conditions reveal that the polymer enantioselectivity is proportional to its head-to-tail linkages with regard to regioselective ring-opening of the epoxide.
These new catalyst systems predominantly exceed the previously much-studied SalenCr(Ⅲ)X systems in catalytic activity,polymer enantioselectivity and stereochemistry control.Continuous determination of a polymer chain end group(initiating and chain growth species) at various times by electrospray ionization mass spectrometry(ESI-MS) indicates that DMAP predominately plays an initiator of polymer-chain growth in the copolymerization process.Electrospray ionization mass spectrometry(ESI-MS) in combination with FT-IR spectra study suggest that the coordination modes of these two chromium complexes with DMAP and the induction periods of CO_2/PO copolymerization and the rate of polymer-chain growth with these two catalyst systems are very different.Furthermore,a straightforward approach to insight into the details of copolymerization of CO_2 and epoxide by SalanCr(Ⅲ)X/DMAP catalyst system is provided.
引文
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