Department of Chemistry, Room 6-331, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
Acc Chem Res. 2014 Aug 19;47(8):2457-66. doi: 10.1021/ar500139s. Epub 2014 Jun 6.
Some of the most readily available and inexpensive monomers for ring-opening metathesis polymerization (ROMP) are norbornenes or substituted norbornadienes. Polymers made from them have tacticities (the stereochemical relationship between monomer units in the polymer chain) that remain after the C═C bonds in the polymer backbone are hydrogenated. Formation of polymers with exclusively a single structure (one tacticity) was rare until approximately 20 years ago, when well-defined ROMP catalysts based on molybdenum imido alkylidene complexes that contain a chiral biphenolate or binaphtholate ligand were shown to yield cis,isotactic-poly(2,3-dicarbomethoxynorbornadiene) and related polymers through addition of the monomer to the same side of the M═C bond in each step. Over the past few years, molybdenum and tungsten monoaryloxide pyrrolide (MAP) imido alkylidene initiators have been found to produce cis,syndiotactic polynorbornenes and substituted norbornadienes through addition of the monomer to one side of the M═C bond in one step followed by addition to the other side of the M═C bond in the next step. This "stereogenic metal control" is possible as a consequence of the fact that the configuration of the stereogenic metal center switches with each step in the polymerization. Stereogenic metal control also allows syndiotactic polymers to be prepared from racemic monomers in which enantiomers of the monomer are incorporated alternately into the main chain. Because pure trans polymers have not yet been prepared through some predictable mechanism of stereochemical control, it seems unlikely that all four basic polymer structures from a single given monomer can be prepared simply by choosing the right initiator. However, because tactic, and relatively oxygen-stable, hydrogenated polymers are often a desirable goal, the ability to form pure cis,isotactic polymers (through enantiomorphic site control) and cis,syndiotactic polymers (through stereogenic metal control) is sufficient for preparing hydrogenated polymers with a single structure. It is hoped that the principles of forming polymers that have a single structure through ring-opening metathesis polymerization will be general for a relatively large number of monomers and that some important problems in ROMP polymer chemistry can benefit from knowledge of polymer structure at a molecular level. With an increase in knowledge concerning the mechanistic details of polymerization by well-defined initiators, more elaborate ROMP polymers and copolymers with stereoregular structures may be possible.
对于开环易位聚合（ROMP），一些最容易获得和最便宜的单体是降冰片烯或取代降冰片二烯。由它们制成的聚合物在聚合物主链中的 C═C 键加氢后仍具有立构规整性（聚合物链中单体单元的立体化学关系）。在大约 20 年前，基于包含手性联苯酚或联萘酚配体的钼亚氨基烷基化物的确定的 ROMP 催化剂的出现之前，形成具有单一结构（单一立构规整性）的聚合物是罕见的，这些催化剂通过在每个步骤中向 M═C 键的同一侧添加单体来生成顺式、全同立构-聚（2,3-二羧酸甲酯降冰片二烯）和相关聚合物。在过去的几年中，发现钼和钨单芳氧代吡咯烷（MAP）亚氨基烷基化物引发剂可以通过在一个步骤中向 M═C 键的一侧添加单体，然后在下一个步骤中向 M═C 键的另一侧添加单体来制备顺式、间同立构聚降冰片烯和取代降冰片二烯。这种“手性金属控制”是可能的，因为在聚合过程中每一步都伴随着手性金属中心的构型切换。手性金属控制还允许从外消旋单体中制备间同立构聚合物，其中单体的对映异构体交替掺入主链中。由于通过某种可预测的立体化学控制机制尚未制备出纯反式聚合物，因此似乎不太可能通过选择合适的引发剂简单地制备出来自单个给定单体的所有四种基本聚合物结构。然而，由于 tactic 和相对稳定的氢化聚合物通常是一个理想的目标，因此形成纯顺式、全同立构聚合物（通过对映体选择性位控制）和顺式、间同立构聚合物（通过手性金属控制）的能力足以制备具有单一结构的氢化聚合物。人们希望通过开环易位聚合形成具有单一结构的聚合物的原则对于相对大量的单体是通用的，并且 ROMP 聚合物化学中的一些重要问题可以从分子水平的聚合物结构知识中受益。随着对通过确定的引发剂聚合的机械细节的了解的增加，可能会有更精细的 ROMP 聚合物和具有立体规则结构的共聚物。
