Toward the Copolymerization of Propylene with Polar Comonomers,Accounts of Chemical Research

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Toward the Copolymerization of Propylene with Polar Comonomers
Accounts of Chemical Research ( IF 16.4 ) Pub Date : 2020-11-26 , DOI: 10.1021/acs.accounts.0c00628
Stephen L. J. Luckham ₁ , Kyoko Nozaki ₁

Affiliation

Polyolefins are produced in vast amounts and are found in so many consumer products that the two most commonly produced forms, polyethylene (PE) and polypropylene (PP), fall into the rather sparse category of molecules that are likely to be known by people worldwide, regardless of their occupation. Although widespread, the further upgrading of their properties (mechanical, physical, aesthetic, etc.) through the formation of composites with other materials, such as polar polymers, fibers, or talc, is of huge interest to manufacturers. To improve the affinity of polyolefins toward these materials, the inclusion of polar functionalities into the polymer chain is essential. The incorporation of a functional group to trigger controlled polymer degradation is also an emerging area of interest. Currently practiced methods for the incorporation of polar functionalities, such as post-polymerization functionalization, are limited by the number of compatible polar monomers: for example, grafting maleic anhydride is currently the sole method for practical functionalization of PP. In contrast, the incorporation of fundamental polar comonomers into PE and PP chains via coordination insertion polymerization offers good control, making it a highly sought-after process. Early transition metal catalysts (which are commonly used for the production of PE and PP) display poor tolerance toward the functional groups within polar comonomers, limiting their use to less-practical derivatives. As late transition metal catalysts are less-oxophilic and thus more tolerant to polar functionalities, they are ideal candidates for these reactions. This Account focuses on the copolymerization of propylene with polar comonomers, which remains underdeveloped as compared to the corresponding reaction using ethylene. We begin with the challenges associated with the regio- and stereoselective insertion of propylene, which is a particular problem for late transition metal systems because of their propensity to undergo chain walking processes. To overcome this issue, we have investigated a range of metal/ligand combinations. We first discuss attempts with group 4 and 8 metal catalysts and their limitations as background, and then focus on the copolymerization of propylene with methyl acrylate (MA) using Pd/imidazolidine–quinolinolate (IzQO) and Pd/phosphine–sulfonate (PS) precatalysts. Each generated regioregular polymer, but while the system featuring an IzQO ligand did not display any stereocontrol, that using the chiral PS ligand did. A further difference was found in the insertion mode of MA: the Pd/IzQO system inserted in a 1,2 fashion, while in the Pd/PS system a 2,1 insertion was observed. We then move onto recent results from our lab using Pd/PS and Pd/bisphosphine monoxide (BPMO) precatalysts for the copolymerization of propylene with allyl comonomers. These P-stereogeneic precatalysts generated the highest isotacticity values reported to date using late transition metal catalysts. This section closes with our work using Earth-abundant nickel catalysts for the reaction, which would be especially desired for industrial applications: a Ni/phosphine phenolate (PO) precatalyst yielded regioregular polypropylene with the incorporation of some allyl monomers into the main polymer chain. The installation of a chiral menthyl substituent on the phosphine allowed for moderate stereoselectivity to be achieved, though the applicable polar monomers currently remain limited. The Account concludes with a discussion of the factors that affect the insertion mode of propylene and polar comonomers in copolymerization reactions, beginning with our recent computational study, and finishing with work from ourselves and others covering both comonomer and precatalyst steric and electronic profiles with reference to the observed regioselectivity.

中文翻译：

丙烯与极性共聚单体的共聚

聚烯烃的生产量很大，并且在如此众多的消费产品中被发现，以至于两种最常见的生产形式，聚乙烯（PE）和聚丙烯（PP）属于相当稀少的分子类别，全世界人们可能都知道，不管他们的职业。尽管已经广泛使用，但通过与其他材料（例如极性聚合物，纤维或滑石粉）形成复合材料来进一步提高其性能（机械，物理，美观等）仍是制造商的极大兴趣。为了提高聚烯烃对这些材料的亲和力，必须在聚合物链中包含极性官能团。引入官能团以引发受控的聚合物降解也是关注的新兴领域。引入极性官能团（例如聚合后官能化）的当前实践方法受到相容的极性单体数量的限制：例如，接枝马来酸酐目前是PP进行实际官能化的唯一方法。相反，通过配位插入聚合将基本的极性共聚单体掺入PE和PP链可提供良好的控制，使其成为备受追捧的过程。早期过渡金属催化剂（通常用于生产PE和PP）显示出对极性共聚单体中官能团的耐受性差，从而将它们的使用限制为不太实用的衍生物。由于后期过渡金属催化剂的亲氧性较低，因此对极性官能度的耐受性更高，因此它们是这些反应的理想选择。该报告着重于丙烯与极性共聚单体的共聚，与使用乙烯的相应反应相比，丙烯共聚仍未开发。我们从与丙烯的区域和立体选择性插入相关的挑战入手，这对于后期过渡金属系统来说是一个特殊的问题，因为它们倾向于经历链走过程。为了克服这个问题，我们研究了多种金属/配体组合。我们首先讨论使用第4和第8组金属催化剂的尝试及其局限性，然后重点讨论使用Pd /咪唑烷-喹啉酸酯（IzQO）和Pd /膦-磺酸盐（PS）预催化剂使丙烯与丙烯酸甲酯（MA）共聚。每个生成的区域规则聚合物，但是，虽然具有IzQO配体的系统没有显示任何立体对照，但使用手性PS配体却可以。在MA的插入模式中发现了另一个区别：以1,2方式插入Pd / IzQO系统，而在Pd / PS系统中观察到2,1插入。然后，我们将使用Pd / PS和Pd /一氧化二膦（BPMO）预催化剂进行丙烯与烯丙基共聚单体共聚的实验室最新结果。使用后期过渡金属催化剂，迄今为止，这些P-立体生成的预催化剂产生了最高的全同立构规整度值。本节结束于我们使用富含地球的镍催化剂进行反应的工作，这对于工业应用而言尤其理想：Ni /膦酚盐（PO）预催化剂可生成规整聚丙烯，并将一些烯丙基单体掺入主聚合物链中。尽管目前适用的极性单体仍然受到限制，但在膦上安装手性薄荷基取代基可实现中等的立体选择性。该帐户最后讨论了影响丙烯和极性共聚单体在共聚反应中插入方式的因素，首先是我们最近的计算研究，然后以我们自己和其他人的工作（涉及共聚单体和前催化剂的空间和电子分布）作为参考。观察到的区域选择性。尽管目前适用的极性单体仍然有限。该帐户最后讨论了影响丙烯和极性共聚单体在共聚反应中插入方式的因素，首先是我们最近的计算研究，然后以我们自己和其他人的工作（涉及共聚单体和前催化剂的空间和电子分布）作为参考。观察到的区域选择性。尽管目前适用的极性单体仍然有限。该帐户最后讨论了影响丙烯和极性共聚单体在共聚反应中插入方式的因素，首先是我们最近的计算研究，然后以我们自己和其他人的工作（涉及共聚单体和前催化剂的空间和电子分布）作为参考。观察到的区域选择性。

更新日期：2021-01-19

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