Pd-Catalyzed CO/Vinyl Arene Copolymerization: when the Stereochemistry is Controlled by the Comonomer

The control of the stereochemistry
of macromolecules is a very
important goal, and coordination-insertion polymerization is superior
with respect to the other polymerization methods for its achievement.
In this contribution, we focus on Pd­(II) homogeneous catalysts for
the stereocontrolled synthesis of CO/vinyl arene polyketones. We developed
a library of aldo- and keto-iminopyridine ligands N-N′ that
feature an α- or β-naphthyl or anthracenyl moiety on the
imino nitrogen atom (Nimm). With such ligands, the Pd­(II)
monocationic complexes [Pd­(CH3)­(CH3CN)­(N-N′)]­[PF6] were synthesized. NMR spectroscopy shows that in solution,
each complex exists as an equilibrium mixture of cis and trans stereoisomers,
the latter having the CH3 ligand opposite to the Pd–Nimm bond. The isomeric population depends on the N-N′
ligand: an almost 1:1 ratio is found for the ketimine complexes, whereas
those with the aldimines show a preference for the trans geometry.
These complexes generate very efficient catalysts for the CO/vinyl
arene copolymerization. Catalyst performances depend both on the nature
of N-N′ and of the vinyl arene comonomer. The ketimine-based
catalysts are more stable and more productive than the aldimine counterpart,
leading to prevailingly syndiotactic macromolecules of high Mw (up to 280 kDa). The aldimine derivatives
produce copolymers with isotactic and syndiotactic stereoblocks of
different lengths depending on the vinyl arene. The effect of the
prochiral monomer on the copolymer tacticity is steric in nature as
demonstrated by the stereochemistry of the obtained CO/4-fluorostyrene
polyketone, whose synthesis is reported here for the first time. As
a conclusion, we have now demonstrated that when catalysts with nonsymmetric
ancillary ligands are used, and stereoisomers are present, the stereochemistry
of the copolymerization is driven by both the catalyst isomeric distribution
and the prochiral comonomer.

高分子的立体化学（stereochemistry）调控是极具重要性的研究目标，而配位插入聚合（coordination-insertion polymerization）在实现该目标方面优于其他聚合方法。本研究聚焦于用于CO/乙烯基芳族聚酮（CO/vinyl arene polyketones）立体可控合成的钯(II)（Pd(II)）均相催化剂（homogeneous catalysts）。我们开发了一类醛亚胺与酮亚胺吡啶配体N-N′，这类配体在亚胺氮原子（Nimm）上带有α-或β-萘基或蒽基基团。基于此类配体，我们合成了单阳离子钯(II)配合物[Pd(CH3)(CH3CN)(N-N′)][PF6]。核磁共振光谱（NMR spectroscopy）表征显示，溶液中每种配合物均以顺式与反式立体异构体（cis and trans stereoisomers）的平衡混合物形式存在，其中反式异构体的甲基配体（CH3 ligand）位于与Pd-Nimm键相对的位置。异构体分布取决于配体N-N′的结构：酮亚胺类配合物的异构体比例接近1:1，而醛亚胺类配合物则更倾向于形成反式构型。此类配合物可作为高效的CO/乙烯基芳族共聚催化剂，其催化性能同时取决于配体N-N′与乙烯基芳族共聚单体（comonomer）的性质。酮亚胺基催化剂相较于醛亚胺基催化剂具有更高的稳定性与催化活性，所制备的高分子以间同立构（syndiotactic）为主，且重均分子量（Mw）可达280 kDa。醛亚胺类配合物所制备的共聚物则带有不同长度的等规立构（isotactic）与间同立构立体嵌段（stereoblocks），其嵌段长度取决于所用的乙烯基芳族单体。前手性单体（prochiral monomer）对共聚物立构规整度（tacticity）的影响本质上为空间位阻效应，这一点可通过本文首次报道合成的CO/4-氟苯乙烯聚酮的立体化学得到证实。综上，本研究证实，当使用带有非对称辅助配体（ancillary ligands）且存在立体异构体的催化剂时，共聚反应的立体化学由催化剂的异构体分布与前手性共聚单体共同决定。