Controlling the stereoselectivity of rac-LA polymerization by chiral recognition induced the formation of homochiral dimeric metal alkoxides

Paweł Horeglad , M. Cybularczyk , A. Litwińska , A. M. Dąbrowska , Maciej Dranka , Grażyna Żukowska , M. Urbańczyk , M. Michalak

Abstract

Using dimeric dialkylgallium and dialkylindium alkoxide catalysts for the polymerization of rac-lactide (rac-LA), we have shown for the first time that the formation of homochiral dimeric species [Me2MOR]2 (M = Ga, In), induced by chiral recognition of monomeric Me2MOR units in the presence of Lewis base, leads to an increase of the heteroselectivity of the ring opening polymerization (ROP) of rac-LA, and therefore provides a new tool for controlling the stereoselectivity of the polymerization of heterocyclic monomers. To explain the origin of the heteroselectivity of the [Me2Ga(μ-OCH(Me)CO2Me)]2/Lewis base system in the ROP of rac-LA, structure of (S,S)-[Me2Ga(μ-OCH(Me)CO2Me)]2 ((S,S)-1) and rac-[Me2Ga(μ-OCH(Me)CO2Me)]2 (1) in the absence and presence of tertiary amines and pyridines was investigated. Studies were further extended by analysis of the structure/activity data for both (S,S)-[Me2In(μ-OCH(Me)CO2Me)]2 ((S,S)-2) and rac-[Me2In(μ-OCH(Me)CO2Me)]2 (2). Contrary to gallium complex 1, which exists in a solution as equimolar mixture of homo- and heterochiral diastereomers, an excess of homochiral (R*,R*)-2 species was observed in the case of 2. For both the Ga and In complexes, the interaction of amines with the metal center increased the tendency for the formation of homochiral species with retention of the dimeric structure in the solution. This tendency was additionally demonstrated by the structure of model dialkylgallium (3) and indium (4) complexes with monoanionic ligands possessing chiral centers in the α-position to the alkoxide oxygen and pyridine functionalities. The polymerization of rac-LA with gallium and indium catalysts (S,S)-1 and (S,S)-2 resulted in the formation of heterotactically enriched polylactide (PLA) (Pr = 0.50–0.85) and (Pr = 0.54–0.72), respectively. The heteroselectivity of the investigated systems was in line with the excess of the homochiral catalytic species. The higher activity of homochiral species activated by amines resulted in a positive non-linear effect between an excess of homochiral (R*,R*)-1 or (R*,R*)-2 catalysts and the heterotacticity of the obtained PLA. The observed dependence of stereoselectivity of rac-LA polymerization on the excess of homochiral species was similar to the asymmetric amplification in enantioselective organic catalysis; however, it is exceptional in polymerization processes.
Author Paweł Horeglad - [Uniwersytet Warszawski (UW)]
Paweł Horeglad,,
-
- Uniwersytet Warszawski
, M. Cybularczyk
M. Cybularczyk,,
-
, A. Litwińska
A. Litwińska,,
-
, A. M. Dąbrowska
A. M. Dąbrowska,,
-
, Maciej Dranka KChNTCS
Maciej Dranka,,
- Chair Of Inorganic Chemistry And Solid State Technology
, Grażyna Żukowska KChNTCS
Grażyna Żukowska,,
- Chair Of Inorganic Chemistry And Solid State Technology
, M. Urbańczyk
M. Urbańczyk,,
-
, M. Michalak
M. Michalak,,
-
Journal seriesPolymer Chemistry, ISSN 1759-9954
Issue year2016
Vol7
No11
Pages2022-2036
Publication size in sheets0.7
DOIDOI:10.1039/c5py02005k
URL http://pubs.rsc.org/en/Content/ArticleLanding/2016/PY/C5PY02005K#!divAbstract
Languageen angielski
File
c5py02005k.pdf 2.99 MB
Score (nominal)45
ScoreMinisterial score = 45.0, 28-11-2017, ArticleFromJournal
Ministerial score (2013-2016) = 45.0, 28-11-2017, ArticleFromJournal
Publication indicators WoS Impact Factor: 2016 = 5.375 (2) - 2016=5.123 (5)
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