Abstract
The combination of controlled anionic polymerization and subsequent introduction of hydrogen bonding groups was established to form thermo-reversible, supramolecular networks. Several polyisoprene-block-polystyrene-block-polyisoprene (ISI) copolymers—with polystyrene (PS) as the main block, and consequently giving the decisive material characteristics—were synthesized. The novel modification approach to post-functionalize the polyisoprene (PI) end-blocks and to introduce different motifs, which are able to form self-complementary hydrogen bonds, was attained. In the first step, hydroxylation was accomplished using 9-borabicyclo[3.3.1]nonane. Starting from the hydroxylated polymer, esterification with succinic anhydride was implemented to form an ester group with carboxylic end-group (-O-CO-CH2-CH2-COOH). In a second approach, 1,1’-carbonyldiimidazole was used as coupling agent to introduce various types of diamines (diethylenetriamine, triethylentetramine, and 2,6-diaminopyridine) to prepare urethane groups with amine end-group (-O-CO-NH-R-NH2). 1H NMR spectroscopy was used to confirm the successful synthesis and to calculate the degree of functionalization Df. Differential scanning calorimetry (DSC) showed a difference of the glass transition temperature Tg between unfunctionalized and functionalized block copolymers, but no greater influence between the different types of modification, and thus, on the Tg of the PS block. In temperature dependent FTIR spectroscopy, reversible processes were observed.
Highlights
Supramolecular polymers are formed by the assembly of lower-molecular-weight building blocks to macromolecules via non-covalent interactions
Block copolymers were synthesized from styrene (≥99% with 4-tert-butylcatechol as stabilizer, Sigma-Aldrich, Schnelldorf, Germany) and isoprene (99% with 99%, with 3,5-di-tert-butyl-4-hydroxytoluene as stabilizer, VWR chemicals) using sec-butyllithium as initiator
Several asymmetric polyisoprene-block-polystyrene-block-polyisoprene (ISI) triblock copolymers and polystyrene-block-polyisoprene (SI) diblock copolymers were synthesized by sequential anionic polymerization in THF [33]
Summary
Supramolecular polymers are formed by the assembly of lower-molecular-weight building blocks to macromolecules via non-covalent interactions. For their formation different non-covalent interactions can be used. Apart from ligand-metal complexation or π–π-stacking, especially hydrogen bonding is a promising approach [1,2,3]. An important characteristic of supramolecular polymers is their reversibility. Depending on the used modification, a responsiveness towards additives, pH value or temperature is created and a dynamic system is formed. Most of the investigated supramolecular polymers consist of lower-molecular-weight building blocks or of statistically functionalized polymers with higher molecular weights [4,5]. The position of Materials 2018, 11, 1608; doi:10.3390/ma11091608 www.mdpi.com/journal/materials
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