Synthese und Charakterisierung von Homo- und Blockcopolymeren aus 5-(N,N-Dimethylamino)isopren
Ruth Bieringer (01/1999-01/1999)
Support: Axel H. E. Müller
The present work focuses on the homo- and block copolymerization of 5-(N,N-dimethylamino)-isoprene (IUPAC: N,N-dimethyl-2-methylene-3-butene-1-amine), a comparatively new and up to date rarely investigated monomer. 5-(N,N-dimethylamino)-isoprene can be polymerized following anionic polymerization techniques and the polar tertiary amine functionality besides promising interesting properties can serve as a site for further chemical modification.
Through the alkyl-lithium initialized anionic polymerization of 5-(N,N-dimethylamino)-isoprene in toluene highly regio- and stereoregular polymers are obtained. Thus the monomer units of polymers with molecular weights of which exceeding 25 kg/mol are linked almost exclusively via 4,1-enchainment of which 95 % are found to be in the trans configuration. Polymerization kinetics as well as GPC results showing that poly[5-(N,N-dimethylamino)-isoprene)s possess molecular weight distributions ranging from 1.2 to 1.4 indicate a complex growth mechanism with differently associated species being involved.
Poly[5-(N,N-dimethylamino)-isoprene] synthezised in this way is semicrystalline having a glass transition temperature of 30 °C and a broad melting range at 70 °C. DSC, WAXS as well as polarized light microscopy proves the existence of two different crystalline modifications. The observed polymorphism is very similar to the one known from structurally related trans-polyisoprenes.
With the help of sequential anionic polymerization 16 triblock copolymers consisting of 5-(N,N-dimethylamino)-isoprene, styrene and tert.-butyl methacrylate are synthezised with varying compositions and molecular weights. The polydispersity indices of these triblock copolymers range from 1.23 to 1.65 being thus relatively broad due to the above mentioned broader molecular weight distribution of the PDMAi block and because of the slow exchange reaction from PDMAi to polystyrene in the course of the polymerization.
DSC investigations of these systems show complete phase separation of the PDMAi block in all cases; the necessary resolution for the distinction between the close glass transition temperatures of PS and PtBMA can be achieved neither with DSC nor with rheological methods. The PDMAi block crystallizes in all triblock copolymers where its volume fraction exceeds 12 % provided that the sample has been prepared from solution; polymorphism is encountered in the triblock copolymers likewise. Sample preparation from the melt inhibits the crystallization of the PDMAi block due to conformative restrictions of the neighboring glassy polystyrene block.
Transmission electron microscopy reveals a variety of interesting morphologies in these materials, the interpretation of which is seldom easy because of poor long range order and comparatively high polydispersities. As a result of the relatively small incompatibility between PS and PtBMA as compared to PDMAi core shell structures are observed in a rather broad composition region. In these morphologies PtBMA forms the core and PS the shell of the structure, which is dispersed in a PDMAi matrix.
The quantitative saponification of these triblock copolymers results in triblock copolyampholytes consisting of basic poly[5-(N,N-dimethylamino)-isoprene], hydrophobic polystyrene and acidic polymethacrylic acid. Titrations of the fully protonated materials show two inflection points; analogous to ordinary amino acids the deprotonation of the carboxylic acid functionalities takes place well in advance of the deprotonation of the amine hydrochloride functionalities. At the isoelectric point a thermally stabilized polyelectrolyte complex is formed as a result of strong electrostatic interactions of the two polyelectrolyte endblocks of the polyzwitterion. The pH-dependent aggregation behavior of these triblock copolyampholytes in solution is exemplarily investigated on Ai57S11MAA32 using freeze fracture and dynamic light scattering techniques. Monodisperse vesicles are obtained for a pH-value of 4 as well as 11. Whether miscibility of the two polyelectrolyte endblocks is induced at the isoelectric point as a consequence of the negative mixing enthalpy due to strong electrostatic interactions can not be proven unambiguously.
Using trimethyloxonium tetrafluoroborate the methylation of the amine function of poly[5-(N,N-dimethylamino)-isoprene] can easily be achieved together with the saponification of the poly(tert.-butyl methacrylate) if appropriate reaction conditions are chosen to yield triblock copolymers with a quaternized amine block.