Abstract:The synthesis of different photo-reactive poly(alkenyl norbornenes) and poly(oxonorbornenes) containing benzophenone (BP) via ring-opening metatheses polymerization (ROMP) is described. These polymers are UV irradiated to form well-defined surface-attached polymer networks and hydrogels. The relative propensity of the polymers to cross-link is evaluated by studying their gel content and its dependency on BP content, irradiation wavelength (254 or 365 nm) and energy dose applied (up to 11 J·cm −2 ). Analysis of the UV spectra of the polymer networks demonstrates that the poly(oxonorbornenes) show the expected BP-induced crosslinking behavior at 365 nm, although high irradiation energy doses and BP content are needed. However, these polymers undergo chain scission at 254 nm. The poly(alkenyl norbornenes), on the other hand, do not cross-link at 365 nm, whereas moderate to good cross-linking is observed at 254 nm. UV spectra demonstrate that the cross-linking at 254 nm is due to BP cross-linking combined with a [2 + 2] cylcoaddition of the alkenyl double bonds. This indicates limitations of benzophenone as a universally applicable cross-linking for polymer networks and hydrogels.

Poly(N,N-bis(2-methoxyethyl)acrylamide) (PbMOEAm) featuring two classical chemical motifs from non-ionic water-soluble polymers, namely, the amide and ethyleneglycolether moieties, was synthesized by reversible addition fragmentation transfer (RAFT) polymerization. This tertiary polyacrylamide is thermoresponsive exhibiting a lower critical solution temperature (LCST)–type phase transition. A series of homo- and block copolymers with varying molar masses but low dispersities and different end groups were prepared. Their thermoresponsive behavior in aqueous solution was analyzed via turbidimetry and dynamic light scattering (DLS). The cloud points (CP) increased with increasing molar masses, converging to 46 °C for 1 wt% solutions. This rise is attributed to the polymers’ hydrophobic end groups incorporated via the RAFT agents. When a surfactant-like strongly hydrophobic end group was attached using a functional RAFT agent, CP was lowered to 42 °C, i.e., closer to human body temperature. Also, the effect of added salts, in particular, the role of the Hofmeister series, on the phase transition of PbMOEAm was investigated, exemplified for the kosmotropic fluoride, intermediate chloride, and chaotropic thiocyanate anions. A pronounced shift of the cloud point of about 10 °C to lower or higher temperatures was observed for 0.2 M fluoride and thiocyanate, respectively. When PbMOEAm was attached to a long hydrophilic block of poly(N,N-dimethylacrylamide) (PDMAm), the cloud points of these block copolymers were strongly shifted towards higher temperatures. While no phase transition was observed for PDMAm-b-pbMOEAm with short thermoresponsive blocks, block copolymers with about equally sized PbMOEAm and PDMAm blocks underwent the coil-to-globule transition around 60 °C.

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The thermosensitive aggregation behavior in an aqueous solution of a library of amphiphilic BAB* copolymers is studied, where "A" represents a long permanently hydrophilic poly(N,N-dimethylacrylamide) (pDMAm) block, "B" represents a permanently hydrophobic end with an n-dodecyl chain, and "B*" represents a thermoresponsive (TR) block featuring a lower critical solution temperature (LCST). Four polyacrylamides are employed for B*, namely, poly(N-n-propylacrylamide) (pNPAm), poly(Nisopropylacrylamide) (pNiPAm), poly(N,N-diethylacrylamide) (pDEAm), and poly(N-acryloylpyrrolidine) (pNAP), which differ with respect to the hydrophilicity of their amide side chains and LCST behavior. While blocks A and B were kept constant, the lengths of the TR blocks were varied systematically. These amphiphilic copolymers were studied as a function of concentration and temperature via light and neutron scattering (static light scattering (SLS), dynamic light scattering (DLS), small-angle neutron scattering (SANS)). For sufficiently long pNiPAM and pDEAm blocks (DP n > 40), a pronounced hydrophobic effect at temperatures above the LCST transition results in well-structured, ordered aggregates. Thus, the aggregation can be controlled by the choice and length of the TR block, thereby elucidating a so far hardly explored class of temperature-sensitive polymeric amphiphiles.

Temperature control of rheological properties of aqueous solutions can be achieved by the addition of amphiphilic polymers that show temperature-dependent self-assembly. For this purpose, we explored three sets of acrylamide-based block copolymers with BAB*-, B 2 AB*-, and B(AB*) 2 -type architectures, where "B" represents a permanently hydrophobic unit, "A" is a permanently hydrophilic block, and "B*" is a thermoswitchable block, which undergoes a phase transition of the lower critical solution temperature (LCST) type. Depending on the specific polymer architecture and choice of the thermoresponsive block, the viscosity of their aqueous solutions can augment substantially with increasing temperature. The macroscopic rheological changes were correlated with the results of static and dynamic light scattering (SLS, DLS) and small-angle neutron scattering (SANS) experiments, showing a clear correlation with the mesoscopic organization of the respective systems. Complementary studies with the fluorescence probe Prodan also revealed a clear correlation of the enhanced viscosity to the formation of hydrophobic domains of the thermoresponsive block. Accordingly, the appropriate design of such "smart" copolymer thickeners enables the tuning of the viscoelastic properties of aqueous solutions.