Controlled free-radical polymerization at high pressures: Synthesis and properties of poly(alpha-substituted acrylates).

摘要

The free-radical polymerization of a series of α-substituted acrylic acid derivatives has been achieved by using hydrostatic pressure as a kinetic and thermodynamic driving force. It is shown that conducting polymerization in a range of pressures between 1 and 9 kbar dramatically improves the polymerizabilities of the investigated acrylates, opening the way to the synthesis of high-molecular weight polymers in short reaction times. The polymerizabilities of α-alkylacrylates at ambient pressure, obtained by extrapolation from high-pressure kinetic data, correlate well with Meyer's steric parameters for the relevant α-alkyl group. While the polymerization of α-alkylacrylates with linear alkyl groups proceeded via a vinyl double-bond addition in a traditional way, methyl α-isobutylacrylate polymerized though alternating steps of addition and 1,5-hydrogen transfer from the penultimate unit to provide an isomeric α-branched polymer structure.; Structure-property relationships for the synthesized poly(α-substituted acrylates) have been investigated. Increasing the size of the α-alkyl group decreases thermodynamic stability of the poly(α-alkylacrylates), and facilitates their thermal and photo-degradation. Conformational transitions of poly(α-alkylacrylic acid)s in aqueous solutions are shown to be highly dependent on the size of the α-alkyl group. The transition pH can be fine-tuned by adjusting the total hydrophobic content of the polymer via copolymerization of α-alkylacrylic acid with different alkyl substituents.; A high-pressure reversible addition-fragmentation chain transfer (HP-RAFT) protocol for controlled/living free-radical polymerization has been developed. It is demonstrated that this HP-RAFT technique can be used to livingly polymerize sterically hindered monomers, such as methyl ethacrylate, to provide polymers with low polydispersities, controlled molecular weights and end-groups. A methodology for the synthesis of well-defined poly(ethacrylic acid) has been developed.; The controlled polymerization of polystyrene-methacrylate macromonomers has been achieved by HP-RAFT, providing densely branched comb-like polymers with controlled molecular weight characteristics. The synthesis of linear-comb diblock copolymers is also achievable by this technique.; The HP-RAFT polymerization of traditional monomers, such as methyl methacrylate (MMA), has been investigated. It is demonstrated that the methodology allows for the synthesis of ultra-high molecular weight polymers with low polydispersities. The technique was used to obtain well-defined PMMAs with molecular weights of more than one million.