Infoscience

Thesis

Synthesis and characterization of novel polyelectrolytes based on double-charged cationic monomers

Bis-1,3(N,N,N-trimethylammonium)-2-propylmethacrylate dichloride (di-M) and bis-1,3(N,N,N-trimethylammonium)-2-propylacrylate dichloride (di-A), two new double-charged monomers, were the subject of the research reported in this thesis. These monomers provided the opportunity to study polymerization processes while very strong electrostatic interactions are present, under conditions for which data has not yet been reported. Moreover, linear flexible synthetic polyelectrolytes (PEL) with a charge density in the range of DNA molecules became available. The monomers were used to synthesize homo-, co-, and terpolymers and to study their physico-chemical properties. As prerequisite for the polymerization studies, basic characteristics of the monomers have been determined, including the solubility, partial specific volume and the viscosity of monomer solutions as a function of the concentration. The solubility in water is 1.9 mol·L-1 for di-M and 2.1 mol·L-1 for di-A. 2-2'-Azobis(2-methylpropionamidine)dihydrochloride (AMPHC) was selected as most suitable initiator. Screening revealed limited degree of polymerization and higher basic hydrolysis stability for polydi-M than for polydi-A. Homopolymerization and homopolymers. Thermally initiated free radical polymerization was studied in aqueous solution for the monomer concentration range 0.85 to 1.6 mol·L-1 and revealed non-ideality with monomer reaction orders as high as 4.4 for di-M and 3.5 for di-A, and 0.59 for the initiator. Autoacceleration was observed from less than 10% conversion onwards. The onset depends on the monomer and initiator concentration and the total ionic strength. This phenomenon was not observed for any other ionic monomer under similar conditions. Monomer association and electrostatic effects decreasing the free volume during the polymerization are suggested to govern the monomer exponent and autoacceleration. The low overall activation energy, Ea= 31.5 kJ·mol-1 for di-M, and low pre-exponential factor of the Arrhenius equation support this hypothesis. Counterion activity measurements confirmed strong counterion condensation as expected for a charge distance of 0.12 nm. The exponent of an [η]-Mn relationship for polydi-M in 0.5 M NaCl was determined as 0.96. Copolymerization and copolymers. The radical copolymerization of di-M with acrylamide (A), acryloyloxyethyltrimethylammonium chloride (Q), and diallyldimethylammonium chloride (DADMAC) yielded novel PEL and allowed for assessing the influence of the charge density and charge location on the monomer reactivity. The reactivity of A and Q was higher than the reactivity of di-M, however, the difference diminished with increasing ionic strength. The difference of the reactivity ratios for di-M/A was less than for di-M/Q. Despite 4-fold les cationic charge, the reactivity of DADMAC was much lower than di-M demonstrating the dominating influence of the location of the growing radical. The counterion activity correlates well with the linear average charge density. However, deviation from theoretical values suggests an impact of the charge distribution. Terpolymers. The composition of novel terpolymers containing A, Q and di-M was analyzed combining FTIR and potentiometric titration. The analysis revealed deviations from the initial monomer feed composition due to different monomer reactivity. The differences became more pronounced when di-M increased in the monomer feed. Lower intrinsic viscosity for higher fractions of di-M in the feed confirmed the lower reactivity of di-M. The counterion activity was lower than theoretically expected and did not correlate with the average charge density. Heterogeneous charge distribution was hypothesized as reason. The results of the thesis contribute to progress in basic research of PEL, while also an impact on the design of polymerization processes and materials development is expected.

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