Free‐Radical Termination Kinetics Studied Using a Novel SP‐PLP‐ESR Technique

Effect of Very High Charge Density and Monomer Constitution on the Synthesis and Properties of Cationic Polyelectrolytes

The free-radical homopolymerization of 1,3-bis(N,N,N-trimethylammonium)-2-propylmethacrylate dichloride (di-M) and 1,3-bis(N,N,N-trimethylammonium)-2-propylacrylate dichloride (di-A) in aqueous solution yields cationic polyelectrolytes (PEL) with theoretical/structural charge spacing of only ≈0.12 nm. The high charge density causes condensation of ≈82% of the chloride counterions. The high level of counterion condensation reduces the ionic strength in the polymerizing batch when the monomer molecules connect to PEL chains. This has the consequence that the hydrodynamic and excluded volume of the PEL molecules will change. Studies of the free radical polymerization revealed non-ideal polymerization kinetics already at low conversion and additionally autoacceleration above a certain monomer concentration and conversion. Similar autoacceleration was not observed for monomers yielding PEL with charge spacing of 0.25 or 0.5 nm. Coulomb interactions, monomer association, steric effects, and specific features of the monomer constitution have been evaluated concerning their contributions to the concentration dependence and conversion dependence of kinetic parameters. The different backbone constitutions of di-M and di-A not only influence the polymerization kinetics but also equip poly(di-M) with higher hydrolytic stability. The experimental results confirm the impact of electrochemical parameters and the necessity to reconsider their inclusion in kinetic models.

Theoretical Validation of Single-Pulse Pulsed-Laser Polymerization as a Method for Investigating Chain-Length-Dependent Termination

Ideally, single-pulse pulsed-laser polymerization (SP PLP) experiments are marked by a simple relation between time and radical chain length. Thus time-resolved data from such experiments have the potential to directly reflect the chain-length dependence of the termination reaction. However this capability is hostage to various assumptions. These are investigated by performing a series of theory vs. theory comparisons. On the one hand, assumption-free simulations of the kinetics of SP PLP experiments are carried out. The results thus generated are compared with the predictions of equations derived by making assumptions. In this way it is possible to gauge the rectitude of data-analysis methods based on the equations. In turn, the following assumptions are investigated: (1) That of radicals being of length 0 at time 0; (2) That of all radicals terminating at the same rate at any instant; (3) That of transfer being negligible; (4) That of termination being described by a simple power-law; and (5) That of propagation being chain-length independent in rate. All these assumptions are probed in relation to analysis of SP PLP kinetic data, while assumptions (1) and (4) are looked at in relation to the analysis of molecular weight distributions. While some surprising findings do emerge, for the most part it is found that data-analysis methods are reasonably robust, even if they will generally result in underestimation of the true strength of the chain-length dependence of termination. Further, it is possible to make these methods even more accurate by adopting some refinements that are recommended. In general the outstanding potential of SP PLP experiments for investigating the chain-length dependence of the termination reaction is validated.

Chain-Length-Dependent Termination in Acrylate Radical Polymerization Studied via Pulsed-Laser-Initiated RAFT Polymerization

The chain-length dependence of the termination rate coefficient, kt, in methyl acrylate (MA) and dodecyl acrylate (DA) radical polymerization has been determined via the single pulse pulsed-laser polymerization near-infrared reversible addition–fragmentation chain transfer (SP-PLP-NIR-RAFT) technique. Polymerization is induced by a laser SP and the resulting decay in monomer concentration, cM, is monitored via NIR spectroscopy with a time resolution of microseconds. A RAFT agent ensures the correlation of radical chain length and monomer-to-polymer conversion. The obtained rate coefficients for termination of two radicals of approximately the same chain length, i, are represented by power-law expressions, kt(i,i) ∝ i–α. For both monomers, composite model behaviour of kt(i,i) showing two distinct chain length regimes is observed. The exponent αs referring to short chain lengths is close to unity, whereas the exponent αl, which characterizes the chain-length dependency of large radicals, is slightly above the theoretical value for coiled chain-end radicals. The crossover chain length, ic, which separates the two regions, decreases from MA (ic = 30) to DA (ic = 20). The results for MA and DA are consistent with earlier data reported for butyl acrylate. There appears to be a correlation of αs and ic with chain flexibility.