Wolpers, Arne

English

In the present thesis, various novel iodine-mediated reversible-deactivation radical polymerization (RDRP) techniques were investigated. In addition, effects of instrumental band broadening (BB) in size-exclusion chromatography (SEC) on molar-mass analysis of polymers were theoretically evaluated. Mechanistically, in iodine-transfer polymerizations (ITPs) and reverse ITPs (RITPs), control of chain-growth is generally achieved by a degenerative chain transfer (DT) of iodine end-groups between two macromolecules. While in ITP systems, iodine is introduced by an iodo chain-transfer agent (CTA), in RITP systems, more highly activated iodo CTAs are generated in situ. In the case of reversible chain-transfer catalyzed polymerizations (RTCPs), a catalyst added to an ITP or RITP system is proposed to lead to an additional reversible chain-transfer mechanism of iodine between a macromolecule and a catalyst molecule, resulting in an improved chain-growth control. In this thesis, for poly(methyl methacrylate) (polyMMA) produced by several RITP-based RTCPs, the beneficial impact of the used catalysts on chain-growth control was confirmed by molar-mass analysis via SEC. End-group analysis via electrospray ionization mass spectrometry (ESI-MS) indicated that chain-growth control is exclusively achieved by terminal iodine atoms, supporting the proposed RTCP mechanism and potentially enabling further polymer processing by end-group conversion reactions. Contribution of the catalysts to either initiation or termination reactions—leading to their undesired depletion or to rate retardation—can also be excluded. In addition to the thorough ESI-MS studies, a strategy was presented to effectively reduce the adverse inhibition period of RITP by the use of a radical-initiator cocktail. In a novel approach, the application of UV initiation was developed for iodine-mediated polymerizations. Systems of n-butyl acrylate (BA), n-butyl methacrylate (BMA) and styrene (St) were irradiated in the presence of an iodo CTA and a UV initiator. Their respective potentials to generate well-defined iodine-functionalized polymer at room temperature was studied. The underlying mechanism and kinetics of the systems were thoroughly investigated both analytically and by kinetic simulations. While UV polymerization of St at room temperature is slow, systems of BA and BMA produced well-defined polymer within a few hours (BMA) or even less (BA). Polymerizations were governed by irradiation, with no polymer generated in the dark periods, demonstrating high potential as photoswitchable “on–off” systems. Furthermore, UV irradiation was shown to be highly beneficial for polymerizations of BMA. Significant incessant cleavage of the weak tertiary carbon–iodine bond in polyBMA and successive ultrafast reversible termination with free iodine constitute an additional reversible deactivation mechanism coexisting with DT, thus boosting chain-growth control. UV initiation at elevated temperatures was presented as a tool to increase polymerization rates while fully retaining molar-mass control. In the second part of this thesis, BB effects in the ubiquitously employed SEC technique were investigated with special regard to the analysis of polymer obtained from (quasi-)living polymerizations ((Q)LPs), such as well-controlled RDRPs. To this end, the influence of several experimental operating parameters on the extent of BB was determined by SEC analysis of narrow-distribution polymer. The shape of BB (the so-called BB function, BBF) is known to be skewed. An isolated quantification of symmetric broadening and asymmetric skewing was achieved by using the exponentially-modified Gaussian (EMG) model as BBF. It was shown that (i) a change of the analyte’s injection mass only affects symmetric broadening and (ii) a change of either the flow rate of the eluent or the column temperature only affects skewing. This correspondence between the two parameters and their independent physical drivers underlines the applicability of the EMG model and is in accordance with existing theories of BB mechanisms. The impact of BB on molar-mass analysis was then evaluated by simulating a series of SEC experiments. This included both (i) the simulation of the calibration process with narrow-distribution polymer standards and (ii) the subsequent molar-mass determination of simulated analytes’ molar-mass distributions (MMDs). For respective sets of simulations, predetermined extents of BB (EMG) were systematically applied. Obtained characteristic values of the analytes’ MMDs—i.e., the number-average molar mass, Mn, and the dispersity, D—for the cases of no BB were then compared to the respective cases of applied BB. It was found that BB, in particular skewing and especially during the calibration process, leads to a significant experimental underestimation of the Mn values. This effect becomes disproportionately more pronounced with larger Mn values. In combination with potentially increasing D values, these trends echo those commonly obtained from the analysis of RDRP systems, suggesting that BB has been making a hitherto unsuspected contribution to deviations from ideal behavior in such systems. The impact on molar-mass determination was well-understood and universally quantified over the complete molar mass range for given extents of BB. Methods to fully or partially correct SEC results by taking into account the examined effects were suggested

eDiss Georg-August-University Göttingen

Advances in chain-growth control and analysis of polymer: boosting iodine-mediated polymerizations and mastering band-broadening effects in size-exclusion chromatography

https://ediss.uni-goettingen.de/bitstream/11858/00-1735-0000-0023-9654-7/1/Diss-ArneWolpers.pdf

Advances in chain-growth control and analysis of polymer: boosting iodine-mediated polymerizations and mastering band-broadening effects in size-exclusion chromatography

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eDiss Georg-August-University Göttingen