The synergy of experiments and modeling to identify all monomer sequences and functional groups in copolymerization processes

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A detailed mechanistic study of bulk MADIX of styrene and its chain extension

The microstructural evolution of individual macrospecies during bulk macromolecular design by interchange of xanthates (MADIX) of styrene with (O-ethyl xanthate)-2-ethyl propionate as an initial agent (R0X) and its chain extension with fresh styrene or n-butyl acrylate (nBuA) is visualized in silico, allowing an unbiased (co)polymer product quality labelling according to monomer sequences and end-groups. Degenerative transfer coefficients for both exchange with R0X (Ctr,0) and macro-RAFT agent (Ctr) are reported (Ctr,0 = 0.80 ± 0.02; Ctr: 0.44 ± 0.07) by applying multi-response regression analysis to the experimental data on the RAFT agent and styrene conversion, number and mass average molar masses, and end-group functionality (EGF). The EGF data are obtained by combining dialysis to remove residual R0X species and elemental analysis. It is shown that the MADIX mechanism can be properly understood only by explicitly acknowledging the differences in exchange reactivities and that the macroradical homopolymer CLD follows a Flory–Schulz distribution, which is an exception for controlled reversible addition–fragmentation chain transfer polymerization. Moreover, for the selected monomer conversion ranges, both “blocks” of the chain extension are formed through a single exchange

Model-based design of MADIX under bulk and solution conditions

Macromolecular design by interchange of xanthates (MADIX) is a less studied controlled radical polymerization technique from a mechanistic and modeling point of view. In this contribution, MADIX of styrene and chain extension toward the synthesis of block copolymers is investigated, with azobisisobutyronitrile as conventional radical initiator and O-ethylxanthyl ethyl propionate as initial RAFT agent (R0X). Degenerative transfer coefficients for both the exchange with R0X and macro-RAFT agent are reported and their difference is highlighted to be relevant for the kinetic description. The model validity is supported by measurement of end-group functionality (EGF) data considering elemental analysis. Novel mechanistic insights are that in contrast to typical reversible addition fragmentation chain transfer (RAFT) polymerizations the macroradical CLD follows a Schulz-Flory distribution and that both during the homopolymerization and the chain extensions an exchange, so with monomer incorporation, only takes place once [1]. [1] D.J.G. Devlaminck, P.H.M. Van Steenberge, M.-F. Reyniers, D.R. D’hooge, Polym Chem. 2017, 8, 694

Deterministic modeling of degenerative RAFT miniemulsion polymerization rate and average polymer characteristics : invalidity of zero–one nature at higher monomer conversions

The polymerization rate and average polymer characteristics of degenerative reversible addition–fragmentation chain transfer (RAFT) miniemulsion polymerization of methyl methacrylate with cyanoprop-2-yl dithiobenzoate as initial RAFT agent (R0X) and potassium persulfate as initiator are studied at 333 K up to monomer conversions of 95%, considering a two-dimensional Smith–Ewart model. This model accounts for the number of macroradicals and R0 radicals per nanoparticle, an average particle size between 50 and 500 nm, targeted chain lengths (TCLs) between 50 and 600, exit/entry of R0 radicals, and the possible influence of diffusional limitations on termination and RAFT transfer at the microscale. The accuracy of the microscale model parameters is highlighted by a successful description of bulk literature data, and the interphase mesoscale parameters are determined based on literature miniemulsion data at various average particle sizes. It is demonstrated that at high monomer conversions it is not afforded to assume zero–one kinetics due to diffusional limitations on termination. With larger average chain lengths this deviation is more pronounced and further accelerated by diffusional limitation on RAFT transfer. Even though the miniemulsion kinetics are faster than the bulk counterpart, retardation due to consecutive entry/exit events of R0 radicals can be observed as long as R0X is present

New kinetic and mechanistic insights for MADIX of styrene and its chain extension

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