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Step-growth and chain-growth methods are two main relevant mechanisms of polymerization. Polymers with distinctly different structures in terms of repeat unit functionality, molecular weight and polydispersity can be synthesized by these methods. As these molecular structures relate to higher-level macromolecular considerations, such as chain-chain interactions and the development of material morphology, it is important that the mechanism be understood for any system under study¹⁴³. According to Carother’s equation in step-growth polymerization high molar weight (MW) polymers are obtained at high extents of conversion (Fig. S11). This method requires high monomer purity. The excess of any one monomer type limits the MW considerably. In chain-growth mechanism, a reactive intermediate is first created in an initiation step and subsequently propagates via repeated monomer addition to provide a macromolecule.

Chapter II. Chain growth vs step-growth.

Figure S11. Graph of degree of polymerization versus monomer conversion (p) for step-growth (black line) and chain-growth (red line) reaction mechanisms.

For decades the synthetic routes to soluble CPs were dominated by TMC crosscouplings (e.g., Stille and Suzuki couplings) which typically follow a step-growth polycondensation mechanism. CPs prepared via step-growth polycondensations frequently suffer from structural irregularity, a low degree of control over molecular weight, MW distribution and end-group functionality. This often results in batch-to-batch variations and altered material properties, which is undesirable for applications. Simultaneous control over all of these factors is required to achieve reproducible material properties¹⁷⁰. This course changed in 2004, when McCullough¹⁷¹ and Yokozawa¹⁷² independently identified a living, chain-growth method (now referred to as Kumada catalyst-transfer polycondensation (KCTP), also referred to as GRIM) for synthesizing poly(3-hexylthiophene) (Fig. S12). These initial reports sparked a flurry of activity in the field¹⁴⁴.

Nowadays, CTP is a rapidly developing polymerization method, as it allows, in many cases, the abovementioned limitations of step-growth polymerizations to be overcome. CTP provides a straightforward access to well-defined conjugated homopolymers (e.g., polythiophenes, polyfluorenes, polyphenylenes, etc.), alternate donor-acceptor copolymer and all-conjugated block polymers^144,170. An important prerequisite of the chain-growth mechanism in metal-catalyzed polycondensations is the use of AB-type monomers, having two potentially self-reactive groups in the same molecule.

The majority of researchers in the field accept a mechanism that proceeds through a Ni-polymer π-complex, which enables the active catalyst to stay associated with the growing polymer chain and facilitates chain propagation. As for the parent Kumada crosscoupling

Chapter II. Chain growth vs step-growth.

Page 36 reaction, the catalytic cycle of the polycondensation involves oxidative addition (OA), transmetalation, and reductive elimination (RE) elementary steps (Fig. S12).

Figure S12. Proposed chain-growth mechanism for KCTP reaction of P3HT.

Working group of Dr Anton Kiriy has been focusing on developing new methods for controlled preparation of π-conjugated polymers of various architectures since 2007. The published papers pay particular attention to gain a mechanistic understanding of the chain-growth process, surface-initiated polymerization, exploring new catalysts and ligands, synthesizing previously inaccessible materials by Kumada and Suzuki CTPs. It was shown that the Ni(0) species formed at the reductive elimination step associates with the nearest thiophene ring, forming an π-complex, then «walks» toward a growing polymer chain end, and finally inserts into the terminal C–Br bond¹⁷³. Thus, the rare for polycondensations chain-growth mechanism in this case is due to a unique propensity of coordination-unsaturated Ni(0) species to form π-complexes with the polymerized chain and transfer intramolecularly via a «ring-walking» process¹⁷⁴ rather than to diffuse to another chains or monomer molecules, in such a way contributing to the step-growth mechanism.

Ex-situ initiation strategy was developed for the applicability of KCTP in the synthesis of complex architectures of CPs such as polymer brushes and well-defined block-copolymers^175,176. The most universal and selective ex-situ initiation approach utilizes Et2Ni(bipy) as the activator¹⁷⁷. The broad applicability and high functional group tolerance of the Et2Ni(bipy) approach was illustrated by the synthesis of starpolymers¹⁷⁸, rod-coil block copolymers^179,180 and polymer brushes177,181–184.

Chapter II. Chain growth vs step-growth.

Despite the progress with electron-rich thiophene monomer, the polymerization of electron-poor monomers remains a significant challenge. Kiriy at al. extended the scope of chain-growth KCTP to obtain well-defined conjugated DA alternate copolymers comprising of NDI-unit as an acceptor and thiophene as the donor^185,186. The first surface initiated and site specific Suzuki chain-growth polycondensation to fast and selectively graft polyfluorene from functionalized surfaces was reported¹⁸⁷. Chain-growth Suzuki polycondensation proceeds with AB-type monomers having organoboron and halogen groups in the same monomer molecule. To ensure the intramolecular catalyst-transfer mechanism and, hence, the chain-growth propagation, a «stickiness» of the catalyst to the polymerized π-conjugated chain was provided by Pd complex ligated by bulky and electron rich ^tBu3P ligand. It was found to be the best catalytic system for chain-growth Suzuki polycondensation.

A plethora of conjugated chain architectures have been prepared since the discovery of the chain-growth mechanism of Ni-initiated polymerization of P3HT^10,144,170. However, step-growth Stille and Suzuki polymerizations still remain the most universal tool in the synthesis of a variety of CPs. Future studies should focus on broadening the scope of monomers capable of undergoing CTP by exploring alternative metals, ligands, additives, and transmetalating agents. More generally, a greater mechanistic understanding of “failed”

polymerizations should guide these efforts. Among the most promising building blocks to be involved into controlled CTP are, for example, cyclopentadithiophene-, dithienosilole-, benzodithiophene-based monomers, etc., as well as a variety of dyes, such as diketopyrrolopyrroles, indigos and isoindigos, perylene bisimide and squaraine.

Undergoing CTP, some of undesired reactions (e.g., catalyst exchanging between polymer chains, catalyst coordinating with solvents or un/activated monomer, slow precatalyst initiation) can be minimized by adjusting the catalyst concentration, solvent identity or reaction temperature. Even better approach is to modify the catalyst structure, as it has the largest influence over the living, chain-growth behavior. Recent studies have shown that instead of searching for a universal catalyst, one should tune the catalyst’s electronic and steric properties for each monomer. Three parameters can be altered to tune a catalyst’s reactivity: the ancillary ligand, the reactive ligand, and the transition metal¹⁸⁸.

Based on the preceding, this work focuses on the ongoing efforts to polymerize electron-poor DA-monomers, namely bithiophene-naphthalene diimide (NDIT2) and bithiophene-isoindigo (iIT2) in chain-growth manner. For this purpose, highly efficient Pd/P^tBu3-catalyst is to be used and chain-growth reaction behaviour is expected.

Chapter II. State-of-the-Art. Benchmark solution-processable polymers.