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1. Achieving molecular weight distribution shape control and broad dispersities using RAFT polymerizations

   https://doi.org/10.1039/D1PY00399B  

   Rosenbloom, Stephanie I.; Sifri, Renee J.; Fors, Brett P. (January 2021, Polymer Chemistry)

   null (Ed.)

   Reversible addition–fragmentation chain-transfer (RAFT) polymerizations are one of the most versatile and powerful polymerization techniques for the synthesis of complex macromolecular architectures. While RAFT polymerizations often give polymers with narrow molecular weight distributions (MWDs), commodity plastics often have broad MWDs to give targeted properties and processability. Thus, new methods to precisely control both MWD breadth and shape are essential for fine-tuning polymer properties for next generation materials. Herein, we report a simple method for controlling polymer MWD features in thermally activated radical RAFT and redox activated cationic RAFT polymerizations by means of metered additions of chain transfer agents. 

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2. Predictive design of polymer molecular weight distributions in anionic polymerization

   https://doi.org/10.1039/c9py00074g  

   Domanskyi, Sergii; Gentekos, Dillon T.; Privman, Vladimir; Fors, Brett P. (January 2020, Polymer Chemistry)

   Molecular weight distributions (MWD) have a substantial impact on a diverse set of polymer physical and rheological properties, from processability and stiffness to many aspects of block copolymer microphase behavior. The precise MWD compositions of these polymers can be modularly controlled through temporal initiation in anionic polymerizations by metered addition of a discrete initiating species. With the technique described in this work, we identify initiator addition profiles through theoretical modeling which can be used to prepare any desired arbitrary MWD. This kinetic model reproduces experimental MWDs with high fidelity. Our modeling strategy incorporates a detailed kinetic description of polymer initiation and propagation, including the association and dissociation equilibria of the living polymer chain ends. We simplify the kinetic model by incorporating the aggregation phenomena into an effective propagation rate constant k p , allowing it to vary with the polymer chain length ( i ). Importantly, this model also yields the ability to predict MWDs at any arbitrary value of monomer conversion during the polymerization. Lastly, we simulate MWDs for a variety of new, yet unmeasured, initiator addition profiles, demonstrating the predictability of this approach. 

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3. General route to design polymer molecular weight distributions through flow chemistry

   https://doi.org/10.1038/s41467-020-16874-6  

   Walsh, Dylan_J; Schinski, Devin_A; Schneider, Robert_A; Guironnet, Damien (June 2020, Nature Communications)

   Abstract The properties of a polymer are known to be intrinsically related to its molecular weight distribution (MWD); however, previous methodologies of MWD control do not use a design and result in arbitrary shaped MWDs. Here we report a precise design to synthesis protocol for producing a targeted MWD design with a simple to use, and chemistry agnostic computer-controlled tubular flow reactor. To support the development of this protocol, we constructed general reactor design rules by combining fluid mechanical principles, polymerization kinetics, and experiments. The ring opening polymerization of lactide, the anionic polymerization of styrene, and the ring opening metathesis polymerization are used as model polymerizations to develop the reactor design rules and synthesize MWD profiles. The derivation of a mathematical model enables the quantitative prediction of the experimental results, and this model provides a tool to explore the limits of any MWD design protocol. 

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4. Effect of Molecular Weight on the Morphology of a Polymer Semiconductor–Thermoplastic Elastomer Blend

   https://doi.org/10.1002/aelm.202201055  

   Peña‐Alcántara, Amnahir; Nikzad, Shayla; Michalek, Lukas; Prine, Nathaniel; Wang, Yunfei; Gong, Huaxin; Ponte, Elisa; Schneider, Sebastian; Wu, Yilei; Root, Samuel_E; et al (January 2023, Advanced Electronic Materials)

   Abstract Polymer semiconductors (PSCs) are essential active materials in mechanically stretchable electronic devices. However, many exhibit low fracture strain due to their rigid chain conformation and the presence of large crystalline domains. Here, a PSC/elastomer blend, poly[((2,6‐bis(thiophen‐2‐yl)‐3,7‐bis(9‐octylnonadecyl)thieno[3,2‐b]thieno[2′,3′:4,5]thieno[2,3‐d]thiophene)‐5,5′‐diyl)(2,5‐bis(8‐octyloctadecyl)‐3,6‐di(thiophen‐2‐yl)pyrrolo[3,4‐c]pyrrole‐1,4‐dione)‐5,5′‐diyl]] (P2TDPP2TFT4) and polystyrene‐block‐poly(ethylene‐ran‐butylene)‐block‐polystyrene (SEBS) are systematically investigated. Specifically, the effects of molecular weight of both SEBS and P2TDPP2TFT4 on the resulting blend morphology, mechanical, and electrical properties are explored. In addition to commonly used techniques, atomic force microscopy‐based nanomechanical images are used to provide additional insights into the blend film morphology. Opposing trends in SEBS‐induced aggregation are observed for the different P2TDPP2TFT4 molecular weights upon increasing the SEBS molecular weight from 87 to 276 kDa. Furthermore, these trends are seen in device performance trends for both molecular weights of P2TDPP2TFT4. SEBS molecular weight also has a substantial influence on the mesoscale phase separation. Strain at fracture increases dramatically upon blending, reaching a maximum value of 640% ± 20% in the blended films measured with film‐on‐water method. These results highlight the importance of molecular weight for electronic devices. In addition, this study provides valuable insights into appropriate polymer selections for stretchable semiconducting thin films that simultaneously possess excellent mechanical and electrical properties. 

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5. A general model for the ideal chain length distributions of polymers made with reversible deactivation

   https://doi.org/10.1039/D1PY01331A  

   Kearns, Madison M.; Morley, Colleen N.; Parkatzidis, Kostas; Whitfield, Richard; Sponza, Alvaro D.; Chakma, Progyateg; De Alwis Watuthanthrige, Nethmi; Chiu, Melanie; Anastasaki, Athina; Konkolewicz, Dominik (February 2022, Polymer Chemistry)

   Polymer molecular weight, or chain length distributions, are a core characteristic of a polymer system, with the distribution being intimately tied to the properties and performance of the polymer material. A model is developed for the ideal distribution of polymers made using reversible activation/deactivation of chain ends, with monomer added to the active form of the chain end. The ideal distribution focuses on living chains, with the system having minimal impact from irreversible termination or transfer. This model was applied to ATRP, RAFT, and cationic polymerizations, and was also used to describe complex systems such as blended polymers and block copolymers. The model can easily and accurately be fitted to molecular weight distributions, giving information on the ratio of propagation to deactivation, as well as the mean number of times a chain is activated/deactivated under the polymerization conditions. The mean number of activation cycles per chain is otherwise difficult to assess from conversion data or molecular weight distributions. Since this model can be applied to wide range of polymerizations, giving useful information on the underlying polymerization process, it can be used to give fundamental insights into macromolecular synthesis and reaction outcomes. 

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