Sequential Polymerization from Complex Monomer Mixtures: Access to Multiblock Copolymers with Adjustable Sequence, Topology, and Gradient Strength

Abstract

Switchable polymerization shows considerable potential for simulating the molecular precision of natural biopolymers, such as nucleic acids or proteins, to synthesize highly sequence-controlled macromolecules but is mainly limited to three-and four-component systems. To expand the scope to systems with up to five components, we established a reactivity gradient among 12 monomers, including cyclic anhydrides, cyclic esters, and epoxides. Highly selective competitive anhydride/epoxide, selfswitchable cyclic anhydride/epoxide/cyclic ester, and competitive cyclic ester/trimethylene carbonate copolymerizations were achieved using a simple alkyl metal carboxylate catalyst. Anhydrides gave access to gradient copolymers with reactivity ratios of 2 < r 1 < 5, 0.7 > r 2 > 0.3 giving medium-gradient copolymers and r 1 > 400, r 2 < 0.03 giving block copolymers. Further, the anhydride reactivity was predicted using 13 C NMR chemical shifts. This comonomer library will allow more complex copolymer structures with adjustable sequence, topology, and gradient strength to be predicted and prepared.

Understanding differences in rate versus product determining steps to enhance sequence control in epoxide/cyclic anhydride copolymers

“…The nonterminal kinetic model has also been used to successfully describe reaction kinetics of other anionic copolymerization's (ring opening and others). 20,22,25,26 Additionally, if the initial monomer feed ratio ( f monomer ) remains relatively unchanged throughout polymerization, eqn (1) can be used to determine the monomer composition ( F monomer ) based on reactivity ratios and f monomer .…”

“…20 Sataoh and coworkers also observed gradient copolymers from mixtures of one epoxide and two anhydrides, signifying that anhydride choice is vitally important. 22…”

“…20 Sataoh and coworkers also observed gradient copolymers from mixtures of one epoxide and two anhydrides, signifying that anhydride choice is vitally important. 22 First, it was important to see if the YCl 3 •6H 2 O/[PPN]Cl catalytic system could achieve similar block morphologies as Satoh and workers, so polymerization of a comparable set of monomers was pursued first (BO/CPMA/SA, SA = succinic anhydride). The reaction of BO/CPMA/SA was performed at 80 °C using the YCl 3 •6H 2 O/[PPN]Cl system, and utilizing a nonterminal compositional fit model, reactivity ratios were calculated for both anhydrides via analysis of aliquots (r SA = 130 and r CPMA = 0.04).…”

“…20 13 C NMR analysis of cyclic anhydrides, as previously reported in Macromolecules, 2023, 56, 92-103, can give a general qualitative trend of anhydride reactivity ratios. 22 But, as discussed above, the choice of temperature, catalyst, and epoxide will all impact the reactivity ratios and therefore only general trends can be implicated. Scaled up reactions were then conducted to determine if the reactivity ratios of reactions on a smaller scale could inform reactions with more dilute catalyst loadings.…”

“…Satoh and coworkers have shown how multiblock polymers, either block-like or gradient, can be achieved through one-pot mixtures of epoxides/cyclic anhydrides/cyclic esters/cyclic carbonates using simple pivalate salts, with one epoxide and up to six anhydrides. [20][21][22] These studies identified that certain monomer pairs proceeded to polymerization preferentially before others, leading to block copolymer formation. These reports show that there is promise for sequence-controlled block polyesters via ROCOP of epoxides and cyclic anhydrides and that further investigations are warranted.…”