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Unraveling the History and Revisiting the Synthesis of Degradable Polystyrene Analogues Via Radical Ring-Opening Copolymerization with Cyclic Ketene Acetals

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Unraveling the History and Revisiting the Synthesis of Degradable Polystyrene Analogues Via Radical Ring-Opening Copolymerization with Cyclic Ketene Acetals materials Article Unraveling the History and Revisiting the Synthesis of Degradable Polystyrene Analogues via Radical Ring-Opening Copolymerization with Cyclic Ketene Acetals Alexander W. Jackson , Srinivasa Reddy Mothe , Lohitha Rao Chennamaneni , Alexander van Herk * and Praveen Thoniyot * Institute of Chemical and Engineering Sciences (ICES), 1 Pesek Road, Jurong Island, Singapore 627833, Singapore; [email protected] (A.W.J.); [email protected] (S.R.M.); [email protected] (L.R.C.) * Correspondence: [email protected] (A.v.H.); [email protected] (P.T.) Received: 24 April 2020; Accepted: 14 May 2020; Published: 19 May 2020 Abstract: Degradable analogues of polystyrene are synthesized via radical ring-opening (co)polymerization (rROP) between styrene and two cyclic ketene acetals, namely 2-methylene-1,3-dioxepane (MDO) and 5,6-benzo-2-methylene-1,3-dioxepane (BMDO). This approach periodically inserts ester bonds throughout the main chain of polystyrene, imparting a degradation pathway via ester hydrolysis. We discuss the historical record of this approach, with careful attention paid to the conflicting findings previously reported. We have found a common 1H NMR characterization error, repeated throughout the existing body of work. This misinterpretation is responsible for the discrepancies within the cyclic ketene acetal (CKA)-based degradable polystyrene literature. These inconsistencies, for the first time, are now understood and resolved through optimization of the polymerization conditions, and detailed characterization of the degradable copolymers and their corresponding oligomers after hydrolytic degradation. Keywords: cyclic ketene acetal; degradable; radical-ring opening polymerization; styrene 1. Introduction The accumulation of man-made materials in the natural world is a topic of great concern. Currently, the chief culprit is plastic, specifically its increasing presence in our oceans [1]. Reducing, reusing and refurbishing plastic to the utmost extent possible will certainly play a crucial role in minimizing this problem. Nonetheless, there remains a need to develop polymer-based plastics which are easier to chemically breakdown before recycling, or transformation into other building blocks. The ultimate goal for polymer/plastic chemists working in this area is to increase the feasibility of circular economies. Free radical polymerization (FRP) is an extremely attractive approach when synthesizing polymers, as it is relatively cheap, experimentally straightforward and readily performed in aqueous media. FRP can provide access to a wide range of plastic-based materials due to the vast array of tried-and-tested vinyl monomers available. These benefits have resulted in FRP being employed to synthesize approximately 40–45% of all industrial polymers [2]. Perhaps the only drawback to chain-growth FRP is the resulting all carbon–carbon main-chain polymer, which furnishes polymer chains with incredible chemical stability. This chemical robustness is a double-edged sword, as it is responsible for both their highly desirable physical properties, and simultaneously their resistance to chemical breakdown and subsequent poor recyclability. With this in mind, one of the important tasks facing contemporary polymer chemists is to prepare degradable polymers via FRP. One of the essential objectives is to introduce chemically Materials 2020, 13, 2325; doi:10.3390/ma13102325 www.mdpi.com/journal/materials Materials 2020, 13, 2325 2 of 14 Materials 2020, 13, x FOR PEER REVIEW 2 of 13 cleavableto prepare bonds degradable within thepolymers main chainvia FRP. of FRPOne of polymers the essential which objectives can facilitate is to introduce their conversion chemically into functionalcleavable building bonds within blocks, the after main post-use chain of collection. FRP polymers Of course, which thiscan incorporationfacilitate their ofconversion cleavable into links shouldfunctional not significantly building blocks, reduce after the post-use physical collection properties. Of of course, the resultant this incorporation material, a of rather cleavable challenging links balancingshould not act. significantly reduce the physical properties of the resultant material, a rather challenging balancingIn the early act. 1980s, the group of William J. Bailey published pioneering research describing the synthesisIn ofthe linear early aliphatic1980s, the polyesters group of William via radical J. Bailey ring-opening published polymerization pioneering research (rROP) describing of cyclic ketene the acetalssynthesis (CKAs) of (Figurelinear aliphatic1a) [ 3–9]. polyesters The most valuablevia radical aspect ring-opening of this chemistry polymerization is the ability (rROP) to copolymerize of cyclic CKAsketene with acetals conventional (CKAs) (Figure vinyl 1a) monomers, [3–9]. The thereby most va periodicallyluable aspect insertingof this chemistry hydrolytically is the ability cleavable to estercopolymerize bonds within CKAs the with main conventional chain of typically vinyl monomers, all carbon–carbon thereby periodically main-chain inserting polymers. hydrolytically In doing so, a levelcleavable of chemical ester bonds degradability within the can main be impartedchain of ty intopically otherwise all carbon–carbon non-degradable main-chain vinyl polymers polymers. under In FRPdoing conditions. so, a level Since of itschemical discovery, degradability CKA chemistry can be hasimparted been widelyinto otherwise adopted non-degradable to prepare degradable vinyl analoguespolymers of under many FRP conventional conditions. vinylSince polymersits discovery, [10, 11CKA]. Polystyrene chemistry has is abeen common widely building adopted block to forprepare a large rangedegradable of plastic-based analogues materials,of many conventional including food vinyl packaging. polymers Needless [10,11]. toPolystyrene say, the ability is a to common building block for a large range of plastic-based materials, including food packaging. synthesize a degradable polystyrene analogue would be a welcomed development in the effort towards Needless to say, the ability to synthesize a degradable polystyrene analogue would be a welcomed tackling the build-up of plastic in the environment. Indeed, degradable analogues of polystyrene have development in the effort towards tackling the build-up of plastic in the environment. Indeed, been reported via copolymerization between styrene and CKA monomers. Interestingly, the literature degradable analogues of polystyrene have been reported via copolymerization between styrene and reported thus far is full of conflicting outcomes, specifically around the level of CKA incorporation CKA monomers. Interestingly, the literature reported thus far is full of conflicting outcomes, andspecifically subsequently around the degreethe level of polystyreneof CKA incorporation degradability. and This subsequently retrospective the articledegree makes of polystyrene sense of the contradictorydegradability. data This previously retrospective published, article makes by revisiting sense of the contradictory rROP copolymerization data previously between published, styrene 1 andby CKA revisiting monomers. the rROP We copolymerization have found a betwH NMReen styrene characterization and CKA monomers. error, repeated We have throughout found a 1H the existingNMR bodycharacterization of work, that error, has repeated resulted inthroughout a significant the misinterpretation existing body of work, of the CKA-basedthat has resulted degradable in a polystyrenesignificant results. misinterpretation of the CKA-based degradable polystyrene results. FigureFigure 1. 1. (a)( aCyclic) Cyclic ketene ketene acetal acetalradical radicalring-opening ring-opening polymerization polymerization (rROP) affording (rROP) an aliphatic affording anpolyester; aliphatic synthesis polyester; of degradable synthesis of a polystyrene degradable analogue a polystyrene via radical analogue ring-opening via radical copolymerization ring-opening copolymerizationwith (b) 2-methylene-1,3-dioxepane with (b) 2-methylene-1,3-dioxepane (MDO) affording (MDO)poly(MDO- affordingco-styrene) poly(MDO- and (c)co 5,6-benzo-2--styrene) and (c)methylene-1,3-dioxepane 5,6-benzo-2-methylene-1,3-dioxepane (BMDO) affording (BMDO) poly(BMDO- affordingco-styrene). poly(BMDO- co-styrene). Essentially,Essentially, the the copolymerizations copolymerizations previouslypreviously described in in the the literature literature are are performed performed for for extendedextended periods periods of of time time with with high high levels levels of of radical radical initiator, initiator, often often resulting resulting inin aa significantsignificant degreedegree of monomerof monomer composition composition drift. drift. Typically, Typically, styrene styrene is primarilyis primarily consumed consumed first first with with a a very very lowlow levellevel of CKAof CKA incorporation, incorporation, followed followed by by the the polymerization polymerization of of the the CKACKA monomermonomer with with a avery very low low level level of of styrenestyrene incorporation. incorporation. At At first first glance, glance, thethe purifiedpurified samples display display high high levels levels of of ester ester incorporation, incorporation, butbut the the majority majority of of polystyrene polystyrene contains contains aa veryvery lowlow degree of of CKA CKA units. units. In In order order to to truly truly confirm confirm the degree of CKA
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