Polyesters by a Radical Pathway: Rationalization of the Cyclic Ketene Acetal Efficiency

Radical ring-opening polymerization (rROP) of cyclic ketene acetals (CKAs) combines the advantages of both ring-opening polymerization and radical polymerization thereby allowing the robust production of polyesters coupled with the mild polymerization conditions of a radical process. rROP was recently rejuvenated by the possibility to copolymerize CKAs with classic vinyl monomers leading to the insertion of cleavable functionality into a vinyl-based copolymer backbone and thus imparting (bio)degradability. Such materials are suitable for a large scope of applications, particularly within the biomedical field. The competition between the ring-opening and ring-retaining propagation routes is a major complication in the development of efficient CKA monomers, ultimately leading to the use of only four monomers that are known to completely ring-open under all experimental conditions. In this article we investigate the radical ring-opening polymerization of model CKA monomers and demonstrate by the combination of DFT calculations and kinetic modeling using PREDICI software that we are now able to predict in silico the ring-opening ability of CKA monomers.     

A systematic investigation of the ring size effects on the free radical ring-opening polymerization (rROP) of cyclic ketene acetal (CKA) using both experimental and theoretical approach

Radical ring-opening polymerization (rROP) reaction of cyclic ketene acetals (CKA) is an interesting route to biodegradable polymers. Contrary to their tremendous potential, fundamental understanding of their reaction kinetics and thermodynamics is still limited. We present experimental and theoretical investigations for rROP reactions of CKA to systematically elucidate the effects of monomer ring sizes on the homopolymerization. We aim to provide insights on the structural-reactivity relationship of CKA by studying the thermodynamics and kinetics of the forward ring-opening propagation reactions and key side reactions, namely ring-retained propagation and radical back-biting reaction leading to branching. Experimental results show that for the CKA with smaller ring sizes, significant amount of ring-retained side products are formed when up to 90% of the monomers are converted. However, for the larger ring sizes (7 and 8 membered), almost complete ring-opening polymerization with <1% of ring-retained products are formed. Density functional theory (DFT) calculations show that kinetic effects from the collision frequency dominate in differentiating between ring-opening propagation, ring-retained propagation, and backbiting. The results corroborate well with experiments and reports in the literature. Our systematic study from the first principle and experimental validation provide insights into CKA rROP to apply radical polymerization to generate biodegradable polymers.     

Free‐radical polymerization of dioxolane and dioxane derivatives: Effect of fluorine substituents on the ring opening polymerization

Partially fluorinated and perfluorinated dioxolane and dioxane derivatives have been prepared to investigate the effect of fluorine substituents on their free‐radical polymerization products. The partially fluorinated monomer 2‐difluoromethylene‐1,3‐dioxolane (I) was readily polymerized with free‐radical initiators azobisisobutyronitrile or tri(n‐butyl)borane–air and yielded a vinyl addition product. However, the hydrocarbon analogue, 2‐methylene‐1,3‐dioxolane (II), produced as much as 50% ring opening product at 60 °C by free‐radical polymerization. 2‐Difluoromethylene‐4‐methyl‐1,3‐dioxolane (III) was synthesized and its free‐radical polymerization yielded ring opening products: 28% at 60 °C, decreasing to 7 and 4% at 0 °C and −78 °C, respectively. All the fluorine‐substituted, perfluoro‐2‐methylene‐4‐methyl‐1,3‐dioxolane (IV) produced only a vinyl addition product with perfluorobenzoylperoxide as an initiator. The six‐membered ring monomer, 2‐methylene‐1,3‐dioxane (V), caused more than 50% ring opening during free‐radical polymerization. However, the partially fluorinated analogue, 2‐difluoromethylene‐1,3‐dioxane (VI), produced only 22% ring opening product with free‐radical polymerization and the perfluorinated compound, perfluoro‐2‐methylene‐1,3‐dioxane (VII), yielded only the vinyl addition polymer. The ring opening reaction and the vinyl addition steps during the free‐radical polymerization of these monomers are competitive reactions. We discuss the reaction mechanism of the ring opening and vinyl addition polymerizations of these partially fluorinated and perfluorinated dioxolane and dioxane derivatives. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5180–5188, 2004     

Oxygen‐Triggered Switchable Polymerization for the One‐Pot Synthesis of CO2‐Based Block Copolymers from Monomer Mixtures

Switchable polymerization provides the opportunity to regulate polymer sequence and structure in a one‐pot process from mixtures of monomers. Herein we report the use of O₂ as an external stimulus to switch the polymerization mechanism from the radical polymerization of vinyl monomers mediated by (Salen)Co^III?R [Salen=N,N′‐bis(3,5‐di‐tert‐butylsalicylidene)‐1,2‐cyclohexanediamine; R=alkyl] to the ring‐opening copolymerization (ROCOP) of CO₂/epoxides. Critical to this process is unprecedented monooxygen insertion into the Co?C bond, as rationalized by DFT calculations, leading to the formation of (Salen)Co^III?O?R as an active species to initiate ROCOP. Diblock poly(vinyl acetate)‐b‐polycarbonate could be obtained by ROCOP of CO₂/epoxides with preactivation of (Salen)Co end‐capped poly(vinyl acetate). Furthermore, a poly(vinyl acetate)‐b‐poly(methyl acrylate)‐b‐polycarbonate triblock copolymer was successfully synthesized by a (Salen)cobalt‐mediated sequential polymerization with an O₂‐triggered switch in a one‐pot process.     

Radical Copolymerization of Vinyl Ethers and Cyclic Ketene Acetals as a Versatile Platform to Design Functional Polyesters

Free‐radical copolymerization of cyclic ketene acetals (CKAs) and vinyl ethers (VEs) was investigated as an efficient yet simple approach for the preparation of functional aliphatic polyesters. The copolymerization of CKA and VE was first predicted to be quasi‐ideal by DFT calculations. The theoretical prediction was experimentally confirmed by the copolymerization of 2‐methylene‐1,3‐dioxepane (MDO) and butyl vinyl ether (BVE), leading to r _MDO=0.73 and r _BVE=1.61. We then illustrated the versatility of this approach by preparing different functional polyesters: 1) copolymers functionalized by fluorescent probes; 2) amphiphilic copolymers grafted with poly(ethylene glycol) (PEG) side chains able to self‐assemble into PEGylated nanoparticles; 3) antibacterial films active against Gram‐positive and Gram‐negative bacteria (including a multiresistant strain); and 4) cross‐linked bioelastomers with suitable properties for tissue engineering applications.     

Computational evaluation of radical ring‐opening polymerization

The tendencies of ring‐opening processes in radical ring‐opening polymerizations were evaluated by AM1 and PM3 semi‐empirical calculations and 6‐31G*‐level calculations based on the density functional theory (DFT) B3LYP models. Sixteen cyclic monomers bearing vinyl or exomethylene groups were categorized into ring‐opening and no‐ring‐opening monomers by the evaluation of the differences of the internal energies and the lengths of the cleaving bonds between the ground states of the initial radicals and the activated states in the ring‐opening processes. Although the semi‐empirical calculations not parameterized to radical reactions resulted in the moderate categorization of the ring‐opening monomers, the DFT calculation clearly distinguished the ring‐opening and no‐ring‐opening monomers. The ring‐opening tendencies were also evaluated with the changes in the internal energies throughout the ring‐opening processes, but this method could not group the ring‐opening and no‐ring‐opening monomers clearly. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2827–2834, 2007     

Free radical accelerated cationic polymerizations

The simultaneous photoinitiated cationic polymerizations of epoxides and vinyl ethers in the presence of diaryliodonium salt photoinitiators results in an acceleration of the ring‐opening epoxide polymerization and a deceleration of the vinyl ether polymerization. These effects are seen both in mixtures of the two monofunctional monomers as well as in hybrid monomers which bear vinyl ether and epoxide groups in the same molecule. A combination of two mechanisms have been proposed to account for these effects. The reversible conversion of alkoxycarbenium to oxiranium ions results in a two‐stage reaction in which first, the epoxide, then the vinyl ether polymerization takes place. Free radical chain induced decomposition of the diaryliodonium salt produces a large incremental number of carbenium ion species which results in the acceleration effect.     

Synergistic interaction of epoxides and N‐vinylcarbazole during photoinitiated cationic polymerization

A kinetic study was conducted of the independent photoinitiated cationic polymerization of a number of epoxide monomers and mixtures of these monomers with N‐vinylcarbazole. The results show that these two different classes of monomers undergo complex synergistic interactions with one another during polymerization. It was demonstrated that N‐vinylcarbazole as well as other carbazoles are efficient photosensitizers for the photolysis of both diaryliodonium and triarylsulfonium salt photoinitiators. In the presence of large amounts of N‐vinylcarbazole, the rates of the cationic ring‐opening photopolymerization of epoxides are markedly accelerated. This effect has been ascribed to a photoinitiated free‐radical chain reaction that results in the oxidation of monomeric and polymeric N‐vinylcarbazole radicals by the onium salt photoinitiators to generate cations. These cations can initiate the ring‐opening polymerization of the epoxides, leading to the production of copolymers. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3697–3709, 2000     

Evaluation of peroxide initiators for radical polymerization‐based self‐healing applications

The suitability of various peroxide initiators for a radical polymerization‐based self‐healing system is evaluated. The initiators are compared using previously established criteria in the design of ring opening metathesis polymerization‐based self‐healing systems. Benzoyl peroxide (BPO) emerges as the best performing initiator across the range of evaluation criteria. Epoxy vinyl ester resin samples prepared with microcapsules containing BPO exhibited upwards of 80% healing efficiency in preliminary tests in which a mixture of acrylic monomers and tertiary amine activator was injected into the crack plane of the sample after the initial fracture. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2698–2708, 2010     

A New Additive for Controlled Radical Polymerization

A new versatile additive for the controlled radical polymerization of numerous vinyl monomers under mild conditions is presented. The addition of 1,1‐diphenylethene (DPE) to a conventional radical polymerization system consisting of initiator and monomer results in a controlled behavior of the polymerization. Block copolymers are obtained by simply heating the resulting polymers in the presence of a second monomer.     

Ultra‐Fast Synthesis of Multivalent Radical Nanoparticles by Ring‐Opening Metathesis Polymerization‐Induced Self‐Assembly

We report the straightforward, time‐efficient synthesis of radical core–shell nanoparticles (NPs) by polymerization‐induced self‐assembly. A nitroxide‐containing hydrophilic macromolecular precursor was prepared by ring‐opening metathesis copolymerization of norbornenyl derivatives of TEMPO and oligoethylene glycol and was chain‐extended in situ with norbornene in ethanolic solution, leading to simultaneous amphiphilic block copolymer formation and self‐assembly. Without any intermediate purification from the monomers to the block copolymers, radical NPs with tunable diameters ranging from 10 to 110 nm are obtained within minutes at room temperature. The high activity of the radical NPs as chemoselective and homogeneous, yet readily recyclable catalysts is demonstrated through oxidation of a variety of alcohols and recovery by simple centrifugation. Furthermore, the NPs show biocompatibility and antioxidant activity in vitro.     

ABA and Star Amphiphilic Block Copolymers Composed of Polymethacrylate Bearing a Galactose Fragment and Poly(ε‐caprolactone)

Novel ABA and star amphiphilic block copolymers of poly(vinyl sugars) with biodegradable hydrophobic poly(ε‐caprolactone) segments are presented. They were prepared by a combination of ring‐opening polymerization of ε‐caprolactone and atom‐transfer radical polymerization of methacrylate‐bearing isopropylidene‐protected galactose. Subsequently, the protecting groups of the sugar fragments were removed by treatment with 80% formic acid.     

Free radical ring‐opening polymerization of cyclic allylic sulfides: Liquid monomers with low polymerization volume shrinkage

The free radical polymerization of four methylated cyclic allylic sulfides was examined with reference to their polymerization volume shrinkage and the effect of ring size on reactivity. The compounds examined were 2‐methyl‐5‐methylene‐1,3‐dithiane ( 5 ) (solid), 2‐methyl‐6‐methylene‐1,4‐dithiepane ( 6 ) (liquid), 6‐methyl‐3‐methylene‐1,5‐dithiacyclooctane ( 7 ) (liquid), and 6,8‐dimethyl‐3‐methylene‐1,5‐dithiacyclooctane ( 8 ) (liquid). The monomers were stable materials not requiring any special handling or storage conditions. They were polymerized in bulk using thermal azobisisobutyronitrile (AIBN, VAZO88) and photochemical initiators (Ciba DAROCUR 1173) and in benzene solutions (AIBN, 70 °C). The six‐membered ring monomer 5 was unreactive whereas seven‐membered ring monomer 6 polymerized to high conversion in bulk. In addition, 6 did not polymerize in benzene solution at 70 °C at [ 6 ] = 1.25M. Eight‐membered ring monomers 7 and 8 polymerized in bulk to complete conversion with thermal and photochemical initiators to give lightly crosslinked materials. Near complete conversion to soluble polymers could be obtained in solution polymerizations in benzene. Soluble polymers were also obtained in photochemical initiated bulk polymerizations by lowering initiator concentrations or length of irradiation. The methyl substituent had no effect on which allylic carbon–sulfur bond fragmented in the ring‐opening step. The polymerization volume shrinkages of monomers 7 and 8 were 1.5 and 2.4% respectively and together with monomer 4 (1.5–2.0% shrinkage) are the best available liquid free radical ring‐opening monomers that can be polymerized in bulk at room temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 202–215, 2001     

One‐step synthesis of block copolymers using a hydroxyl‐functionalized trithiocarbonate RAFT agent as a dual initiator for RAFT polymerization and ROP

A range of well‐defined block copolymers were synthesized using 4‐cyano‐4‐(dodecylsulfanylthiocarbonyl)sulfanylpentanol (CDP) as a dual initiator for reversible addition‐fragmentation chain transfer (RAFT) polymerization and ring‐opening polymerization (ROP) in a one‐step process. Styrene, (meth)acrylate, and acrylamide monomers were polymerized in a controlled manner for one block composed of vinyl monomers, and δ‐valerolactone (VL), ε‐caprolactone (CL), trimethylene carbonate (TMC), and L ‐lactide (LA) were used for the other block composed of cyclic monomers. Diphenyl phosphate was used as a catalyst for the ROP of VL, CL, and TMC, and 4‐dimethyamino pyridine for the ROP of LA. These catalysts did not interfere with RAFT polymerization and the synthesis of various block copolymers proceeded in a controlled manner. CDP was found to be a very useful dual initiator for a one‐step synthesis of various block copolymers by a combination of RAFT polymerization and ROP. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Expected and unexpected reactions in ring‐opening (co)polymerization

This review covers most of the authors' work on ring‐opening polymerization and copolymerization of heterocyclic monomers during the time of their cooperation since 1985. The mechanistic aspects of anionic ring opening polymerization of cyclic carbonates with a variety of functional groups are described first. By sequential polymerization of first styrene, methyl methacrylate or suitable heterocyclic monomers and then secondly a cyclic carbonate, the site transformation is highlighted. The influence of the chemical nature of macroinitiators with identical active sites on the course of polymerization of cyclic carbonates was studied for poly(ethylene oxide), poly(tetrahydrofuran), and poly(dimethylsiloxane) macroinitiators. For the copolymerization of cyclic carbonates with lactones and lactide the dependence of the polymer microstructure on the polymerization conditions is discussed on the basis of the copolymerization mechanism. The copolymerization of cyclic carbonates with ε‐caprolactam and with tetramethylene urea results in an alternating copolymer, i. e. a poly(ester urethane) and an [m, n]‐polyurethane, respectively, the key step being the insertion of the lactam or the cyclic urea into the carbonate chain. The cationic ring opening polymerization of cyclic six and seven membered carbamates leading to [4]‐ and [5]‐polyurethane with uniform microstructure is reported with respect to kinetic, mechanistic, and thermodynamic aspects. This new access to [n]‐polyurethanes by a chain growth reaction allows the synthesis of well defined polymer architectures with polyurethane sequences. Sequential polymerization of tetrahydrofuran and the cyclic carbamate with mono‐ and bifunctional initiators leads to the respective A–B and B–A–B block copolymers. Site transformation from the oxonium to the immonium active species is the key step in the polymerization mechanism. Finally, mechanistic aspects of the ring‐opening polymerization of cyclic ester‐amides are presented.     

Synthesis of Polyethylene with In‐Chain α,β‐Unsaturated Ketone and Isolated Ketone Units: Pd‐Catalyzed Ring‐Opening Copolymerization of Cyclopropenone with Ethylene

Although various functionalized units can be incorporated into polyolefins by transition metal catalyzed coordination copolymerizations of nonfunctionalized olefins with polar functional monomers, the incorporated functional units are largely limited to a C1 unit from either CO or C2 units from vinyl monomers. Reported here is the Pd‐catalyzed copolymerization of ethylene with cyclopropenone, leading to incorporation of C3 units with functional groups, α,β‐unsaturated ketones, in the chain. Coordination‐insertion of the carbonyl group and ring opening of the strained three‐membered ring are proposed as the key steps in the mechanism. Under different reaction conditions an isolated ketone structure was afforded as the major carbonyl unit, and could be generated by the copolymerization of ethylene with CO formed in situ from cyclopropenone.     

Solvent‐free ring‐opening polymerization of cyclohexene oxide catalyzed by palladium chloride

In this study, we have for the first time demonstrated that palladium chloride (PdCl₂) is an efficient catalyst for ring‐opening polymerization of cyclohexene oxide in a solvent‐free condition. The polymerization product was in atactic structure, and reaction conditions, such as reaction temperature, time, and catalyst amount, showed effects on polymerization conversion yield, turnover number, and number‐average molecular weight of the resulting poly(cyclohexene oxide). PdCl₂ catalysis follows a cationic ring‐opening mechanism. The polymerization result is highly determined by the chemical structure of the monomers.     

Synthesis of hyperbranched polymers by self‐addition free radical vinyl polymerization of photo functional styrene

The hyperbranched polystyrenes are prepared by the self‐addition free radical vinyl polymerization of N,N‐diethylaminodithiocarbamoylmethylstyrene (DTCS). DTCS monomers play an important role in this polymerization system as an inimer that is capable of initiating living radical polymerization of the vinyl group. The compact nature of the hyperbranched macromolecules is demonstrated by viscosity measurements compared to the linear analogues.     

Synthesis of a soluble vinyl‐type polynorbornene with a half‐titanocene/methylaluminoxane catalyst

Norbornene polymerization was performed with monocyclopentadienyltitanium tribenzyloxide activated with methylaluminoxane (MAO). The catalyst afforded a pure vinyl‐type polymer at temperatures below 80 °C and at appropriate MAO concentrations. However, at higher temperatures or high MAO concentrations, a portion of the titanium species was pyrolyzed to form an alkylidene compound that catalyzed the ring‐opening metathesis polymerization of norbornene. As a result, both vinyl‐type and ring‐opening polymers were produced under the reaction conditions. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1421–1425, 2002     

Synthesis of epoxy monomers that undergo synergistic photopolymerization by a radical‐induced cationic mechanism

A series of novel, cycloaliphatic, cationically photopolymerizable epoxide monomers bearing benzyl ether groups were prepared. These monomers display a considerable enhancement in the rate of their cationic ring‐opening polymerizations in comparison with monomers that do not contain such groups. In this article, a synergistic free‐radical mechanism is proposed that accounts for this effect, and supporting evidence is offered for its verification. During UV irradiation of an onium salt cationic photoinitiator, the aryl radicals that are generated abstract labile benzyl hydrogens present in such monomers to generate the corresponding carbon‐centered radicals. Subsequently, these radicals are oxidized to benzyl carbocations by the onium salt via a nonphotochemical chain process. The observed increase in the rate and extent of the cationic ring‐opening polymerization of the epoxide monomers is due to the aforementioned mechanism, which effectively increases the number of reactive cationic species present during polymerization. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3578–3592, 2001