Reactive Chain Ends Research Articles

Selective electrochemical degradation of bottlebrush elastomers

AbstractWe introduce a simple synthetic strategy to selectively degrade bottlebrush networks derived from well‐defined poly(4‐methylcaprolactone) (P4MCL) bottlebrush polymers. Functionalization of the hydroxyl groups present at the terminal ends of P4MCL side chains with α‐lipoic acid resulted in bottlebrush polymers having a range of molecular weights (Mn = 45–2200 kg mol−1) and a tunable number of reactive dithiolane chain ends. These functionalized chain ends act as efficient crosslinkers due to radical ring‐opening of the dithiolane rings under UV light. The resulting redox‐active disulfide crosslinks enable mild electrochemical or chemical degradation of the SS crosslinks to regenerate the starting bottlebrush polymer. P4MCL side chains and the disulfides can be degraded simultaneously using harsher reducing conditions. This combination of bottlebrush architecture with facile disulfide crosslinking presents a versatile platform for preparing highly tunable elastomers that undergo controlled degradation under mild conditions.

Design and analysis of biodegradable and potentially biobased multiphase systems from starch and co-polyesters

To develop tailored and performing biodegradable multilayer systems with polyester as cap layers and thermoplastic starch (TPS)/polyester blends in the core, several TPS-based blends, with varying formulations and several biodegradable and potentially biobased polyesters, were first elaborated. Poly(butylene adipate-co-terephthalate) (PBAT), poly(butylene succinate) (PBS) and poly(butylene succinate-co-adipate) (PBSA) were tested. The TPS-based blends for the core layer were compatibilized with a multifunctional epoxide styrene-acrylic reactive chain extender. As evidenced by tensile tests, the compatibilization was more effective with blends based on PBAT, probably due to its higher number of reactive chain ends. Multilayer films comprising neat PBAT cap layers and a TPS/PBAT blend, compatibilized or not, as a core layer were then elaborated by coextrusion. Thanks to the PBAT cap layers, the multilayer films had improved thermal stability and water vapor barrier properties, while TPS in the core layer added oxygen barrier properties to the global systems. In connection with the applications, the biocompatibility was also demonstrated through cell growth tests. The suitability of such multilayer films for biodegradable packaging linked to biomedical applications was thereby confirmed.

Self‐healing and reprocessing of saturated hydroxyl‐terminated polybutadiene‐based networks enabled by dynamic covalent chemistry

AbstractHydroxyl‐terminated polybutadiene (HTPB) is found in many applications due to its ease of manufacturing, useful mechanical properties over a wide temperature range, and reactive hydroxyl chain ends. Typically, HTPB is crosslinked with isocyanates to form polyurethane thermosets. Limitations of this approach include the use of toxic isocyanates and the oxidative instability of backbone alkenes. In this work, saturated HTPB is used to form reprocessable covalent adaptable networks that are capable of stress relaxation and reprocessing, without relying on isocyanates or unstable alkenes. This approach introduces dynamic chemistry to the HTPB network via chain extension and subsequent crosslinking with 4‐methyl caprolactone (4mCL) and a novel bislactone crosslinker. Using benzenesulfonic acid (BSA) as a transesterification catalyst, stress relaxation times range from 150 to 8 min at temperatures of 70 to 100 °C. Despite crosslinking, these networks behave elastically, as evidenced by strain‐at‐break values of 93% for pristine samples, and dynamically, as shown by a strain‐at‐break of 72% after reprocessing the damaged samples. Shape reprogramming is also demonstrated by straining the crosslinked networks and heating to elevated temperatures where bond exchange occurs. These findings illustrate the advantageous properties that can be achieved by using cheap commodity building blocks to achieve dynamic properties. We anticipate that valorizing commodity polymers into reprocessable thermosets will be of utility in applications that lack other viable recycling pathways.

(Third Place Poster Award) Synthesis and Characterization of Block Copolymer Dispersants for Semiconducting Single-Walled Carbon Nanotubes

To leverage the full potential of the outstanding electronic properties of semiconducting single-walled carbon nanotubes (s-CNTs), it is essential to disperse and sort high purity s-CNTs from a heterogeneous mixture. Due to the inherent insolubility of CNTs, various organic dispersing agents that non-covalently interact with CNTs have been successfully used. These dispersants include small molecule surfactants and aromatic compounds, conjugated polymers, DNA, and polyelectrolytes. Of the macromolecular dispersants, polymers with conjugated backbones are widely studied because they can wrap around or strongly adsorb on CNTs to create very stable dispersions in organic solvents. Most of these conjugated polymers are based on polythiophenes or polyfluorenes, which depending on the structure and composition can sort specific chiralities of s-CNTs with over 99.99% semiconducting purity.In this work, we explore conjugated polymer-based block copolymers for sorting and dispersing s-CNTs. Non-conjugated block copolymer-based dispersants have been reported by us and others with varying degrees of success in aqueous and organic solvents. Here we present our recent studies on the synthesis of ABA triblock copolymer-based CNT dispersants where B is the conjugated block and A is the non-conjugated block. The synthesis of the ABA triblock can be achieved by separately synthesizing the rod segments and the coil segments, each with reactive chain end functionalities, and then coupling the rod segments with the coil segments via reaction of the chain ends using, for example, a click reaction. Alternatively, the rod segment or the coil segment can be synthesized first and used as a macroinitiator for the polymerization of the other segment. Each of these two methods offers some advantages and disadvantages. A series of these ABA triblocks with fixed B block length and varying A block lengths were synthesized and evaluated for their ability to sort and disperse ArcD and HiPCO CNTs. A second series of ABA triblocks with fixed A block lengths but increasing B block lengths were also synthesized. We will present the challenges associated with the synthesis and purification of these block copolymer dispersants. As the middle-conjugated B block wraps around the CNTs, we anticipate the two peripheral A blocks to interact with the organic solvent toluene to further solubilize the CNTs. Our studies show that the presence of the A blocks significantly increases the sorting efficiency while preserving the chirality selectiveness of the B block. As the length of the B block increases, the sorting yields can be significantly improved as well. We further discuss the role of the dispersity of the B block if any on the dispersion of s-CNTs.

Suppressing Creep and Promoting Fast Reprocessing of Vitrimers with Reversibly Trapped Amines

AbstractWe report a straightforward chemical strategy to tackle current challenges of irreversible deformation in low Tg vitrimers at operating temperature. In particular, vinylogous urethane (VU) vitrimers were prepared where reactive free amines, necessary for material flow, were temporarily shielded inside the network backbone, by adding a small amount of dibasic ester to the curing mixture. The amines could be released as reactive chain ends from the resulting dicarboxamide bonds via thermally reversible cyclisation to an imide moiety. Indeed, (re)generation of the required nucleophilic amines as network defects ensured reprocessing and rapid material flow at higher temperature, where exchange dynamics are (re)activated. As a result, VU vitrimers were obtained with limited creep at service temperature, yet with good reprocessability at elevated temperatures. Thus, by exerting strong control on the molecular level over the availability of exchangeable functional groups, a remarkable improvement of VU properties was obtained.

Suppressing Creep and Promoting Fast Reprocessing of Vitrimers with Reversibly Trapped Amines.

We report a straightforward chemical strategy to tackle current challenges of irreversible deformation in low Tg vitrimers at operating temperature. In particular, vinylogous urethane (VU) vitrimers were prepared where reactive free amines, necessary for material flow, were temporarily shielded inside the network backbone, by adding a small amount of dibasic ester to the curing mixture. The amines could be released as reactive chain ends from the resulting dicarboxamide bonds via thermally reversible cyclisation to an imide moiety. Indeed, (re)generation of the required nucleophilic amines as network defects ensured reprocessing and rapid material flow at higher temperature, where exchange dynamics are (re)activated. As a result, VU vitrimers were obtained with limited creep at service temperature, yet with good reprocessability at elevated temperatures. Thus, by exerting strong control on the molecular level over the availability of exchangeable functional groups, a remarkable improvement of VU properties was obtained.

Brønsted Acid Catalyzed Stereoselective Polymerization of Vinyl Ethers.

Isotactic poly(vinyl ether)s (PVEs) have recently been identified as a new class of semicrystalline thermoplastics with a valuable combination of mechanical and interfacial properties. Currently, methods to synthesize isotactic PVEs are limited to strong Lewis acids that require a high catalyst loading and limit the accessible scope of monomer substrates for polymerization. Here, we demonstrate the first Brønsted acid catalyzed stereoselective polymerization of vinyl ethers. A single-component imidodiphosphorimidate catalyst exhibits a sufficiently low pKa to initiate vinyl ether polymerization and acts as a chiral conjugate base to direct the stereochemistry of monomer addition to the oxocarbenium ion reactive chain end. This Brønsted acid catalyzed stereoselective polymerization enabled an expanded substrate scope compared to previous methods, the use of chain transfer agents to lower catalyst loading, and the capability to recycle the catalyst for multiple polymerizations.

Cationic living polymerization of cyclic dithiocarbonates involving sulfide-migration

Ring-opening polymerization of cyclic dithiocarbonates was attempted by varying reaction conditions (temperature, concentration, and solvents) to achieve a high polymer conversion. The participation of neighboring sulfide groups in the ring opening was demonstrated by selective sulfide migration and evaluated by composition analysis of the resulting copolymer. The migration was accelerated by electron-enriching substituents on the aryl sulfide and led to the formation of homopolymers with alkyl sulfides. The isomerization of the monomer encountered in the polymerization reduced with sulfide migration and exclusively occurred as substituted by a trityl sulfide. Three catalytic isomerization pathways were proposed for the polymerization mechanism, wherein the electronic and steric contributions of a substituted sulfide group were considered to rationalize the competitive formation of polymers and isomers. The polymerization of a monomer carrying two neighboring groups of butyl sulfide and benzoate proceeded to homopolymerization involving the migration of a sulfide group. The preferential migration was confirmed by the isolation of a six-membered cyclic carbonate as the main fragment product of depolymerization that occurred in the presence of potassium tert-butoxide. A reactive chain end in living polymerization was modified by an in situ reaction with sodium dithiocarbamate. A linear correlation between experimental and theoretical number average molecular weights of the prepared polymers was investigated with polymer conversion variance. The neutralized chain-end improved the chemical stability of the prepared poly(dithiocarbonate)s, and a stable polydispersity was obtained. The thermal decomposition temperature, Td (10% loss), of a polydithiocabonate was measured at 189 °C and the corresponding value after end-capping of the polymer was 285 °C.

Review on transesterification in polycarbonate–poly(butylene terephthalate) blend

AbstractVarious polycarbonate ‐ poly(butylene terephthalate) polyester (PC‐PBT) blend prepared by reactive melt blending primarily in extruder with range of temperature and time are discussed in this review article. In the melt blended PC‐PBT blend system, transesterification plays a major role in formation of PC‐PBT blend. Transesterification reactions between these two polymers have been analyzed by proper polymer sample, end‐capped or having reactive chain end group. The kinetics of mechanism is a function of temperature and PC‐PBT ratio. Trace catalyst residue present in PBT catalyzes the transesterification reaction. As the extent of transesterification is increased, it forms various composition of copolymer. Inhibition in the transesterification reaction of PC‐PBT blend has been observed when different type of alkyl, aryl, and alkyl‐aryl group of phosphite are introduced into it, otherwise there has been collateral change in the morphology, thermal, and mechanical property of blend. Therefore, this PC‐PBT blend has been an interesting topic of research in academia and industries.

Replication of Sequence Information in Synthetic Oligomers.

ConspectusThe holy grail identified by Orgel in his 1995 Account was the development of novel chemical systems that evolve using reactions in which replication and information transfer occur together. There has been some success in the adaption of nucleic acids to make artificial analogues and in templating oligomerization reactions to form synthetic homopolymers, but replication of sequence information in synthetic polymers remains a major unsolved problem. In this Account, we describe our efforts in this direction based on a covalent base-pairing strategy to transfer sequence information between a parent template and a daughter copy. Oligotriazoles, which carry information as a sequence of phenol and benzoic acid side chains, have been prepared from bifunctional monomers equipped with an azide and an alkyne. Formation of esters between phenols and benzoic acids is used as the equivalent of nucleic base pairing to covalently attach monomer building blocks to a template oligomer. Sequential protection of the phenol side chains on the template, ester coupling of the benzoic acid side chains, and deprotection and ester coupling of the phenol side chains allow quantitative selective base-pair formation on a mixed sequence template. Copper catalyzed azide alkyne cycloaddition (CuAAC) is then used to oligomerize the monomers on the template. Finally, cleavage of the ester base pairs in the product duplex by hydrolysis releases the copy strand. This covalent template-directed synthesis strategy has been successfully used to copy the information encoded in a trimer template into a sequence-complementary oligomer in high yield.The use of covalent base pairing provides opportunities to manipulate the nature of the information transferred in the replication process. By using traceless linkers to connect the phenol and benzoic acid units, it is possible to carry out direct replication, reciprocal replication, and mutation. These preliminary results are promising, and methods have been developed to eliminate some of the side reactions that compete with the CuAAC process that zips up the duplex. In situ end-capping of the copy strand was found to be an effective general method for blocking intermolecular reactions between product duplexes. By selecting an appropriate concentration of an external capping agent, it is also possible to intercept macrocyclization of the reactive chain ends in the product duplex. The other side reaction observed is miscoupling of monomer units that are not attached to adjacent sites on the template, and optimization is required to eliminate these reactions. We are still some way from an evolvable synthetic polymer, but the chemical approach to molecular replication outlined here has some promise.

Hyperbranched Macromolecules: From Synthesis to Applications.

Hyperbranched macromolecules (HMs, also called hyperbranched polymers) are highly branched three-dimensional (3D) structures in which all bonds converge to a focal point or core, and which have a multiplicity of reactive chain-ends. This review summarizes major types of synthetic strategies exploited to produce HMs, including the step-growth polycondensation, the self-condensing vinyl polymerization and ring opening polymerization. Compared to linear analogues, the globular and dendritic architectures of HMs endow new characteristics, such as abundant functional groups, intramolecular cavities, low viscosity, and high solubility. After discussing the general concepts, synthesis, and properties, various applications of HMs are also covered. HMs continue being materials for topical interest, and thus this review offers both concise summary for those new to the topic and for those with more experience in the field of HMs.

Modification of Silica Nanoparticles with Miktoarm Polymer Brushes via ATRP

Silica nanoparticles were successfully modified with miktoarm brushes via atom transfer radical polymerization (ATRP) using three different approaches. In the first approach: “graft onto and from”, a poly(tert-butyl acrylate) (PtBA) macroinitiator was grafted onto the surface of a monomer-modified silica nanoparticle. Then, polystyrene (PSt) brush was grafted from the surface-tethered reactive chain end. In the second approach: “two-step reverse ATRP”, the PtBA and poly(n-butyl acrylate) (PBA) brushes were consecutively grafted from initiator-modified silica particles via ATRP. The polymerization was initiated from the silica surface via a two-step controlled thermal decomposition of surface-tethered diazo initiator moieties. In the third method: “diblock first”, a diblock copolymer of poly(tert-butyl acrylate) and poly(glycidyl methacrylate) (PtBA-b-PGMA) was grafted onto amine-modified silica particles. The diblock copolymer was covalently attached to the silica surface via interaction between surface-tethered amine groups and the short reactive block containing glycidyl groups. Next, the polystyrene brushes were grafted from surface-tethered reactive chain end. The materials prepared by three different approaches were characterized using gel permeation chromatography (GPC) and thermogravimetric analysis (TGA). The PtBA brushes were hydrolyzed under acidic conditions to form poly(acrylic acid) (PAA) brushes. The resulting materials were imaged using atomic force microscopy (AFM) and transmission electron microscopy (TEM).

Molecularly imprinted polymers by reversible chain transfer catalysed polymerization

Molecularly imprinted polymers (MIPs) are tailor-made biomimetic receptors obtained by polymerization in the presence of molecular templates. They contain binding sites for target molecules with affinities and specificities on a par with those of antibodies, hormone receptors or enzymes. Mainly synthesized via radical polymerization of vinyl monomers, MIPs have also recently taken advantage of the introduction of modern methods of controlled/living radical polymerization (CRPs). In this paper, we report for the first time the use for MIP synthesis of a recently developed CRP method based on an iodide-transfer polymerization with a reversible activation mechanism, referred to as Reversible chain-Transfer Catalysed Polymerization (RTCP). Using S-propranolol as model template, we show that this technique is a convenient method for the synthesis of MIPs, both in the format of bulk polymers and nanoparticles, yielding polymers with reactive chain ends for an easy surface functionalization.

Investigation of the Local Environment of Hydrophobic End Groups on Polyethylene Glycol (PEG) Brushes Using Fluorometry: Relationship to Click Chemistry Conjugation Reactions on PEG-Protected Nanoparticles.

Targeted nanoparticles often require conjugating targeting ligands to polyethylene glycol (PEG) chains of a nanoparticle's dense protecting corona. "Click" chemistries are commonly employed for their bioorthogonality, with strain-promoted azide-alkyne cycloadditions (SPAAC) increasingly chosen to avoid cytotoxic copper catalysts. However, conjugation becomes compromised if reactive PEG chain ends cannot encounter their reaction counterparts. We use fluorescence to probe the location of Nile Red, methylpyrene, and butylpyrene, dyes with comparable hydrophobicities to SPAAC alkynes (logP = 3.2-5.7), tethered to PEG chains on 100 nm NPs. Using fluorescence peak shifts, we find that Nile Red resides 43% of the time in the 5k PEG corona and 57% at the more hydrophobic nanoparticle core. Increasing the PEG MW to 67k doubles the corona dye fraction to 86% (14% core). More hydrophobic methylpyrene and butylpyrene, monitored with I1/I3 ratios, reside 1% in the corona (99% core). These results explain difficulties with using SPAAC reactions for conjugating large ligands to nanoparticles with PEG coronae.

Stackable, Covalently Fused Gels: Repair and Composite Formation

Combining modeling and experiment, we created multilayered gels where each layer was “stacked” on top of the other and covalently interconnected to form mechanically robust materials, which could integrate the properties of the individual layers. In this process, a solution of new initiator, monomer, and cross-linkers was introduced on top of the first gel, and these new components then underwent living (co)polymerization to form the subsequent layer. We simulated this process using dissipative particle dynamics (DPD) to isolate factors that affect the formation and binding of chemically identical gel as well as incompatible layers. Analysis indicates that the covalent bond formation between the different layers is primarily due to reactive chain-ends, rather than residual cross-linkers. In the complementary experiments, we synthesized multilayered gels using either free radical (FRP) or atom transfer radical polymerizations (ATRP) methods. Polymerization results demonstrated that chemically identical mat...

Transamidation determination and mechanism of long chain-based aliphatic polyamide alloys with excellent interface miscibility

Transamidation between PA1012 and PA612 was investigated with combination techniques including differential scanning calorimetry (DSC), rheometry, nuclear magnetic resonance (NMR) and variable-temperature Fourier transform infrared spectroscopy (VT-FTIR). Based on the increase of storage modulus with sweep time, the presence of reactive chain ends were proved, promoting chain growth in either polyamide and interchange reaction in binary blend at high temperature. NMR and VT-FTIR testing signals sufficiently convinced the expected interchange reaction. Quantitative data analysis of DSC and NMR provided direct evidences for evaluating the exchange degree. Various experimental parameters were systematically considered, including reaction temperature and time, blending composition and categories of polyamides. Raising the reaction temperature or prolonging the reaction time would accelerate the transamidation rate and degree. The interfacial miscibility between two LCPA components could be improved by the formation of copolyamides. This work provides a quantitative evaluating method to detect the transamidation extent between two aliphatic LCPAs that overcomes the traditional characterization limitation which is only feasible for whose polyamide component chain is with large volume structure like benzene ring.

Flow Microreactor Synthesis of Fluorine-Containing Block Copolymers

Living anionic polymerization of perfluoroalkyl methacrylates initiated by 1,1-diphenylhexyllithium was conducted without adding lithium chloride in an integrated flow microreactor system. The high degree of polydispersity index (PID) control was achieved at 0°C (2-(nonafluorobutyl)ethyl and 2-(tridecafluorohexyl)ethyl methacrylates) or −40°C (2,2,2-trifluoroethyl methacrylate). The subsequent reaction of a reactive polymer chain end with alkyl methacrylates or tert-butyl acrylate led to the formation of fluorine-containing block copolymers with narrow PDI.

Self-Assembly Assisted Polypolymerization (SAAP) of Diblock Copolymer Chains with Two Reactive Groups at Its Insoluble End

Preparation of diblock copolymers, (≡,N3)-poly(N-isopropylacrylamide)-b-poly(N,N-dimethylacrylamide) [(≡,N3)-PNIPAM-b-PDMA] and (≡,N3)-polystyrene-b-PNIPAM [(≡,N3)-PS-b-PNIPAM], with reactive alkyne and azide at one end using a trifunctional agent enables us to study how their self-assembly in a selective solvent affects interchain coupling, i.e., the self-assembly assisted polypolymerization (SAAP). As expected, (≡,N3)-PNIPAM-b-PDMA chains self-assemble into a micelle-like core–shell structure with a PNIPAM core in water at 50 °C. The coupling of as many as 17 PNIAPM ends together led to star-like chains, independent of the copolymer concentration, while the coupling efficiencies at lower temperatures (with no self-assembly) and in good solvents are much lower. These star-like chains remember their “birth” state in water and undergo the intrachain contraction to form single-chain micelles instead of large multichain aggregates. On the other hand, (≡,N3)-PS-b-PNIPAM exists as individual chains in THF, a mixture of unimers and micelles in 2-propanol, the core–shell micelles in methanol, and irregular aggregates in water. Only in methanol, the coupling efficiency is notably improved. The addition of water into 2-propanol enhances the self-assembly and so does the interchain coupling. The current study shows that even the solvophobic interaction makes the insoluble blocks less mobile inside the core and decreases the collision probability of reactive chain ends, the self-assembly still concentrates the reactive ends together and assists the coupling if the selective solvent is properly chosen.

Synthesis and temperature gradient interaction chromatography of model asymmetric star polymers by the “macromonomer” approach

We describe herein the synthesis and characterisation of a series of asymmetric three arm polystyrene stars via the “macromonomer” approach. The stars have been designed as model polymers to probe branched polymer dynamics and in particular to establish the chain-length of side-arm which precipitates a change in the rheological properties of the resulting polymers from “linear-like” to “star-like”. Thus, a homologous series of three arm stars have been prepared in which the molar mass of two (long) arms are fixed at 90,000gmol−1 and the molar mass of the remaining (short) arm is varied from below the entanglement molecular weight (Me) to above Me. The arms were prepared by living anionic polymerisation, resulting in well-defined chain lengths with narrow molecular weight distribution. In contrast to the usual chlorosilane coupling approach, the macromonomer approach involves the introduction of reactive chain-end functionalities on each of the arms, either through the use of a functionalised (protected) initiator or a functional end-capping agent, which allows the stars to be constructed by a simple condensation coupling reaction. In this study we will compare the relative efficiency of a Williamson and ‘click’ coupling reaction in producing the stars. Most significantly, although this approach maybe a little more time-consuming than the more common silane coupling reaction, in the present study the “long” arm may be produced in sufficient quantity such that all of the asymmetric stars are produced with long arms of identical molecular weight – the only remaining variable being the molecular weight of the short arm. This will allow for a far more robust interpretation of the resulting characterisation of the dynamic properties. Temperature gradient interaction chromatography was used alongside size exclusion chromatography to characterise the structural dispersity of the resulting stars and establish the degree of structural homogeneity.

Limitations of cyclodextrin‐mediated RAFT homopolymerization and block copolymer formation

ABSTRACTThe design and synthesis of a new hydrophobic monomer, that is, 4‐(tert‐butyl)phenyl 6‐acrylamidohexanoate (TBP‐AA‐HO) and its ability to form supramolecular host/guest complexes with β‐cyclodextrin (CD) is described. The aqueous CD‐mediated reversible addition fragmentation chain transfer (RAFT) polymerization affords molecular masses up to 8600 g mol−1 with polydispersities between 1.2 and 1.4. The surprisingly low molecular weights for higher monomer/chain transfer agent (CTA) ratios are investigated by comparing results obtained from free radical and RAFT radical polymerization in aqueous and organic media. The results indicate a steric hindrance caused by attached CD molecules on the growing polymer chain leading to stagnation of the polymerization process due to a restricted accessibility of the reactive chain end. This hypothesis is supported by matrix‐assisted laser desorption/ionization time of flight mass spectrometry. Furthermore, the CD‐mediated synthesis of amphiphilic diblock copolymers in variable aqueous media is described. Hydrophilic poly(N,N‐dimethylacrylamide) macro‐CTAs with different molecular weights are used to polymerize TBP‐AA‐HO at 50 °C. The diblock copolymers are analyzed by 1H‐nuclear magnetic resonance spectroscopy and size exclusion chromatography. The results confirm the polymer structure and reveal similar limitations of chain growth as observed for the CD‐mediated homopolymerization with a limit of 7000 g mol−1 for efficient chain extension. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2504–2517