Degradable and Recyclable Polymers by Reversible Deactivation Radical Polymerization
2022; Chinese Chemical Society; Volume: 4; Issue: 7 Linguagem: Inglês
10.31635/ccschem.022.202201987
ISSN2096-5745
AutoresMichael R. Martinez, Krzysztof Matyjaszewski,
Tópico(s)Conducting polymers and applications
ResumoOpen AccessCCS ChemistryMINI REVIEW14 Jul 2022Degradable and Recyclable Polymers by Reversible Deactivation Radical Polymerization Michael R. Martinez and Krzysztof Matyjaszewski Michael R. Martinez Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213 Google Scholar More articles by this author and Krzysztof Matyjaszewski *Corresponding author: E-mail Address: [email protected] Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202201987 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Reversible deactivation radical polymerization (RDRP) provides unprecedented control over polymer composition, size, functionality, and topology. Various materials, such as linear polymers, star polymers, branched polymers, graft polymers, polymer networks, and hybrid materials, have been prepared by RDRP. The ability to control polymer topology also enabled precision synthesis of well-defined polymer topologies with degradable functional groups located at specific locations along a polymer chain. This review outlines progress in the synthesis of degradable polymers designed by RDRP, organized by topology and synthetic route. Recent progress in the depolymerization of polymers using RDRP mechanisms is highlighted and critically discussed. Download figure Download PowerPoint Introduction Plastics have become an integral part of our world since their discovery more than 100 years ago. They are used in components of electronics, transportation, packaging, biomedical devices, infrastructure, textiles, and other diverse applications due to their low cost, light weight, and tunable mechanical properties. The ubiquitous nature of plastics is reflected in their history and projections of future growth. More than 360 million metric tons of synthetic plastic was produced in 2018.1 The global plastic market size grew to $579.7 billion (U.S.) in 2020 and is expected to continue growing to $750.1 billion (U.S.) by 2028.2 The exponential growth of the plastics industry has, unfortunately, also led to environmental concerns through its reliance on fossil fuels as feedstocks and pollution into the environment. An estimated 6300 Mt of plastic waste was generated worldwide between 1950 and 2015, and the majority was discarded in landfills or the environment.3 To this extent, we must critically assess recent and past progress in the synthesis of degradable and sustainable high-performance polymers to inform future work in this area.4–7 A significant fraction of commercial synthetic polymers are produced via conventional radical polymerization (RP) of vinyl monomers, such as various olefins, vinyl acetate, vinyl lactams, styrene, acrylates, and methacrylates (Figure 1).8,9 The widespread use of RP is largely enabled by a broad functional group compatibility and tolerance to moisture and protic media. Polymers prepared by RP follow a standard chain growth mechanism consisting of initiation, propagation, transfer, and termination. Radicals are generated by slow decomposition of a radical initiator, followed by rapid propagation, and are terminated by biradical termination or transfer. Slow decomposition of initiator and lack of end-group retainment in the absence of a chain transfer agent (CTA) provide minimal control over the molecular weight and end-group functionality of polymers produced by RP. Figure 1 | Polymerizable monomers by radical polymerization. The monomers denoted with a colored * were reported to be polymerizable by the respective RDRP method. Download figure Download PowerPoint The invention of living ionic polymerizations led to advancements in polymer chemistry through optimization of polymerization kinetics. A living polymerization is a chain-growth polymerization which proceeds in the absence of chain-breaking events.10,11 Polymers produced in a living process can have uniform chain length if the rate of initiation is higher than the rate of propagation. This enables the synthesis of low-dispersity polymers with chain-end functionality, as well as with complex topologies such as brushes, stars, and branched polymers.12,13 However, application of living ionic polymerizations are limited by the sensitivity of the cationic and anionic propagating species to specific functional groups and moisture. A compromise between the robust nature of radical polymerization and precision of a living polymerization was achieved through reversible deactivation radical polymerizations (RDRP). RDRP provides control over molecular structures in radical polymerizations through reversible dissociation and combination with a thermally labile nitroxide adduct in a nitroxide-mediated polymerization (NMP) (Figure 2a), degenerative chain transfer with a CTA in radical addition-fragmentation transfer (RAFT) (Figure 2c), and by halogen atom transfer with a transition metal complex in an atom transfer radical polymerization (ATRP) (Figure 2b).14–18 The fraction of terminated chains is significantly reduced in the presence of a large amount of dormant species. RDRP achieves uniform growth of polymer chains by maintaining a fast rate of initiation relative to propagation. These factors enable precise control over polymer chain length, dispersity, and chain-end functionality for monomers and reaction conditions suitable for radical polymerization. The use of multifunctional initiators enables the synthesis of branched polymer topologies, such as stars and molecular bottlebrushes, by tethering multiple initiators to a core or backbone.8,19–25 Initiators can also be installed on the surface of various inorganic and organic scaffolds in the preparation of various hybrid composites.8,19,22,24,26,27 Figure 2 | The general mechanism of RDRP mediated by (a) reversible decomposition and combination in a NMP; (b) reversible halogen atom transfer in an ATRP; (c) degenerative transfer with a CTA in a RAFT polymerization. Download figure Download PowerPoint There has been significant progress in expanding the monomer scope, environmental impact, and cost efficiency of RDRP methods over the last two decades.4,9,28,29 RDRP of styrene, acrylates, and methacrylates continue to be the state-of-the-art. However, progress has been made in the polymerization of challenging monomers, such as conjugated dienes30–32 and vinyl chloride (Figure 1).33–36 New methods of initiation by photoinduced electron/energy transfer RAFT and photoinduced ATRP enable polymerization under oxygen atmosphere upon exposure to low-intensity visible light.37–39 Catalyst regeneration in the presence of external reducing agents or radical initiators enables well-controlled ATRP with a catalyst concentration as low as 10 ppm.40 Polymers prepared by RDRP are used commercially as high-performance adhesives, sealants, rheology modifiers, surface modifiers, latex binders, chromatographic supports, solid polymer electrolytes, and smart (bio)materials.41–43 The ability to precisely incorporate degradable and reversible bonds into high-performance materials is an attractive opportunity for industry. Polyolefins are often cracked to lower molecular weight waxes and fuels under harsh conditions because they lack the degradable bonds necessary for selective bond scission under mild conditions.44,45 Polyesters, such as poly(lactic acid) and poly (glycolic acid), have beneficial degradable properties, but applications have been limited by their high cost and poor mechanical properties.46,47 There has been recent progress in improving the properties of polyesters which can match the properties of commodity plastics and in the upcycling of plastic waste.48–53 All of these efforts continue to be difficult challenges for our field to overcome.50,54–58 This review focuses on the incorporation of degradable functional groups into high-performance polymers prepared by reversible deactivation radical polymerization. The first portion of this review provides an overview of linear polymers prepared by RDRP of degradable monomers.59 Precise control over end-groups and topologies enables control over both polymer structure and functionality at the junctions of branched polymers. The following sections are organized by polymer topology, starting from linear polymers and then (hyper)branched polymers, star polymers, molecular bottlebrushes, polymer networks, and composites. The final section of this review summarizes recent work on the depolymerization of polymers to monomers by depolymerizations mediated by RDRP mechanisms. Polymerizations and Copolymerizations of Degradable Monomers Radical ring-opening polymerization (RROP) has emerged as a powerful method to install heteroatom-containing functional groups into a vinyl polymer backbone. The resulting copolymers have a statistical amount of (often) hydrolyzable functional groups along the backbone which enables polymer degradation to oligomers with low molecular weight (Scheme 1). Scheme 1 | Degradable copolymer prepared by copolymerization of a vinyl monomer (black dot) with a degradable comonomer (red dot) can be cleaved into lower molecular weight oligomers after scission of the degradable bonds along the backbone. Download figure Download PowerPoint The amount and position of the degradable bonds along a copolymer backbone are dictated by the reactivity ratios of the monomers and the propensity of the monomer to polymerize via vinyl addition or RROP. Incorporation of esters by RROP RROP of cyclic ketene acetals (CKAs) is the most common method used to statistically incorporate ester functionalities into a vinyl polymer backbone.59 Incorporation of ester functional groups enables degradation by hydrolysis along the backbone, which is impossible for vinyl polymers with only C–C bonds in the backbone. The CKA functionality is typically less reactive than other vinyl comonomers, which leads to compositional drift during copolymerization which enriches vinyl repeat unit content at the beginning of the polymerization and ring-opened repeat units near the end of the reaction.60 Thus, degradation of copolymers prepared by RROP of two monomers with poor compatibility yields ill-defined lower molecular weight oligomers,61 which may influence their biodegradability.62 The 2-methylene-1,3-dioxepane (MDO) CKA was effectively copolymerized with monomers such as vinyl acetates, methacrylates, and pyrrolidones, via RAFT polymerization.63,64 The reactivity of MDO is lower than the comonomers, which leads to slow and incomplete incorporation of MDO into the backbone at high loadings. The ester functional groups along the backbone are resistant to hydrolysis against potassium carbonate, which enabled selective hydrolysis of poly(vinyl chloroacetate)-co-poly(2-methylene-1,3-dioxepane) into poly(vinyl alcohol)-co-poly(2-methylene-1,3-dioxepane)) without degrading the polymer backbone,65 but are degradable in more basic solutions. The addition of a phenyl functional group to the 7-member MDO CKA scaffold improved polymerization control due to better resonance stabilization of the opened radical. The RROP of 5,6-benzo-2-methylene-1,3-dioxepane (BMDO) was nearly quantitative to the ester, and yields a resonance-stabilized primary radical after ring opening (Schemes 2a and 2b). BMDO was successfully homopolymerized by ATRP, RAFT, and NMP to moderate chain lengths with decent control.66–68 Similar to other CKAs, copolymerization of BMDO and most methacrylates have a large difference in reactivity ratios which favor homopolymerization of the methacrylates.69 The copolymerization of pentafluorophenyl methacrylate (PFMA) and BMDO is an exception due to the electron-withdrawing nature of the pentafluoro group in the side chain.70 The large difference in electronics between electron-rich BMDO and electron-poor PFMA led to a near-alternating sequence. Scheme 2 | (a) CKAs polymerized by RDRP include MDO, BMDO, MPDL, and MPDO. (b) RROP of BMDO. Download figure Download PowerPoint The ATRP and NMP of oligo(ethylene oxide) methacrylate (OEOMA) with a 2-methylene-4-phenyl-1,3-dioxolane (MPDL) CKA with a five-membered ring and a phenyl substituent had better living character than MDO and BMDO.71 The reactivity of MPDL was still less favorable than polymerization of OEOMA. However, only a small fraction of degradable monomer might be necessary to induce a large change in polymer topology. P(OEOMA-co-MPDL) statistical copolymers with low mol fraction of MPDL (FMPDL) of 0.036 had a ∼30% reduction in molecular weight (Mn) after hydrolysis against 5% potassium hydroxide in 24 h.72 Further increasing FMDPL to 0.113 and 0.248 resulted in ∼80% and ∼95% reductions in Mn, respectively.73 P(MPDL-co-OEOMA) copolymers had a similar degradation profile to hydrophobic poly(lactic acid) and poly(caprolactone) (PCL) polyesters over a one-year timeframe under physiological conditions.74 Copolymerization of MPDL and ethyl maleimide (EMA) had an alternating sequence.75 The preference for alternation was also attributed to electronics, where the MDPL was the electron-rich donor which had preference for addition to the electron-deficient EMA acceptor. Copolymerization of 5-Methylene-2-phenyl-1,3-dioxolan-4-one (MPDO) with methyl methacrylate (MMA) or styrene led to copolymers with a higher MPDO content than originally in the feed, due to its captodative structure. However, copolymerization of MPDO predominantly proceeded by 1,2-vinyl addition rather than ring opening.76 Incorporation of thioesters via RROP Thioester functional groups are an attractive alternative to polyesters due to their lower reactivity and possibility for enhanced degradability under mild conditions. Thioesters were installed by RROP RAFT of macrocyclic dibenzo[c,e]oxepane-5-thione (DOT) thionolactones with n-butyl acrylate (BA) and t-butyl methacrylate (tBA) comonomers (Scheme 3).77 Characterization of the purified polymers confirmed quantitative ring opening of the thionolactone. Copolymerization of BA with DOT reached near quantitative consumption of thionolactone at loadings below 5 mol %. However, lower incorporation of DOT was observed in copolymerizations with high initial loadings of DOT. Alternating copolymers of thionolactone and maleimide were prepared by RAFT copolymerization with N-methylmaleimide, N-phenylmaleimide, and N-2,3,4,5,6-pentafluorophenylmaleimide.78 Similar to the polyesters, thionolactone functional groups can be degraded by exposure to sodium methoxide and cysteine methyl ester. Scheme 3 | Proposed mechanism for the RROP of DOT.255 Download figure Download PowerPoint Radical ring opening polymerization of macrocycles Macrocyclic monomers with low ring strain were polymerized and copolymerized by RAFT radical ring-opening polymerization. Monomers with ester, disulfide, and thioester functional groups were reported in the literature (Scheme 4a).79 Macrocyclic monomers were designed such that propagation through the allylic sulfide would readily open the macrocycle through beta-scission to produce a new carbon–carbon double bond and thiyl radical capable of addition to other monomers (Scheme 4b).79,80 The macrocycles were copolymerized with MMA, N,N-dimethylaminoethyl methacrylate (DMAEMA), 2-hydroxyethyl methacrylate (HEMA), and 2-hydroxypropyl methacrylate (HPMA).79,81 The copolymers were degraded in solutions of sodium methoxide in tetrahydrofuran (THF),79 and the disulfide-containing polymers were degraded by reduction upon exposure to a tris(2-carboxyethyl)phosphine reducing agent.81 Scheme 4 | (a) Allylic sulfide macrocyclic monomers polymerized by RAFT copolymerization in the literature. (b) Mechanism of RROP of allylic sulfide macrocyclic monomers. (c) RROP by radical cascade ring opening polymerization of an allylic sulfone macrocyclic monomer. Download figure Download PowerPoint The polymerization of macrocyclic allyl alkylsulfone monomers proceeded through a radical cascade process starting with β-elimination of alkylsulfone, followed by α-scission and liberation of gaseous SO2, providing a 2-propionate radical capable of propagation (Scheme 4c).82 The macrocycles had nearly ideal reactivity with acrylic and acrylamide monomers due to the similar structure of the propagating radical after ring opening and loss of SO2.83 The ideal reactivity between these monomers and vinyl comonomers is a significant improvement over other RROP methods because the degradable bonds should be located roughly the same number of repeat units apart. This is anticipated to yield well-defined oligomers after degradation under the appropriate conditions. RDRP of degradable vinyl monomers Some vinyl polymers can be degraded to lower molecular weight oligomers via photoinduced or catalyst-activated bond scission. Poly(vinyl ketone)s are a unique class of materials capable of degradation under ultraviolet (UV) irradiation through Norrish Type I and Norrish Type II reactions upon exposure to UV light, resulting in a degradation of polymer into shorter oligomers.84,85 Methyl vinyl ketone (MVK) and phenyl vinyl ketone (PVK) were polymerized by RAFT and subsequently degraded by photolysis (Figure 3).85,86 Multiblock copolymers of poly(vinyl ketones) with nondegradable polymers provided materials with selectively etchable blocks. The morphology of polystyrene-b-poly(methyl vinyl ketone) thin films changed after etching the poly(methyl vinyl ketone) block by UV light exposure.86 The modulus of triblock copolymers with a poly(n-butyl acrylate) middle block and PVK outer blocks decreased after selectively etching the PVK hard blocks.87 Figure 3 | Structure of MVK and PVK. Download figure Download PowerPoint RAFT copolymerization of methyl α-chloroacrylate (MCA) and MMA provided copolymers with chlorine functionalities along the backbone.88 The chlorine functionality remained intact through the RAFT polymerization, and resembled a α-chloroisobutyrate functionality along the backbone. Activation of the chlorines by ATRP catalysts generated acrylic midchain radicals, which were proposed to degrade through beta-scission and midchain cleavage of the copolymer along the backbone (Schemes 5a and 5b). The macroinitiator degraded from a starting Mp = 14,900 (Ð = 1.62) to oligomers of Mp = 2000 (Ð = 1.32) upon activation with FeCl2 and the tributylamine cocatalyst. Scheme 5 | (a) RAFT polymerization of MMA and MCA. (b) Degradation of a P(MMA-co-MCA) copolymer by activation of chlorine bonds along the backbone with an ATRP catalyst, leading to beta scission into lower molecular weight oligomers. Download figure Download PowerPoint Degradable Polymers from the Backbone via the Use of Bifunctional Initiators Linear polymers with one degradable bond at the center of a linear polymer could be prepared by RDRP using bifunctional initiators.89 The use of a bifunctional initiator enables growth of a polymer on both sides of the degradable bond. Thus, degradation of the central linkage via chemical or external stimuli cleaves the high molecular weight polymer into two linear polymers with half the molecular weight of the precursor (Scheme 6). A diverse library of degradable bonds were installed in the middle of two ATRP initiators (Figure 4). Scheme 6 | Scheme of an RDRP using a red bifunctional initiator installs the degradable bond at the middle of the polymer chain. Exposure to chemical or external stimuli cleaves the polymer into two polymers with half the molecular weight of the precursor. The degraded halves can reform the original polymer if the reaction is reversible. Download figure Download PowerPoint Redox degradable polyacrylates and PSs were prepared by ATRP with bifunctional 2-bromopropionic acid and 2-bromoisobutyrate diesters of bis(2hydroxyethyl) disulfide initiators.90,91 The ATRP reaction can be performed without reduction of the disulfide bond, providing high molecular weight polymers with one central disulfide bond. The disulfide linkage could be reversibly reduced by dithiothreitol (DTT) or tributylphosphine to yield linear polymers with half the molecular weight of the original material. The degraded mercapto-functional oligomers could be recoupled after oxidation in the presence of weak oxidizing agents, such as FeCl3 or iodine, to reform the high molecular weight polymer linked by the disulfide bonds. Figure 4 | Bifunctional ATRP initiators with degraded products and conditions required to cleave the degradable bond. Download figure Download PowerPoint The method of disulfide reduction and oxidation can affect the efficiency of the self-healing reaction. Reductive cleavage/nucleophilic substitution of a poly(methyl acrylate) (PMA) with a central disulfide bond and terminal bromine functionality with diethylamine base and DTT reduces the disulfide bond and installs an additional thiol at the chain-end by substitution of the bromine. Thus, oxidation of telechelic α,ω-bis(thiol)-functionalized PMA with I2 leads to step-growth polymerization by bridging disulfide bonds (Figures 5a and 5b).92 Reduction of the disulfide bond over Bu3SnH and Azobisisobutyronitrile (AIBN) saturates the bromine chain-end while reducing the disulfide bond, yielding two saturated chains with terminal thiol functionality. Oxidation of the polymer produced a new material with comparable molecular weight to the precursor. Figure 5 | (a) Reaction scheme and gel permeation chromatography (GPC) traces illustrating the reduction of a polymer with a central disulfide bond with diethylamine as base and DTT as reducing agent, followed by oxidation to a higher molecular weight polymer containing multiple disulfide bonds. (b) Reaction scheme and GPC illustrating the reduction and hydrogenolysis of the same polymer by Bu3SnH, followed by subsequent oxidation to disulfide polymer, then reduction with DTT in dimethyl sulfoxide. (Mn determined by GPC; values in parentheses determined by 1H NMR integration). Reproduced with permission from ref 92. Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Download figure Download PowerPoint Furfuryl-maleimide Diels–Alder (DA) functionalities enabled linear polymer degradation after thermal and mechanical treatment. Maleimide functionalized PMA was prepared by ATRP of methyl acrylate with a maleimide functionalized α-bromoisobutyrate initiator.93 DA [4+2] cycloaddition between ω-maleimide-PMA and a ω-furan-poly(ethylene glycol) produced diblock copolymers with the furan-maleimide adduct at the center. The retro-Diels Alder reaction at 120 °C cleaved the adduct into the same ω-furan-poly(ethylene glycol) and ω-maleimide-PMA starting materials.93 Ultrasonication of PMA with the same DA adduct led to mechanochemical retro-DA of the polymer into two PMA chains with half the molecular weight of the original polymer.94 Multiblock Backbone Scission via RDRP then Step-Growth Polymerization The chain-end functionality of polymers prepared by RDRP enables installation of various functional groups by substitution chemistries. Multiblock copolymers can be prepared by post-polymerization modification of bifunctional polymers into macromonomers, followed by step-growth polymerization, to yield high molecular weight linear polymers (Scheme 7). The use of reversible, or degradable, bonds in the step-growth polymerization enables degradation from high to lower molecular weight upon exposure to the appropriate chemical or external stimuli. Scheme 7 | RDRP with a bifunctional initiator or CTA installs end-group functionality (blue) at both ends of the polymer. Modification of the end groups (red dots) can enable step-growth polymerization to high molecular weight, with degradable bonds placed between each macromer. Exposure to chemical or external stimuli can cleave the polymer into linear polymers with comparable molecular weight to the precursor. Download figure Download PowerPoint Multisegment degradable polymers were prepared by radical trap-assisted atom transfer radical coupling (RTA-ATRC) of halogenated styrenic and (meth)acrylic polymers with nitroxide coupling agents.95 ATRC is a method of coupling polymer chains by preference of crosstermination by combination.96,97 Step-growth ATRC of α,ω-dihalogenated PS was performed in the presence of a bifunctional (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) adduct with, and without, a central disulfide bond.95,98 PS prepared by ATRC was degraded thermally in the presence of excess TEMPO, due to exchange between the bifunctional crosslinker and the higher concentration of monofunctional TEMPO. High molecular weight polymers were also prepared by step-growth polymerization of a bifunctional 4-phenylene bis(2-bromoisobutyrate) ATRP initiator with a bifunctional TEMPO adduct by radical coupling between activated chain ends and TEMPO radical traps.99 Thermogravimetric analysis of the poly(alkoxyamines) showed considerable mass loss below 200 °C, suggesting poor thermal stability caused by scission of the initiator-alkoxyamine bonds. Nitroso radical traps were also used to install thermally cleavable alkoxyamine functionalities onto styrenic and acrylic polymers by ATRC.100 The coupling of polymer chains with nitroso compounds proceeds in two steps (Scheme 8). In the first step, the polymeric chain-end radical reacts with the nitroso functional group to produce a polymer with nitroxide functionality. The chain-end nitroxide can trap a second polymeric radical to produce alkoxyamine-linked polymer topologies. Thermolysis of the alkoxyamine cleaves the bond and produces two polymer chains with similar Mn and Ð as the starting material. This approach was first outlined in the RTA-ATRC of a monobrominated PMMA-Br precursor with a nitrosobenzene radical trap.100 Brominated PS and PMA were also coupled by RTA-ATRC using nitrosobenzene radical traps, and later work applied the concept to multiblock copolymers.101 Under dilute conditions, the coupling reaction produces cyclic polymers which can be degraded back to linear polymers by thermolysis.102 Scheme 8 | ATRC of bifunctional PS with nitrosobenzene Download figure Download PowerPoint Nucleophilic substitution of halogen chain-end functionality (CEF) can install thiol linkages on telechelic α,ω-dibromo vinyl polymers.90,103 Telechelic polymers with thiol functionalities could also be prepared by aminolysis of trithiocarbonate (TTC; Scheme 9)104 or xanthate RAFT CTAs.105 Oxidative coupling of linear α,ω-dithiol polymers produces high molecular weight linear polymers connected by disulfide bonds, which can be reversibly degraded to the low molecular weight telechelic α,ω-dithiol pre-polymer after reduction. Scheme 9 | Aminolysis of RAFT chain ends enables the synthesis of bifunctional PS with thiol chain ends. Two thiols can couple after oxidation to disulfide bonds to yield high molecular weight PS. The polymer could be degraded back to low molecular weight after reduction of the disulfide bonds. Download figure Download PowerPoint Oxidative step-growth polymerization of α,ω-bis(thiol) polymers enabled the synthesis of degradable random multiblock copolymers. Multiblock telechelic copolymers were prepared by oxidative coupling of α,ω-bis(thiol)-poly(styrene)-b-poly(n-butyl acrylate)-b-poly(tert-butyl acrylate).105 Oxidative coupling of α,ω-bis(thiol) poly(N-isopropyl acrylamide) (PNIPAM) (Mn = 7860, Ð = 1.24) and α,ω-bis(thiol) poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) (Mn = 7780, Ð = 1.18) at a 1:1 weight ratio produced a high molecular weight (Mn = 180,000 and Ð = 5.1) temperature and pH-responsive multiblock copolymer.106 The multiblock copolymers were degraded to lower molecular weight by reduction with DTT. Block Copolymers Degradable functionalities can also be installed using a degradable macroinitiator. This has often involved the use of polymer macroinitiators prepared by ROP, such as poly(lactic acid), polycaprolactone, polypeptides, or polyphosphates as the degradable block (Scheme 10).107–109 Scheme 10 | Use of a degradable macroinitiator (red) enabled
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