Progress in the Preparation of Functional and (Bio)Degradable Polymers via Living Polymerizations

This review presents the latest developments in (bio)degradable approaches and functional aliphatic polyesters and polycarbonates prepared by typical ring-opening polymerization (ROP) of lactones and trimethylene carbonates. It also considers several recent innovative synthetic methods including radical ring-opening polymerization (RROP), atom transfer radical polyaddition (ATRPA), and simultaneous chain- and step-growth radical polymerization (SCSRP) that produce aliphatic polyesters. With regard to (bio)degradable approaches, we have summarized several representative cleavable linkages that make it possible to obtain cleavable polymers. In the section on functional aliphatic polyesters, we explore the syntheses of specific functional lactones, which can be performed by ring-opening copolymerization of typical lactone/lactide monomers. Last but not the least, in the recent innovative methods section, three interesting synthetic methodologies, RROP, ATRPA, and SCSRP are discussed in detail with regard to their reaction mechanisms and polymer functionalities.

Besides the well-known aliphatic polyesters, another family of emerging, highly (bio)degradable and biocompatible aliphatic polycarbonates, poly(trimethylene carbonate) (PTMC), have also been extensively investigated [55]. Being the starting material for trimethylene carbonate (TMC), 1,3-propanediol can be acquired from the degradation of natural carbohydrates or ring-closure carbonylation of carbon dioxide [56]. The analog of TMC monomers is thus referred to as a renewable resource. As shown in Scheme 3a, f-PTMCs can be synthesized by ROP of f-TMCs through either typical organometallic catalysts or organo-catalysts [57]. As illustrated in Scheme 3b, several representative functional groups (FG) on the TMC ring are addressed, including (i) OH, (ii) COOH, (iii) allyl/SH, (iv) propargyl, (v) epoxide, (vi) norbornene, (vii) maleimide, and so on. In order to further graft specific (macro)molecules, post-reactions between FG and PG can be performed via (i) etherification, (ii) esterification, (iii) thiol-ene, (iv) alkyne-azide cycloaddition, (v) epoxide-amine, (vi) Diels-Alder, (vii) Michael addition, and so forth. For example, Harth et al. [58] reported the synthesis of PTMC-based hydrogels with various crosslinking reagents. They first synthesized PTMCs with pendant functional groups of both ethyl ester (i.e., PTMC-pen-(O)COEt) and allylic ester (i.e., PTMC-pen-(O)COCH 2 =). Subsequently, thio-ene click reactions of the (PTMC-=) and (HS-(EG) n -SH) (EG: ethylene glycol; n = 1 or 35) were conducted to obtain PTMC-crosslinked hydrogels. A polyol of branched polyglycidol (PGY) and a transesterification catalyst of zinc acetate (Zn(OAc) 2 ) were further introduced into the hydrogels. Interestingly, the composite underwent a chemical self-modification from the PTMC/PGY/(Zn(OAc) 2 ) hydrogels at high temperature (ca. 120 • C) on the basis of dynamic covalent bonds. Accordingly, the attractive renewable feature of f-TMC monomers and their ability to render diverse pendant functional (macro)molecules on f-PTMCs has led to very high expectations for their application to various practical uses.

Synthesis of Aliphatic Polyesters by Radical Ring-Opening Polymerization (RROP)
Polyesters are typically synthesized by either ROP of lactones/lactides [31] or polycondensation of hydroxyl and acid monomers [59]. In the 1980s, Bailey et al. [60][61][62][63][64] reported the pioneering cases of free radical ring-opening polymerizations (RROPs) of specific cyclic ketene acetal (CKA) and cyclic acrylate (CA) monomers initiated by conventional thermal initiators (i.e., peroxide and azo compounds). These studies established an innovative approach for producing a variety of functional polyesters on the basis of radical chemistry. Thereafter in the 1990s, Endo et al. [65][66][67] and Rizzardo et al. [68] designed a series of specific vinylcyclopropane (VCP) and cyclic allylic sulfide (CAS) monomers, respectively, to produce f-APs as well. RROP of VCPs rendered ester linkages in the backbone and also improved the introduction of other functionalities into the backbone (e.g., olefin and phenyl) and at the pendent sides (e.g., benzyl and ethylene ketal) (Mn = 22,000 and Mw/Mn = 2.05). In the case of the RROP of CASs, polyester backbones with sulfide linkages and pendant double bonds were obtained (Mn = 46,200 and Mw/Mn = 2.3).
The monomers for RROP are classified into two types: the vinyl type such as VCP and the exomethylene type such as CKA. In the RROP mechanism of both monomers, a radical species is added to the carbon-carbon double bond and thus generates a carbon-centered radical, then the radical ringopening reaction generates a new carbon-centered radical via the cleavage of a carbon-carbon bond or a carbon-heteroatom bond (Scheme 4). The polymerization of these monomers proceeds inherently

Synthesis of Aliphatic Polyesters by Radical Ring-Opening Polymerization (RROP)
Polyesters are typically synthesized by either ROP of lactones/lactides [31] or polycondensation of hydroxyl and acid monomers [59]. In the 1980s, Bailey et al. [60][61][62][63][64] reported the pioneering cases of free radical ring-opening polymerizations (RROPs) of specific cyclic ketene acetal (CKA) and cyclic acrylate (CA) monomers initiated by conventional thermal initiators (i.e., peroxide and azo compounds). These studies established an innovative approach for producing a variety of functional polyesters on the basis of radical chemistry. Thereafter in the 1990s, Endo et al. [65][66][67] and Rizzardo et al. [68] designed a series of specific vinylcyclopropane (VCP) and cyclic allylic sulfide (CAS) monomers, respectively, to produce f-APs as well. RROP of VCPs rendered ester linkages in the backbone and also improved the introduction of other functionalities into the backbone (e.g., olefin and phenyl) and at the pendent sides (e.g., benzyl and ethylene ketal) (M n = 22,000 and M w /M n = 2.05). In the case of the RROP of CASs, polyester backbones with sulfide linkages and pendant double bonds were obtained (M n = 46,200 and M w /M n = 2.3).
The monomers for RROP are classified into two types: the vinyl type such as VCP and the exo-methylene type such as CKA. In the RROP mechanism of both monomers, a radical species is added to the carbon-carbon double bond and thus generates a carbon-centered radical, then the radical ring-opening reaction generates a new carbon-centered radical via the cleavage of a carbon-carbon bond or a carbon-heteroatom bond (Scheme 4). The polymerization of these monomers proceeds inherently via the RROP mechanism or a conventional vinyl polymerization mechanism. Only RROP provides an ester structure in the polymer main chain, thus it is essential to use monomers that have high RROP selectivity. Kinetic and thermodynamic factors are required for the selectivity: (i) the ring-opening reaction is accelerated due to the strain of the ring; and (ii) the resulting new carbon-centered radical is stabilized and/or the ring-opening reaction involves thermodynamically favored isomerization of the functional group. via the RROP mechanism or a conventional vinyl polymerization mechanism. Only RROP provides an ester structure in the polymer main chain, thus it is essential to use monomers that have high RROP selectivity. Kinetic and thermodynamic factors are required for the selectivity: (i) the ringopening reaction is accelerated due to the strain of the ring; and (ii) the resulting new carbon-centered radical is stabilized and/or the ring-opening reaction involves thermodynamically favored isomerization of the functional group. Scheme 5a displays representative CKAs with high or exclusive RROP selectivity. One of the most attractive features of RROP of CKA is the copolymerization with conventional vinyl monomers including (meth)acrylates, styrenes, vinyl pyridine, vinyl acetate, and N-vinylpyrrolidone, which can readily provide various (bio)degradable f-PE. The size of the ring of CKA is one of the major factors controlling the selectivity in the mechanism. CKA with strained rings, for example, the 7 and 8membered ones are more favorable in the RROP compared to the stable 5 and 6-membered ones. Namely, the polymerization of the 7-membered ring monomer MDO exclusively provides polyester via the selective RROP; however, the selectivity of the RROP mechanism of the corresponding 6membered ring monomer 2-methylene-1,3-dioxane decreases to about 50%. Another important structural factor is the introduction of a stabilizing group for the carbon-centered radical generated by the ring-opening, which are typically phenyl or alkyl groups. While the ring-opening occurs from both sides of an acetal ring, the introduction of a radical stabilizing group regulates the side of the ring-opening that has the group. In addition, the relative reactivity between CKA and the vinyl monomer is critical in the copolymerization to prepare f-PE. Because the olefin of CKA is very electron-rich, the reaction with propagating radicals formed from common vinyl monomers, which are nucleophilic or amphiphilic, is relatively unfavored [69]. Therefore, the reactivity ratio in the copolymerization of CKA and vinyl monomers is often rCKA << 1 < rvinyl, which impedes the incorporation of the CKA monomer and causes a deviation between the composition of polymer from the monomer feeding ratio. For example, in the copolymerization of MDO and MMA, the reactivity ratios are rMDO = 0.057 and rMMA = 34.12 [70], and the composition of MDO in the resulting polymer is 4% (polymerization at 40 °C) or 30% (at 120 °C) from copolymerization with a monomer feeding ratio of MDO/MMA = 54/46 or 50/50, respectively. On the other hand, the copolymerization of CKA with vinyl acetate (VAc) undergoes in an almost random manner (i.e., more statistical distribution of monomers in the chain), which is confirmed by rCKA = 0.93 and rVAc = 1.71 in the copolymerization with MDO [71], and this provides a copolymer with a homogeneous composition of CKA and VAc that is similar to the monomer feeding ratio. This feature is not limited to VAc, and other vinyl carboxylate monomers also give a copolymer with CKA in an almost random manner. Additionally, Scheme 5a displays representative CKAs with high or exclusive RROP selectivity. One of the most attractive features of RROP of CKA is the copolymerization with conventional vinyl monomers including (meth)acrylates, styrenes, vinyl pyridine, vinyl acetate, and N-vinylpyrrolidone, which can readily provide various (bio)degradable f-PE. The size of the ring of CKA is one of the major factors controlling the selectivity in the mechanism. CKA with strained rings, for example, the 7 and 8-membered ones are more favorable in the RROP compared to the stable 5 and 6-membered ones. Namely, the polymerization of the 7-membered ring monomer MDO exclusively provides polyester via the selective RROP; however, the selectivity of the RROP mechanism of the corresponding 6-membered ring monomer 2-methylene-1,3-dioxane decreases to about 50%. Another important structural factor is the introduction of a stabilizing group for the carbon-centered radical generated by the ring-opening, which are typically phenyl or alkyl groups. While the ring-opening occurs from both sides of an acetal ring, the introduction of a radical stabilizing group regulates the side of the ring-opening that has the group. In addition, the relative reactivity between CKA and the vinyl monomer is critical in the copolymerization to prepare f-PE. Because the olefin of CKA is very electron-rich, the reaction with propagating radicals formed from common vinyl monomers, which are nucleophilic or amphiphilic, is relatively unfavored [69]. Therefore, the reactivity ratio in the copolymerization of CKA and vinyl monomers is often r CKA << 1 < r vinyl , which impedes the incorporation of the CKA monomer and causes a deviation between the composition of polymer from the monomer feeding ratio. For example, in the copolymerization of MDO and MMA, the reactivity ratios are r MDO = 0.057 and r MMA = 34.12 [70], and the composition of MDO in the resulting polymer is 4% (polymerization at 40 • C) or 30% (at 120 • C) from copolymerization with a monomer feeding ratio of MDO/MMA = 54/46 or 50/50, respectively. On the other hand, the copolymerization of CKA with vinyl acetate (VAc) undergoes in an almost random manner (i.e., more statistical distribution of monomers in the chain), which is confirmed by r CKA = 0.93 and r VAc = 1.71 in the copolymerization with MDO [71], and this provides a copolymer with a homogeneous composition of CKA and VAc that is similar to the monomer feeding ratio. This feature is not limited to VAc, and other vinyl carboxylate monomers also give a copolymer with CKA in an almost random manner. Additionally, the copolymerization of vinyl bromobutanoate and MDO [72], with post-modification through alkyne-azide cycloaddition results in PEG-grafted degradable polyester.
Recently, interesting progress in the scope of copolymerization has been reported. Guillaneuf et al. [75] reported the copolymerization of vinyl ether and MDO in a highly random manner. The composition of monomers during the polymerization reaction was found to follow the initial feeding ratio of the monomers. The copolymerization reactivity ratio was r MDO = 0.73 and r vinyl ether = 1.61, which is consistent with of the theoretical calculation of the reaction rate for the α-oxyethyl radical and MDO. The highly random manner could also be related to the fact that vinyl ethers are not homopolymerized by the radical polymerization. The synthetic benefit of vinyl ether for f-PE is the ready availability of various functional monomers, and indeed, Cl, oligo(ethylene glycol) and terminal alkene functionalized vinyl ethers have been shown to give copolymers with MDO. These copolymers were further modified to obtain a fluorescent probe functionalized polymer and a degradable elastomer. Another recent example is the copolymerization of BMDO and maleimide reported by Sumerlin [76]. The copolymerization proceeded with the quantitative ring-opening of BMDO to the ester and in a highly alternating manner. The copolymer was readily functionalized by utilizing N-substituent of maleimide, and the alternating structure might be suitable for fast degradation to low molecular weight fragments. Interestingly, although the alternative copolymerization of other CKAs such as MDO and maleimide has been reported, the selectivity of the ring-opening of CKA was not enough, which indicates that the appropriate combination of monomer and the reactivity is quite important in designing the CKA copolymer as a f-PE. Recently, interesting progress in the scope of copolymerization has been reported. Guillaneuf et al. [75] reported the copolymerization of vinyl ether and MDO in a highly random manner. The composition of monomers during the polymerization reaction was found to follow the initial feeding ratio of the monomers. The copolymerization reactivity ratio was rMDO = 0.73 and rvinyl ether = 1.61, which is consistent with of the theoretical calculation of the reaction rate for the α-oxyethyl radical and MDO. The highly random manner could also be related to the fact that vinyl ethers are not homopolymerized by the radical polymerization. The synthetic benefit of vinyl ether for f-PE is the ready availability of various functional monomers, and indeed, Cl, oligo(ethylene glycol) and terminal alkene functionalized vinyl ethers have been shown to give copolymers with MDO. These copolymers were further modified to obtain a fluorescent probe functionalized polymer and a degradable elastomer. Another recent example is the copolymerization of BMDO and maleimide reported by Sumerlin [76]. The copolymerization proceeded with the quantitative ring-opening of BMDO to the ester and in a highly alternating manner. The copolymer was readily functionalized by utilizing N-substituent of maleimide, and the alternating structure might be suitable for fast degradation to low molecular weight fragments. Interestingly, although the alternative copolymerization of other CKAs such as MDO and maleimide has been reported, the selectivity of the ring-opening of CKA was not enough, which indicates that the appropriate combination of monomer and the reactivity is quite important in designing the CKA copolymer as a f-PE.
In conventional free radical polymerization, radicals are produced in the initiation step (kd = ca. 10 −4 -10 −6 s −1 ) through the decomposition of thermal or photo-initiators. Subsequently, a moderate-tofast chain propagation (depending on the monomers: kp = ca. 10 2 -10 4 M −1 s −1 ) and very fast radical terminations (kt = ca. 10 6 -10 8 M −1 s −1 ) occur [77]. Due to the irreversible radical generating step, rather high concentrations of the unstable species cause a significant number of terminations, which leads to broad molecular weight distributions and uncontrollable kinetics. Although RROP provides an alternative approach to the synthesis of f-APs, the RROP method has to meet some application demands that require a narrow range of molecular weight distributions. In the mid-1990s, reversibledeactivation radical polymerization (RDRP) was discovered and it is still being developed in academia and industry [78]. In RDRP, reagents of dormant molecules and regulators (which act as In conventional free radical polymerization, radicals are produced in the initiation step (k d = ca. 10 −4 -10 −6 s −1 ) through the decomposition of thermal or photo-initiators. Subsequently, a moderate-to-fast chain propagation (depending on the monomers: k p = ca. 10 2 -10 4 M −1 s −1 ) and very fast radical terminations (k t = ca. 10 6 -10 8 M −1 s −1 ) occur [77]. Due to the irreversible radical generating step, rather high concentrations of the unstable species cause a significant number of terminations, which leads to broad molecular weight distributions and uncontrollable kinetics. Although RROP provides an alternative approach to the synthesis of f-APs, the RROP method has to meet some application demands that require a narrow range of molecular weight distributions. In the mid-1990s, reversible-deactivation radical polymerization (RDRP) was discovered and it is still being developed in academia and industry [78]. In RDRP, reagents of dormant molecules and regulators (which act as activators and deactivators) are necessarily present. The key to achieving controlled/living polymerization is to follow a number of general steps [78]: (i) the initiating rate of the reaction between the activators and dormant molecules should be faster than that of the chain propagating rate; (ii) meanwhile, deactivators and active radicals are produced in order to proceed with certain monomer additions; (iii) the concentration of the (macro)radicals is quickly deactivated by deactivators; and (iv) meanwhile, activators and dormant (macro)molecules are reversibly generated so that another cycle can be conducted starting from step (i). In a controlled/living polymerization, the concentrations of active radicals should remain low in order to achieve the suppression of termination reactions and so that all polymer chains can grow evenly and consecutively. A homogeneous dispersion of a regulator in the polymerization mixture provides an effectively controlled/living process. However, the initiating sites (i.e., active centers) can be either homogeneously dispersed in the polymerization mixture or attached to various heterogeneous surfaces of silicon, metals, and plastics (e.g., wafers, plastic tubes, porous materials, (nano)fibers, (nano)particles, etc.).

RROP
AT-RROP Degradable [73] a Specific applications reported in the study [75], or the degradability as the general interest of the application. b FL: fluorescent.

Synthesis of Degradable Polyesters by Atom Transfer Radical Polyaddition (ATRPA)
The normal ATRP and its derivative techniques [80] are based on atom transfer radical addition (ATRA) [101,102]. Recently, extensions of multi-step ATRA created a novel analog of aliphatic polyesters with functional groups on their polymer backbones that can be obtained through manipulation of the different activation/deactivation rate constants of the inimers (i.e., initiator and monomer). In 1997, preliminary research on the preparation of aliphatic polyesters through ATRP of AB-type inimers was reported by Matyjaszewski et al. [103,104]. The studies addressed the possibility of obtaining aliphatic polyesters during atom transfer-induced radical self-condensing vinyl polymerization (ATR-SCVP) of aliphatic ester type inimers. Through the ATR-SCVP of 2-((2-bromopropionyl)oxy)ethyl acrylate (BPEA), topological (hyper)branched polymers comprising both aliphatic polyester and polyacrylates structures were attained. Later, Li et al. coined the term, "atom transfer radical polyaddition" (ATRPA), for the synthesis of perfect linear aliphatic polyesters. Scheme 6 demonstrates the general mechanism of ATRPA, which provides perfect linear aliphatic polyesters (i.e., the R 2 linkage comprising the ester group). The first example of perfect control ATRPA was demonstrated by Kamigaito et al. in 2007 [105]. Through the design of a specific AB type inimer (e.g., allyl 2-chloropropanoate) with a reactive C-Cl bond (i.e., site B), they developed a novel and radical polyaddition method perfectly controlled by transition metals. The specific inimer can be activated by a lower-oxidation state transition metal (e.g., Cu(I), Ru(II), Fe(II), etc.) to form a radical species (i.e., species 1) and react with a double bond (i.e., site A) of another inimer. More importantly, the newly formed single addition radical species (i.e., species 2) can be deactivated by a higher-oxidation state transition metal (e.g., Cu(II), Ru(III), Fe(III), etc.) to obtain a dimer that possesses extremely inactive pendant C-Cl bonds (i.e., site B' on species 3). By perfectly and slowly repeating the single addition of inimers, linear aliphatic polyesters can be obtained [106,107]. Li et al. also designed an AB type inimer (i.e., (4-vinylbenzyl 2-bromo-2-isobutyrate (VBBiB)) [108] or AA/BB paired monomers, i.e., bis(styrenics)/bis(bromoisobutyrates) [109,110]. They made four critical breakthroughs on the basis of the ATRPA technique including: (i) controlling the topology from hyperbranched to linear polymers; (ii) improving the effectiveness of perfect-control ATRPA, i.e., polymerizations were reduced to a few days; (iii) rendering a variety of functional groups into the linear polymer backbone, i.e., diverse functional linkages between R 1 and R 2 ; and (iv) grafting different functional polymers onto the linear polymer backbone, i.e., through post-reactions of C-X bond. In polymerizations of VBBiB in anisole at 0 • C with a homogeneous catalyst system, for example, a step-growth trend was detected. At such a low temperature, high selectivity between the inactive B' and active B sites can be retained, leading to the formation of linear aliphatic polyesters. In polymerizations of VBBiB in toluene at 0 • C with a heterogeneous catalyst system, the deactivation efficiency of the active B• radical (chain-)ends was insufficient, which led to fast conventional free radical polymerizations to produce linear polymers with C-C as the backbones and bromoisobutyryl as the pendant groups. Polymerizations of VBBiB in anisole at high temperatures (i.e., 20-60 • C) with a homogeneous catalyst system resulted in low selectivity between the inactive B' and active B sites, leading to the formation of branched polymers through the mechanism of atom transfer-induced radical self-condensing vinyl polymerization (ATR-SCVP). Further, Kamigaito  observed in the PVBBiB, which resulted in the formation of a five-membered-ring lactone structure (i.e., (5-(4-(bromomethyl)phenyl)dihydro-3,3-dimethylfuran-2(3H)-one)). In the same circumstances, the PVBBPA performed as a stable type polyester. Eventually, post-click reactions were applied to obtain amphiphilic polymer brushes (i.e., aliphatic polyesters as the backbone; hydrophilic poly(ethylene glycol) as the grafting chains). The novel polyesters possess pH-sensitive and reversible thermoresponsive behaviors [110,113]. Therefore, the development of innovative ATRPAs provides an alternative strategy for obtaining f-APs. Scheme 6. General mechanism of transition metal catalyzed atom transfer radical polyaddition (ATRPA).

Synthesis of Degradable Polyesters by Simultaneous Chain-and Step-Growth Radical Polymerization (SCSRP)
Kamigaito et al. developed another novel technique, which was similar but different to ATRPA, for simultaneous chain-and step-growth radical polymerization (SCSRP) [114]. As shown in Scheme 7, a common vinyl (i.e., methyl acrylate (MA)) and an inimer (i.e., compound 1) are both present in the reaction mixture. Mediated by transition metal catalysts (e.g., Cu(I), Ru(II), Fe(II), etc.), the resulting atom transfer reactions can effectively perform simultaneous ATRP (i.e., Path A) and ATRPA (i.e., Path B) mechanisms. The proper design of the inimer (i.e., compound 1) means that the branching reactions via the ATR-SCVP mechanism (i.e., Path D) can almost be suppressed. Eventually, novel polymer structures are obtained that are comprised of both polyvinyl and polyesters as the backbone (Mw = ca. 36,000 and Mw/Mn = 2.01). That is, the chemical structures obtained via the SCSRP of vinyls and effective inimers are similar to the copolymers obtained via the RROP of vinyls and CKAs. However, the CKA monomers have very poor reactivity toward Scheme 6. General mechanism of transition metal catalyzed atom transfer radical polyaddition (ATRPA).
Recently, Huang et al. [87,111,112] designed a highly reactive AB type inimer (i.e., 4-vinylbenzyl 2-bromo-2-phenylacetate (VBBPA)), which significantly improved the reactivity for ATRPA (i.e., polymerizations were reduced to a few hours) but kept high selectivity, to obtain linear aliphatic polyesters. Therefore, high molecular weight aliphatic polyesters (M w = ca. 26,000 and M w /M n = 2.09) can be effectively obtained in three hours. This significant improvement was due to two factors: (i) the activation rate of the VBBPA initiating site is much faster than that of VBBiB (i.e., k a,VBBPA /k a,VBBiB = ca. 2 × 10 3 at 35 • C), which results in highly reactive ATRPA; and (ii) the difference in the activation rate in the C-X groups at the chain ends and the inactive C-X groups on the backbones is extremely large (i.e., k a,C-X(PVBBPA end) /k a,C-X(PVBBPA backbone) = ca. 3 × 10 4 at 35 • C), which results in a highly selective ATRPA. By tracing the ATRPAs of VBBiB and VBBPA, an interesting self-degrading behavior was observed in the PVBBiB, which resulted in the formation of a five-membered-ring lactone structure (i.e., (5-(4-(bromomethyl)phenyl)dihydro-3,3-dimethylfuran-2(3H)-one)). In the same circumstances, the PVBBPA performed as a stable type polyester. Eventually, post-click reactions were applied to obtain amphiphilic polymer brushes (i.e., aliphatic polyesters as the backbone; hydrophilic poly(ethylene glycol) as the grafting chains). The novel polyesters possess pH-sensitive and reversible thermoresponsive behaviors [110,113]. Therefore, the development of innovative ATRPAs provides an alternative strategy for obtaining f-APs.

Synthesis of Degradable Polyesters by Simultaneous Chain-and Step-Growth Radical Polymerization (SCSRP)
Kamigaito et al. developed another novel technique, which was similar but different to ATRPA, for simultaneous chain-and step-growth radical polymerization (SCSRP) [114]. As shown in Scheme 7, a common vinyl (i.e., methyl acrylate (MA)) and an inimer (i.e., compound 1) are both present in the reaction mixture. Mediated by transition metal catalysts (e.g., Cu(I), Ru(II), Fe(II), etc.), the resulting atom transfer reactions can effectively perform simultaneous ATRP (i.e., Path A) and ATRPA (i.e., Path B) mechanisms. The proper design of the inimer (i.e., compound 1) means that the branching reactions via the ATR-SCVP mechanism (i.e., Path D) can almost be suppressed. Eventually, novel polymer structures are obtained that are comprised of both polyvinyl and polyesters as the backbone (M w = ca. 36,000 and M w /M n = 2.01). That is, the chemical structures obtained via the SCSRP of vinyls and effective inimers are similar to the copolymers obtained via the RROP of vinyls and CKAs. However, the CKA monomers have very poor reactivity toward copolymerization with vinyls, which limits the introduction of ester linkage into the polyvinyl backbones. Thus, copolymers with varying compositions of polyvinyl and polyester can be effectively attained via the SCSRP technique [115][116][117]. Zhu et al. [118] performed fast and effective SCSRP of MA and ABP (i.e., allyl 2-bromopropanoate) to obtain P(MA-co-ABP) copolymer (M w = ca. 5100 and M w /M n = 1.78) with both a degradable ester group and an undegradable poly(acrylate) segment. They also identified the α-double bond at the copolymer chain end. Then, efficient thiol-ene click reactions of thiol-terminated PNIPAM (poly(N-isopropyl amide)) and double bond-terminated P(MA-co-ABP)s were performed. Serial novel block copolymers of PNIPAM-b-P(MA-co-ABP) were synthesized and these displayed thermoresponsive properties with lower critical solution temperatures (LCSTs: 34-37 • C). SCSRP successfully linked the undegradable polyvinyl (i.e., C-C linkages in the backbones) and the degradable polyester (i.e., ester groups in the backbones) to prepare functional and eco-friendly commodity plastics.

Conclusions and Outlook
In this report, we first discussed the most recent novel synthetic methods for preparing functional polyesters. Then, recent topics of interest in regard to the synthesis of polyesters, including the use of bio-originated or "sustainable" monomers, organocatalyzed ROP for polyesters, and the efficient functionalization of polyesters are summarized. Finally, recent innovations in the polymer Scheme 7. An exemplified mechanism of simultaneous chain-and step-growth radical polymerization (SCSRP) of MA and inimer 1 [96].

Conclusions and Outlook
In this report, we first discussed the most recent novel synthetic methods for preparing functional polyesters. Then, recent topics of interest in regard to the synthesis of polyesters, including the use of bio-originated or "sustainable" monomers, organocatalyzed ROP for polyesters, and the efficient functionalization of polyesters are summarized. Finally, recent innovations in the polymer chemistry of RROP, ATRPA, and SCSRP methodologies have created a novel series of (bio)degradable and functional polyester-containing polymers. These novel functional polyesters have great potential for application in biomedical, biotechnology, nanomaterials, microelectronics as well as contributing to a circular economy, environmental protection, and agriculture.
The development of degradable, especially biodegradable polymers with functional properties is becoming increasingly important. For example, the European Chemicals Agency recently announced a recommendation to restrict the amount of micro-plastic additives in products and there are other demands for environmental protection. Biodegradable polymers are excluded from these regulations; however, the polymers are required to reach degradability standards that are much stricter than in the past. The regulations regarding use of micro-plastics are also expected to be imposed on various (synthetic) polymer products. Therefore, polymers (as products and additives) with a wide range of functionality and sufficient value, which have low environmental impact and high (bio)degradability are highly desirable in the long run.