New Aldehyde‐Functional Methacrylic Water‐Soluble Polymers

Abstract Aldehyde groups enable facile conjugation to proteins, enzymes, oligonucleotides or fluorescent dyes, yet there are no literature examples of water‐soluble aldehyde‐functional vinyl monomers. We report the synthesis of a new hydrophilic cis‐diol‐based methacrylic monomer (GEO5MA) by transesterification of isopropylideneglycerol penta(ethylene glycol) using methyl methacrylate followed by acetone deprotection via acid hydrolysis. The corresponding water‐soluble aldehyde monomer, AGEO5MA, is prepared by aqueous periodate oxidation of GEO5MA at 22 °C. RAFT polymerization of GEO5MA yields the water‐soluble homopolymer, PGEO5MA. Aqueous periodate oxidation of the terminal cis‐diol units on PGEO5MA at 22 °C affords a water‐soluble aldehyde‐functional homopolymer (PAGEO5MA). Moreover, a library of hydrophilic statistical copolymers bearing cis‐diol and aldehyde groups was prepared using sub‐stoichiometric periodate/cis‐diol molar ratios. The aldehyde groups on PAGEO5MA homopolymer were reacted in turn with three amino acids to demonstrate synthetic utility.

One well-known route to aldehyde-terminated watersoluble polymers is the selective oxidation of the minor fraction of cis-diol units within poly(vinyl alcohol). [42] This water-soluble polymer can be obtained via hydrolysis of poly(vinyl acetate), which contains such cis-diols as defect sites resulting from asmall amount of head-to-head coupling during the free radical homopolymerization of vinyl acetate. [43] Oxidation is readily achieved in aqueous solution under mild conditions using sodium periodate to afford aldehyde-capped poly(vinyl alcohol) chains. [44] Inspired by this well-established chemistry,werecently decided to investigate the periodate oxidation of poly(glycerol monomethacrylate) (PGMA) to produce an aldehyde-functional methacrylic polymer (Scheme 1; Supporting Information). However,p eriodate oxidation of a1 0% w/w aqueous solution of PGMA 39 at 22 8 8Cmerely produced amacroscopic precipitate. This suggests that the target aldehyde-functional methacrylic homopolymer (PAGMA) is actually hydrophobic.I np rinciple,such precipitation could be the result of reaction between the cis-diol and aldehyde units at intermediate conversion.
However,r eaction exotherms (data not shown) and visual inspection of the reaction mixtures suggest that the timescale required for the cis-diol oxidation is much shorter than that for precipitation. Thus,i ts eems more likely that intermolecular crosslinking occurs between geminal diols and aldehydes (Supporting Information, Scheme S1).
In view of these problems,w ed esigned an ew cis-diolbased methacrylic monomer (GEO5MA;S cheme 2a). We envisaged that the pendent oligo(ethylene glycol) moiety in GEO5MA should confer sufficient hydrophilic character to ensure water solubility after converting its terminal cis-diol group into an aldehyde via periodate oxidation to form either AGEO5MA monomer (Scheme 2b)o rt he corresponding PAGEO5MA homopolymer.

Results and Discussion
Thet wo-step synthesis of GEO5MA monomer was conducted on a1 .2 kg scale via 1) transesterification of isopropylideneglycerol penta(ethylene glycol) using methyl methacrylate to afford IPGEO5MA (Scheme 2a)and 2) acid hydrolysis to remove the acetone protecting group (Supporting Information). Thec hemical structure of this new methacrylic monomer was confirmed by 1 Ha nd 13 CNMR spectroscopy ( Figure 1a;S upporting Information, Figure S1a), mass spectrometry,elemental microanalysis and FT-IR spectroscopy (Supporting Information). Thei ntegrated signals in the 1 HNMR spectrum are consistent with the proposed monomer structure.I ts 13 CNMR spectrum contained ten distinct signals.Acharacteristic signal at % 160 ppm was assigned to the ester carbonyl carbon;i ts relatively low intensity is attributed to the slow relaxation time for such quaternary carbon atoms. [45] Thep resence of am ethacrylate group is confirmed by signals at 135 and 127 ppm. Several signals between 62.6 and 71.3 ppm are assigned to the pendent oligo(ethylene glycol) chain and include characteristic signals for the carbons attached to hydroxyl groups. According to mass spectrometry,t he number of ethylene glycol units per oligo(ethylene glycol) group ranged from 2t o 7, with am ean value of 5.
Oxidation of a1 0% w/w aqueous solution of GEO5MA using aN aIO 4 /cis-diol molar ratio of unity (Scheme 2b)l ed to essentially complete oxidation of the terminal cis-diol units within 5min at 22 8 8C, as confirmed by 1 HNMR spectroscopy ( Figure 1). Thes tructure of this new AGEO5MA monomer was confirmed by mass spectrometry,e lemental microanalysis, 1 Hand 13 CNMR (Figure 1b;F igure S1b) and FT-IR spectroscopy (Figure S3a). Tw on ew signals appear at 9.52 and 5.09 ppm in the 1 HNMR spectrum for AGEO5MA, corresponding to an aldehyde group and ageminal diol, respectively.The aldehyde/geminal diol molar ratio was 0.034, which indicates that AGEO5MA exists primarily in its hydrated geminal diol form in D 2 O ( Figure 1b). Similar observations have been reported for other hydrophilic aldehydes in aqueous solution, such as acetaldehyde ( Figure S2). [46][47][48][49] During the periodate oxidation of GEO5MA to form AGEO5MA, the starting material can in principle react with the product to generate dimethacrylate species via (hemi)acetal chemistry. [1] In practice,t he final product contains less than 1% dimethacrylate impurity as estimated by 1 HNMR spectroscopy.T he 13 CNMR spectrum also shows the appearance of two new signals at 169.5 and 88.0 ppm, which correspond to the aldehyde carbon and the geminal diol carbon, respectively.A fter purification by extraction with CH 2 Cl 2 ,t he RAFT aqueous solution polymerization of AGEO5MA was conducted using adicarboxylic acid-functionalized water-soluble RAFT agent (CECPA) to Scheme 2. a) Two-step synthesis of GEO5MA monomer starting from an isopropylidene glycerol precursor as ah ydroxy-functional initiator.T his precursor is then transesterified with methyl methacrylate to produce IPGEO5MA,b efore removing the ketal protecting group with acid to afford GEO5MA monomer.b)Oxidation of GEO5MA in aqueous solution using sodium periodate at 22 8 8Caffords AGEO5MA with formaldehyde as ab y-product. The same selectiveo xidation can be used to convert PGEO5MA homopolymer into PAGEO5MAh omopolymer using identical reaction conditions. target am ean degree of polymerization (DP) of 30 ( Figure 2a). More than 99 %c onversion was achieved and the resulting PAGEO5MA 30 was well-defined, as indicated by its relatively narrow,u nimodal GPC trace (M n = 11 100 gmol À1 ; = 1.18;F igure 2b). 1 HNMR signals for the terminal aldehyde and geminal diol groups were detected for this homopolymer (aldehyde/geminal diol molar ratio = 0.041).
Alternatively,R AFT aqueous solution polymerization of GEO5MA affords anear-monodisperse PGEO5MA 37 homopolymer (M n = 17 200 gmol À1 ; = 1.18). When aN aIO 4 /cisdiol molar ratio of unity was used to derivatize this precursor, essentially complete oxidation was achieved to afford PA -GEO5MA 37 homopolymer within 5min at 22 8 8C ( Table 1, Figure 3). Thel atter product proved to be water-soluble at concentrations of up to 15 %w /w.I ns triking contrast, the product of the oxidation of PGMA 39 homopolymer using as toichiometric amount of periodate,d enoted hereafter as PAGMA 39 ,proved to be water-insoluble when prepared at 1.5 to 10 %w /w (Supporting Information, Table S1). Them uch higher aqueous solubility observed for PAGEO5MA 37 is attributed to the hydrophilic oligo(ethylene glycol) units on each repeat unit. However,only aminor fraction of monomer repeat units may need to be converted into aldehyde groups for certain applications.T hus,p artial oxidation of aP GEO5MA 37 precursor using sub-stoichiometric quantities of NaIO 4 oxidant relative to its cis-diol groups was also investigated (schematic in Figure 3a).
Accordingly,utilizing NaIO 4 /cis-diol molar ratios of 0.10, 0.50 or 0.75 produced aseries of water-soluble P(GEO5MA nstat-AGEO5MA m ) 37 statistical copolymers with approximate degrees of aldehyde functionality of 0.11, 0.49 and 0.78 respectively,asestimated from 1 HNMR spectroscopy studies (Table 1, Figure 3). Thus,t he target degree of aldehyde functionality is always achieved (within experimental error). DMF GPC analyses confirmed that neither partial nor full oxidation of the PGEO5MA 37 homopolymer had asignificant effect on its molecular weight distribution (Table 1; Figure S4). Moreover,u sing as light excess of NaIO 4 relative to the pendent cis-diol groups also resulted in partial loss of the dithiobenzoate end-groups.S imilarly,aPGEO5MA homopolymer (M n = 124.1 kg mol À1 , = 4.55) was synthesized via free-radical polymerization in aqueous solution at 70 8 8Cf or 18 h. Selective oxidation of the cis-diol groups on this homopolymer also had minimal effect on its (broad) molecular weight distribution (Figures S5 and S6).
To investigate the scope of such new water-soluble aldehyde-functional polymers for conjugation with biologically-relevant compounds,P AGEO5MA 37 homopolymer was reacted in turn with three amino acids (glycine,l ysine or cysteine;a mino acid/aldehyde molar ratio = 1.0) to form the corresponding Schiff base,f ollowed by in situ reductive amination using excess NaCNBH 3 (Scheme 3). These aqueous reaction mixtures were stirred at 35 8 8Cf or 48 h, with 1 HNMR spectroscopy studies indicating very high extents of reaction (> 99 %) in each case ( Figure S7). Aqueous GPC analysis of the resulting water-soluble polymers indicated that molecular weight distributions remained relatively narrow after this two-step one-pot derivatization ( Figure S8).
This protocol was then extended to water-soluble diblock copolymers.Aseries of neutral, zwitterionic,c ationic or anionic double-hydrophilic diblock copolymers was prepared in which one of the blocks was PGEO5MA (Scheme 4). For the neutral diblock copolymer, at rithiocarbonate-capped PEG 113 precursor was simply chain-extended via RAFT aqueous solution polymerization of GEO5MA at 50 8 8C. For the synthesis of the ionic diblock copolymers,aPGEO5MA 37 precursor was chain-extended via RAFT aqueous solution polymerization of 2-(methacryloyloxy)ethyl phosphoryl-   Table 2). DMF GPC analysis indicated ah igh blocking efficiency for the RAFT solution polymerization of GEO5MA using the PEG 113 macro-CTAa nd the resulting PEG 113 -PGEO5MA 50 diblock copolymer had ar elatively low dispersity ( = 1.20; Table 2; Figure S9a). However,a queous GPC analysis was required to assess the molecular weight distributions of the ionic diblock copolymers. ( Table 2; Figures S9b-d). Oxida-tion of the pendent cis-diol groups on the PGEO5MA x chains was investigated using aNaIO 4 /cis-diol molar ratio of unity at adiblock copolymer concentration of 20 %w/w.According to 1 HNMR analysis,t he extent of derivatization was at least 99 %inall cases ( Table 2). DMF GPC analysis confirmed that periodate oxidation had minimal effect on the molecular weight distribution ( = 1.22;F igure S9a) in the case of the PEG 113 -PGEO5MA 50 diblock copolymer. Similar results were obtained for the zwitterionic, cationic and anionic diblock copolymers when using aqueous GPC (Table 2; Figures  S9b-d). Scheme 3. Schiff base reaction of PAGEO5MA 37 with an amino acid (e.g.,g lycine, lysine, or cysteine) followed by reductive amination using excess aqueous NaCNBH 3 at 35 8 8Ctoafford aseries of new zwitterionic homopolymers via atwo-step one-potw holly aqueous protocol.

Research Articles Conclusion
In summary,wehave reported the atom-efficient synthesis of an ew cis-diol-based methacrylic monomer (GEO5MA) that is readily converted into ah ydrophilic aldehyde-functional monomer (AGEO5MA) via selective oxidation using NaIO 4 in aqueous solution. Unlike almost all other literature examples of aldehyde-based vinyl monomers,t his latter monomer is water-soluble and can be polymerized with good control via RAFT aqueous solution polymerization. Alternatively,h omopolymerization of the GEO5MA precursor under similar conditions affords awell-defined water-soluble PGEO5MA precursor that can be converted into PAGEO5-MA under mild conditions using as toichiometric amount of NaIO 4 oxidant. On the other hand, using sub-stoichiometric quantities of NaIO 4 relative to the pendent cis-diol units produces ar ange of water-soluble aldehyde-functional statistical copolymers.New PAGEO5MA-based double-hydrophilic diblock copolymers can be prepared and model Schiff base reactions have been conducted in aqueous solution under mild conditions using various amino acids to introduce zwitterionic groups.W eanticipate that this new hydrophilic aldehydic vinyl monomer and its corresponding copolymers will offer ar ange of potential applications in the fields of cell biology and biomaterials.