Acrylic rubbers (ACM) are unsaturated polar rubbers that are characterized with very good swelling resistance in non-polar oils and also in oils containing sulfur substances. Because of the fact they do not have unsaturated bonds in polymer backbone, thus they have good heat, oxygen and ozone resistance. From chemical point of view they are copolymers, eventually terpolymers of different types of acrylic monomers or acrylic monomers and other monomers containing functional groups suitable for curing. The most frequently those are vinyl monomers with reactive atoms of chlorine, epoxy or carboxylic groups. Content of these monomers is low (1-3 % wt.) and has no significant influence on their properties.
ACM are produced by emulsion (exceptionally by suspension) radical polymerization. For its initiation peroxides, azo-compounds or oxidation-reduction initiation systems are used. Because of the fact, that acrylic monomers have ester groups in their molecules, thus pH of reaction medium must be > 7, to avoid their hydrolysis. The same negative influence may have also fillers or other additives of acidic character in their compounds.
ACM properties under low temperatures are influenced mainly by structure of alkyl substituents in their ester groups; generally they are improved with increasing of carbon number in alkyl groups (brittle temperature of poly-n-alkylacrylate is reduced for substituents with carbon number C = 1- 8 from temperature +3 °C down to temperature 65°C). Simultaneously, their polarity is decreasing, and thus their non-polar oil and ageing resistance is decreasing, too. ACM based on polyethyl acrylate are high-polar and very well resistant to oils and increased temperatures, but they have relatively high Tg (approximately -15°C) and their vulcanizates are less flexible under low temperatures. Replacement of ethyl groups by butyl one will reduce Tg to approximately 50°C, their vulcanizates are significantly more flexible under low temperatures, but their swelling resistance in non-polar oils is lower. Polyoctyl acrylate is practically not resistant against non-polar oils. Properties of ACM under low temperatures may be modified by copolymerization of acryl monomers (ethyl acrylate, butyl acrylate, eventually by octyl acrylate or acryl monomer with alkoxy groups) and they can be partially modified also by adding of suitable, less volatile softeners; the best one is of ester type.
The most industrially produced acrylic rubbers are based on ethyl-, butyl- and metoxy ethyl acrylate and monomers containing chlorine or carboxyl groups. Their suitable combination usually compromise between ACM oil resistance, appropriate properties under low temperatures, heat resistance and acceptable curing rate can be achieved.
For cross-linking of ACM rubbers mainly function groups of non-acrylate monomers, despite the fact that under presence of metallic oxides of alkaline character one cannot exclude even self-curing of polyacrylate chains caused by reactions of their ester groups and active hydrogen in a-position against them (Claisen´s condensation) are used. Older types of ACM were cured mostly by diamines or polyamines, present types of rubbers are cured also by combination of polyamines, higher fatty acids or their esters and sulfur, eventually by sulfur donor. Their efficiency is increased by addition of very fast or ultra accelerators. The most often used systems for curing of ACM with epoxy and carboxyl groups is based on quaternary ammonium salts.
Acrylic rubbers are used mainly for production of materials resistant to oils applicable in automotive motors and other mechanical equipments, in form of latexes for impregnation of textile and paper and in form of solutions as adhesive agents and coating compositions.
Examples of the acrylic monomers used during production of ACM
Examples of functional co-monomers used during production of ACM
Design of a monomer having hierarchical reactivity
We focused on monomer 4, in which the B* group is directly connected to the vinyl moiety (Fig. 1b); B* in 4 would be inert under radical or cationic polymerization, because vinyl radicals and cations are unstable. However, once 4 is converted to 5 by reaction with an initiator or the dormant species R-B*, both B* groups in 5 become capable of initiating polymerization, because a stable alkyl radical or cation can now be generated.
The reactivity of 4 and the dormant species of the propagating polymer ends, shown in Fig. 2a, were estimated by density functional theory calculations at the B3LYP/6-31 G(d,p)(C,H) + LAN2DZ(Te) level (Fig. 1c, Supplementary Note 1, and Supplementary Tables 1 and 2). The carbon–tellurium bond dissociation energy (BDE) of vinyl telluride 6a (R = Me) was calculated to be 216 kJ mol−1 (Fig. 1c), and this value is significantly higher than those of 7 (160 kJ mol−1) and 8 (154 kJ mol−1), which are model compounds of the propagating polymer ends. This difference strongly suggests the selective activation of 7 and 8 in preference to 6a under polymerization conditions. Furthermore, the similar BDEs of 7 and 8 ensure the simultaneous activation of the two tellurium groups in 7 once it forms.
Synthesis and characterization of hyperbranched PMA
6a was easily prepared by modifying a previously reported procedure (Supplementary Notes 2, 3, and 4 and Supplementary Figs. 19 and 20)38, 39 and copolymerized with methyl acrylate (MA, 500 equivalents (equiv.)) in the presence of organotellurium CTA 912 at different [6a]/ ratios of 3, 7, 15, 31, and 63 in the presence of azobisisobutyronitrile (AIBN) (0.2 equiv.) as a radical initiator at 60 °C (Fig. 2a, Table 1, runs 1–5, and Supplementary Notes 6–10)40. When all dormant species corresponding to 7 and 8 have equal reactivity, polymerization should yield dendritic HBPs corresponding to the second to sixth dendritic generations (N = 2–6) depending on the [6a]/ ratio, in which the initiating group derived from 9 and branching points derived from 6a are connected by a poly(methyl acrylate) (PMA) chain. Generation N leads to (2N – 1) branching points and [2(N + 1) − 1] branched polymer chains (Fig. 2b). The average number of monomer units () inserted between the branched points can be estimated as
under ideal statistical copolymerization conditions, in which Conv refers to the conversion of the monomer.
Monitoring polymerization by proton nuclear magnetic resonance (1H NMR) and size-exclusion chromatography (SEC) revealed that 6a was quantitatively converted into the copolymer with high MA conversion in all cases, though the addition of 6a significantly slowed the rate of polymerization compared to that when 6a was absent (run 6). This is probably due to the lower activation efficiency of the dormant species corresponding to 7 and 8 than that of the acrylate end having a BDE of approximately 140 kJ mol−1. Fig. 2c–e and Supplementary Fig. 27 show the results of polymerization monitoring for N = 6 ([6a]/ = 63, run 5) corresponding to the sixth dendritic generation. The consumption of 6a and MA followed pseudo-first-order kinetics at the beginning of the polymerization, indicating the occurrence of controlled polymerization, while 6a was consumed slightly faster than MA (Fig. 2c). The polymerization rate gradually decreased with increasing monomer conversion. Therefore, an additional 0.2 equiv. of AIBN was added after 84 h, which significantly increased the reaction rate. Despite the change in the reaction rate, the relative monomer consumption (6a/MA) was constant, indicating the formation of a statistical copolymer.
SEC analyses were carried out after reduction of the methyltellanyl polymer-end group in 10c (X = TeMe) to 10d (X = H) by methyltellanol (MeTeH), which was generated in situ from MeTeSiMe3 (Supplementary Note 5) and methanol41. The number average molecular weight determined by SEC (Mn[SEC]) using poly(methyl methacrylate) (PMMA) standards increased with increasing monomer conversion; Mn[SEC] showed good agreement with the theoretical values (Mn[theo]) only at low monomer conversion (< 20%) and deviated significantly as the monomer conversion increased (Fig. 2d). These results are consistent with the formation of a branched polymer, which has a smaller hydrodynamic volume than linear polymers1. At 95% and 74% conversion of 6a and MA, respectively, Mn[SEC] (0.98 × 104) was significantly smaller than Mn[theo] (3.46 × 104). In contrast, the Mn determined by multi-angle laser light scattering (MALLS, Mn[MALLS]) was 5.39 × 104 (Supplementary Fig. 1), which is close to Mn[theo] (Mn[MALLS] was determined from weight average molecular weight obtained from MALLS (Mw[MALLS]) divided by the polydispersity index (PDI) (Mn/Mw) obtained by SEC. This is because PDI determined by MALLS is low reliability due to the weak intensity of low-molecular weight part as the targeted molecular weight is moderate. Mw(MALLS) and Mn(MALLS) would be also overestimated by the same reason.). The PDI remained below 2.0 throughout the polymerization, which is significantly lower than the PDI obtained by SCVP and SCVCP. In addition, the SEC trace was unimodal throughout the polymerization even at high monomer conversion (Fig. 2e). All results are consistent with the formation of HBPs with well-defined structures.
The generality of this method was supported by the controlled synthesis of HBPs with different dendritic generations (N = 2–6) by changing the [6a]/ ratio from 3 to 63 while keeping the /[MA] ratio constant (runs 1–5). All copolymerization reactions afforded copolymers with unimodal distribution, except for N = 2, in which a small shoulder was observed in the low-molecular-weight region due to the formation of a PMA homopolymer (Fig. 2f). As the current method depends on statistical copolymerization between 6a and MA, a homopolymer inevitably forms for low dendritic generations. However, for N > 3, unimodal SEC traces were observed in all cases. Despite the appearance of a shoulder for N = 2, the PDI was below 2.0. The Mn[SEC]s calibrated using linear PMMA standards decreased with increasing dendritic generation despite the similarity of the Mn[theo]s. In contrast, the Mn[MALLS]s remained almost unchanged, close to the Mn[theo]s (Fig. 2g).
The intrinsic viscosity ([η]) of the HBPs estimated from the Mark–Houwink–Kuhn–Sakurada (MHKS) plot was smaller than that of linear PMA of equal Mw in all samples (Fig. 2h and Supplementary Figs. 15 and 16). Furthermore, the viscosity of the HBPs at the same Mw(MALLS) decreased as the branching number increased. The MHKS exponent α obtained from the slope is 0.36–0.44, which is significantly smaller than that of linear PMA (0.82). All these results support the formation of the desired HBPs.
Control of the branching density and molecular weight, i.e., control of the PMA spacer length between branching points, was demonstrated by changing the MA/9 ratio to 100, 250, and 2000 while maintaining the 6a/9 ratio at 15 (N = 4, Table 1, runs 7–9 and Supplementary Notes 12–14). Quantitative and > 92% conversion of 6a and MA were observed, respectively, and copolymers showing unimodal SEC traces were obtained in all cases (Supplementary Figs. 2–4). In addition, the Mn(SEC)s were smaller than the Mn(theo)s, whereas the Mn(MALLS)s were close to the Mn(theo)s in both cases.
An image of a single polymer molecule was directly observed by atomic force microscopy (AFM) for a polymer sample prepared from 125 and 2000 equiv. of 6a and MA, respectively, relative to 9 (N = 7), which is the highest branched polymer prepared in this study (Fig. 3a and Supplementary Note 15). Many spherical dots corresponding to single polymer molecules were observed (Fig. 3b), and this globular shape further supports the formation of HBPs. The typical height and width of the polymer dot was approximately 4 and 40 nm, respectively, suggesting that the HBPs are flattened on the mica surface (Fig. 3c).
Structural characterization by isotope labeling experiments
To clarify the branched structure and branching efficiency at the molecular level, the polymer-end structure was characterized by a deuterium labeling experiment. In the branched structure, all polymer-end groups should correspond to acrylate moieties, while both acrylate and alkyl ends should be observed if branching does not occur (Fig. 4a). After copolymerization under similar conditions to those given in Table 1, run 4 (/[6a]/[MA] = 1/31/500), the polymer-end groups were reduced by deuterated methyltellanol (MeTeD) prepared from MeTeSiMe3 and deuterated methanol (CH3OD), giving 10d-D (Supplementary Note 24). The 2H NMR spectrum of the polymer showed a single peak at 2.3 ppm, corresponding to acrylate chain ends, while no signal corresponding to alkyl ends was observed (Fig. 4b). The branching efficiency was determined to be > 99%.
The role of vinyl telluride was further investigated with the help of 13C-labeled 6b*, in which the quaternary carbon was selectively labeled by13C. Labeled and naturally abundant 6b* and 6b (Supplementary Notes 25–32 and Supplementary Figs. 14, 21–26), respectively, were copolymerized under the same conditions used for 6a, with /[6b(*)]/[MA] = 1/15/500 (runs 11 and 12 and Supplementary Notes 16 and 17), and the polymer-end groups were reduced by MeTeH. Nearly identical results for the Mn and PDI were obtained regardless of the13C labeling. In the13C NMR spectrum of the13C-labeled polymer, a new peak at 39.4 ppm was observed as the major signal, along with a small peak at 33.4 ppm (Fig. 4c). The major peak at 39.4 ppm was attributed to the quaternary carbon from its disappearance in a distortionless enhancement of the polarization transfer (DEPT) 135° spectrum. The results clearly reveal that vinyl telluride indeed serves as a branching point (Fig. 4d).
The minor signal at 33.4 ppm was assigned to tertiary carbon by the DEPT spectrum. The hydrogen atom at the tertiary carbon most likely originates from the back-biting reaction42, 43 (Fig. 4d). To verify this possibility, the same13C-labeled copolymer was reduced by MeTeD, and the resulting doubly labeled polymer was analyzed by13C and 2H NMR (Supplementary Figs. 17 and 18). The 13C NMR spectrum was identical to that of the non-deuterated 13C-labeled sample, and 2H NMR only showed the acrylate ends. All these results are consistent with the occurrence of the back-biting reaction. Branching should occur from the resulting mid-chain radical, as its structure is similar to that of polymethacrylate chain-end radicals44. All these results indicate that vinyl telluride generates branching points with 100% efficiency.
Expansion of the synthetic scope
To further illustrate the synthetic versatility of the current method, HBPs with different structures were synthesized. For example, the copolymerization of 6a and MA starting from PMA 11 (Mn(SEC) = 1.51 × 104, PDI = 1.12, /[6a]/[MA] = 1/15/340) afforded linear-block-hyperbranched PMA (run 13 and Supplementary Note 18). Furthermore, the same copolymerization utilizing bifunctional and trifunctional initiators 12 and 1345 resulted in dumbbell- and clover-shaped HBPs, respectively (runs 14 and 15 and Supplementary Notes 19 and 20). All polymers exhibited unimodal SEC traces, narrow PDIs, and Mn(MALLS)s close to the Mn(theo)s, while the Mn(SEC)s were significantly smaller than the Mn(theo)s (Supplementary Figs. 8–10). All these results clearly reveal the formation of HBPs with well-controlled structures and demonstrate the synthetic versatility of the current method (Fig. 5).
As the current method relies on a living polymerization, transformation of the living end is possible. For example, the tellurium end group was quantitatively transformed into an amino group by a radical trapping reaction with 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl, giving 10e (run 16, Supplementary Note 23, and Supplementary Fig. 28). Furthermore, monomers having polar functional groups, such as 2-(dimethylamino)ethyl acrylate and N,N-dimethylacrylamide, were used as a comonomer instead of MA, as radical polymerization is highly compatible with polar functional groups. The corresponding HBPs with controlled molecular structures having narrow PDIs were synthesized in both cases (runs 17 and 18, Supplementary Notes 21 and 22, and Supplementary Figs. 11–13). A wide variety of HBPs having different functional groups can be synthesized by changing the comonomer and the method of end-group transformation.