The versatile architectures of polymeric materials impart them with highly programming properties, thus enabling their various applications [
1–
3]. The architectures of polymers include linear, cyclic, hyperbranched, star-shape, and cross-linked structures [
4,
5], which originate from different connection ways of various polymer segments. Among these architectures, branched polymers, owing to their irregular three-dimensional topology, exhibit unique properties compared to their linear counterparts. These include rheological differences such as reduced chain entanglement, as well as higher solubility, weaker mechanical strength, and distinct self-assembly behaviors [
6,
7]. Particularly, branched polymers, owing to their ability to stitch domains together by repeatedly crossing the interface between two homopolymers, are effective compatibilizers for promoting the interfacial adhesion between two immiscible polymers, thus enhancing their mechanical performances [
8–
10]. Therefore, it is important to develop an effective strategy for synthesizing polymers with branched structures to extend the library of branched polymer compatibilizers.
Traditionally, the well-established strategy for synthesizing branched polymers mainly includes grafting strategies, that is, “grafting to”, “grafting through”, and “grafting from” [
11]. However, these strategies often necessitate the pre-synthesis of side- or main-chains, leading to tedious synthetic procedures [
12]. Much effort has been devoted to develop facile and efficient synthetic methods to attain branched polymers [
13–
15]. Two elegant strategies have been established through the: (i)
In situ construction of a reactive group end-capped oligomer as a side chain for synthesizing branched polymers [
16,
17], and (ii) utilization of bifunctional monomers,
i.e., monomers containing two reactive groups, as branching point for synthesizing hyperbranched polymers (HBPs) [
18–
20]. An example of the former is that branched poly(monothiocarbonate) can be accessed from epichlorohydrin and carbonyl sulfide (COS)
via the
in situ synthesis of poly(monothiocarbonate) oligomer end-capped with an epoxy group, which acts as branching point. The terpolymerization of COS with epichlorohydrin and epoxy at elevated temperature generates branched sulfur-containing polymers [
21]. The use of bifunctional monomers for synthesizing HBPs represents a more general method [
22–
25]. For example, 1,3,4-benzene tricarboxylic anhydride was used as a branching point to produce hyperbranched polyether-
b-polycarbonate copolymers [
25]. These well-established synthetic methodologies enable the development of functional HBPs from low-cost, readily available monomers, thereby facilitating the high-value utilization of fundamental compounds.
Epoxides and acrylates are two fundamental chemicals that have been widely used in many areas [
26,
27], with one of the important applications being to produce polymers [
28,
29]. In particular, the direct construction of polymeric materials from epoxides and acrylates represents an elegant strategy for attaining functional polymers [
30–
32]. For example, Wang
et al. described an effective strategy for synthesizing block copolymers comprising polyacrylates and epoxide-derived copolymers using switchable polymerization [
33,
34]. Very recently, we achieved the sequence-controlled copolymerization of epoxides and acrylates using a bimetallic synergistic catalysis strategy [
35], by which multiblock copolymers comprising polyester and polyacrylate segments can be synthesized for use as compatibilizers [
36]. In addition, hyperbranched poly(glycidyl acrylate), which can be used for enhancing the performance of epoxy thermosets, can be synthesized
via a photo-mediated switchable polymerization from epoxide, acrylate, and epoxy-capped acrylate [
37]. Nevertheless, the direct synthesis of HBPs comprising polyether and polyacrylate has not yet been achieved. Our previous work indicated that bimetallic synergistic catalysis was capable of facilitating the copolymerization of epoxides and acrylates for synthesizing block copolymers, wherein the homopolymerization of epoxide proceeded first, followed by the homopolymerization of acrylate with the initiation of an alkoxide [
35]. We envision that the introduciton of glycidyl acrylate (GA), a bifunctional monomer comprising both epoxy and acrylate groups, into the copolymerization of epoxides and acrylates is viable for synthesizing HBPs. In this procedure, the copolymerization of propylene oxide (PO) and GA proceeded first, forming a polyether with attached acrylate groups (
Fig. 1). Then, the copolymerization of the acrylate groups occurred after the epoxide groups were fully consumed with the initiation of alkoxide, generated from the ring-opening polymerization (ROP) of epoxide. Herein, we aimed to synthesize HBPs comprising polyether and polyacrylate through the copolymerization of PO and methyl acrylate (MA) in the presence of GA using a bimetallic catalysis strategy. Varying the GA addition amount enables the production of HBPs with different branching degrees, which can be used as effective compatibilizers for enhancing the mechanical performances of polyether/polyacrylate blends.
Dinuclear SalenCrCl complex (
1) in conjunction with [PPN]Cl (PPN = bis(triphenylphosphine)iminium) was employed to catalyze the copolymerization of PO, GA, and MA with a feed ratio of PO:MA:GA:
1:[PPN]Cl = 1000:1000:3:1:2 at 60 ℃. The polymerization proceeded efficiently, as all the monomers were almost fully consumed within 5 h (
Table 1, entry 1). Hydrogen nuclear magnetic resonance (
1H NMR) spectroscopy characterization revealed that the resulting copolymer comprised poly(propylene oxide) (PPO) and poly(methyl acrylate) (PMA) segments (Fig. S1 in Supporting information). In addition, the number-average molecular weight (
Mn) of the resulting polymer (HBP1) was 19.6 kg/mol with a dispersity (
Ð) of 3.21. In contrast, the copolymerization of PO and MA catalyzed by
1/[PPN]Cl yielded the corresponding PPO-
b-PMA block copolymer (BCP) with an
Mn of 24.3 kg/mol and a narrowed
Ð of 1.56 (entry 2). However, the weight-average molecular weight (
Mw) of HBP1 was higher than that of BCP (62.9
vs. 37.9 kg/mol), indicating the formation of branched structure of resulting copolymers (
Fig. 2a). To verify this hypothesis, detailed comparisons regarding the relationship between
Mw and intrinsic viscosity of HBP1 and BCP was conducted using gel permeation chromatography (GPC) equipped with triple detectors, including refractive index, intrinsic viscosity, and two light scattering detectors. According to Mari–Houwink plots recorded by GPC, the Mark–Houwink exponents (
α) of HBP1 and BCP were 0.36 and 0.79 (
Fig. 2b), respectively, characteristic of a randomly branched and linear polymer. The branched characteristics of HBP1 is further evidenced by the fact that the contraction factor (g′),
i.e., the ratio of the intrinsic viscosities of HBP1 and BCP, decreased with increasing
Mw. Taken together, these results confirmed that HBP1 possesses a hyperbranched structure.
Subsequently, we focused on synthesizing HBPs with different branching degrees by varying the feed ratios of GA. Increasing the GA:
1 feed ratio from 3:1 to 6:1, 9:1, and 12:1 resulted in no obvious change in the
Mns (from ~23.1 kg/mol to 24.8 kg/mol) of the resulting copolymers; whereas,
Ðs increased from 3.21 to 7.66, 8.10, and 12.10, respectively (entries 3–5,
Table 1). Correspondingly,
Mws of the resulting copolymers increased to 189.9, 197.6, and 279.5 kg/mol, respectively, indicating the branched density of the resulting copolymers increased with the GA addition amount. Nevertheless, further improving the GA:
1 feed ratio to 15:1 led to the gelation of the resulting copolymers (entry 6). In addition, at the lower catalyst concentration with a feed ratio of PO:MA:GA:
1:[PPN]Cl = 4000:4000:24:1:2, the copolymerization decelerated, as it required 12 h to attain PO and MA conversions of 99% and 98%, respectively. Compared with the copolymerization with a feed ratio of PO:GA = 1000:6 (HBP2, entry 3),
Mn of the resulting copolymer increased to 34.6 kg/mol, but with a lower
Ð of 4.37, indicating a lower
Mw (151.2
vs. 189.9 kg/mol) of the resulting copolymer (entry 7,
Table 1). This might be attributed to the relatively lower reactivity of GA under a lower catalyst concentration. This hypothesis was further evidenced by the fact that
Ð of the copolymer synthesized with a higher GA feed ratio of 4000:4000:30:1:2 was lower than that of HBP2 (7.66
vs. 7.15) (entry 8,
Table 1). The decelerated copolymerization led to a decreased PO conversion of 72%, producing branched copolymer with a higher
Mw of 320.3 kg/mol. Similarly, the gelation was observed as GA addition was increased to 4000:4000:40:1:2 (entry 9,
Table 1). In addition, the copolymerization was also capable of proceeding under lower catalyst loading, affording a lower PO conversion and branched copolymer with decreased
Mw (entry 10,
Table 1). This can be attributed to the lower transformation efficiency of PO at a lower catalyst concentration.
To shed more light on how the HBPs were produced, the copolymerization process of PO and MA in the presence of GA was monitored
via an intermittent experiment. A series of parallel experiments with a feed ratio of PO:MA:GA:
1:[PPN]Cl = 500:500:3:1:2 were performed and quenched at different times, and the reaction mixtures were extracted for
1H NMR and GPC analyses. Only PO conversion was observed during the initial stage (
Fig. 3a). Notably, no signal representing GA was observed (Fig. S2 in Supporting information), which can be attributed to its concentration being too low to be detected during the polymerization process.
Mns of the resulting polymers increased as PO was consumed and reached 19.6 kg/mol with a
Ð of 1.78 (
Fig. 3b). Then, the transformation of MA occurred after PO was fully consumed. Correspondingly,
Mn of resulting copolymer dramatically increased as MA was consumed, together with an increase in
Ð, indicating the formation of a branched copolymer.
To obtain more structural information on the branched points of the resulting copolymer, the competition rates of the monomer sets PO/GA and GA/MA were explored. The monomer reactivity ratios of PO/GA and GA/MA copolymerizations were established according to the Fineman–Ross equation (Tables S1 and S2 in Supporting information).
rPO and
rGA were determined to be 1.10 and 0.99, respectively, from the copolymerization of PO and GA (
Fig. 3c). The higher reactivity of PO compared with that of GA indicates that the PO insertion is more favored, indicating that GA is distributed at the end of polymer mainchain. Similarly,
rGA and
rMA were determined to be 0.85 and 1.15, respectively, from the copolymerization of GA and MA (
Fig. 3d), indicating that the insertion of MA was more favored. Consequently, the addition of small amounts of GA (up to 0.12%, in molar ratio) led to the formation of HBPs comprising polyether and polyacrylate segments.
Accordingly, the HBPs synthesis mechanism is proposed to be as follows. In the copolymerization process, due to the low addition amount of GA, the random copolymerization of PO and GA occurred first, forming polyether segments with acrylate groups distributed at the end of polyether segments (
Fig. 4). Then, the propagating species of alkoxylate triggered the copolymerization of MA and acrylate groups appended to the polyether segment. Therefore, HBPs comprising polyether and polyacrylate segments were produced from the copolymerization of PO and MA in the presence of GA, in which GA acted as the branching point.
In addition, substituting (
rac)-PO for optical (
S)-PO to copolymerize with MA in the presence of GA (
iPO:GA = 1000:6) produced a HBP (
iHBP1) with an isotactic PPO (
iPPO) segment (Table S3 in Supporting information), of which the isotacticity reached 85% (Fig. S3 in Supporting information). Differential scanning calorimetry (DSC) characterization revealed that
iHBP1 has a melting temperature (
Tm) of 67.5 ℃, ascribed to the
iPPO segment, together with two glass transition temperatures (
Tgs) of −73.4 and 8.6 ℃ (Fig. S4 in Supporting information) [
36,
38]. With this HBP in hand, we attempted to extend its application in facilitating the compatibilization of immiscible
iPPO/PMA blends, in which
iPPO and PMA were synthesized from the respective homopolymerization of (
S)-PO and MA, respectively, thereby improving their mechanical properties (
Fig. 5a). Initially, the thermal and mechanical performances of
iPPO/PMA blends with different ratios were characterized using DSC and universal tensile tests. The
iPPO/PMA blend (70:30 wt%,
B1) has a
Tm and
Tg of 67.6 and 18.4 ℃, respectively (
Fig. 5b), ascribed to
iPPO and PMA. This is in good accordance with the
Tm and
Tg of isolated PPO and PMA, respectively (Fig. S5 in Supporting information). No difference in
Tm and
Tg between the blend and isolated
iPPO and PMA indicates that
B1 has a separated microphase.
Tensile text revealed that
B1 has an ultimate tensile strength (
σb) and strain (
εb) at break of 37.9 MPa and 689%, respectively (
Fig. 5c). Subsequently,
iHBP1 with a weight content of 3 wt% was added to
B1 to improve its mechanical performance (
B2). The addition of
iHBP1 had no obvious influence on the thermal properties of
B2, which exhibited a
Tm of 64.9 ℃ and
Tgs of −66.7 and 18.1 ℃. However,
σb and
εb of
B2 were improved to 53.2 MPa and 889%, respectively. After the addition of 3 wt%
iPPO-
b-PMA block copolymer (
iBCP to
B1),
σB and
εb of
B3 were lower than those of
B2 (53.2
vs. 43.4 MPa and 889%
vs. 761%, respectively). This suggests that the topological structure of the
iPPO-
b-PMA block copolymer significantly influenced its compatibilization efficiency in enhancing the mechanical performance of the
iPPO/PMA blend, with higher branching degrees yielding greater efficiency. To verify this hypothesis, we examined the thermal and mechanical properties of PO/MA blends containing block copolymers with different branching degrees that were synthesized through PO/MA copolymerization in the presence of GA at (
S)-PO:GA feed ratios of 1000:12 (
iHBP2) and 4000:30 (
iHBP3). DCS characterization revealed no obvious change in
Tm and
Tg of the blends (
Fig. 5b). Blends comprising block copolymers with higher branching degrees exhibited enhanced mechanical properties, as
σbs improved from 37.8 (
B1) to 53.5 (
B4) and 57.9 MPa (
B5), with a corresponding increase in
εbs from 689% to 981% and 998%, respectively (
Fig. 5c). These results revealed that block copolymers with higher branching degrees exhibit higher compatibilization efficiency, with
iHBP3 demonstrated to be the most effective compatibilizer.
Subsequently, the influence of the
iHBP3 content on the compatibilization efficiency were explored in detail.
iHBP3 incorporation had no significant effect on the thermal properties of the resulting blends. Two glass transitions (from −65.0 ℃ to −64.0 ℃ and from 16.7 ℃ to 18.4 ℃) and a melting peak near 66.0 ℃ were observed from the second-heating DSC curves for blends containing different amounts of
iHBP3 (
Fig. 6a). Characterizing the blends using dynamic mechanical analyzer (DMA) revealed comparable loss factors (tan
δ) for all blends. However, the storage modulus (
E’) of the blends increased with a higher
iHBP3 content (
Fig. 6b). Notably,
B5 (
B1 containing 3 wt%
iHBP3) achieved the highest
E’ of 4350 MPa at −75.0 ℃. Reducing the amount of
iHBP3 to 1 wt% (
B6) reduced the compatibilization efficiency, yielding a
σb and
εb of 42.0 MPa and 780%, respectively (
Fig. 6c). While incorporating 5 wt%
iHBP3 into
B1 (
B7) enhanced its mechanical properties (
σb = 54.1 MPa,
εb = 963%), these values remained lower than those of
B5. Collectively, these results indicate that 3 wt%
iHBP3 optimally improved the mechanical performance of the
iPPO/PMA blend. To elucidate how
iHBP3 enhances the mechanical performances of the
iPPO/PMA blend, the morphologies of blends containing varying
iHBP3 contents were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM characterization revealed the sea-island structure of the
iPPO/PMA blend, indicating that immiscible domains were formed due to the separated microphase of
iPPO/PMA (
Fig. 6d,
B1). In contrast, incorporating hyperbranched
iPPO-
b-PMA block copolymer improved microphase separation, which was evidenced by the smoother, flatter morphologies observed for the
iPPO/PMA blends containing
iHBP3. Specifically, the blend with 3 wt%
iHBP3 (
B5) exhibited the most uniform microphase, which was further supported by AFM imaging (
Fig. 6e), highlighting its superior ability to promote microphase fusion. These findings indicate that
iHBP3 addition reduces interfacial tension and domain size within the
iPPO/PMA blend, thereby enhancing its mechanical performance.
In summary, we have demonstrated an effective strategy for synthesizing HBPs from epoxides and acrylate via bimetallic synergistic catalysis in the presence of GA. This strategy is supported by the monomer reactivity-determined polymerization in the presence of GA, with the random copolymerization of epoxide and GA occurring first. Notably, the branching degree of the resulting HBP can be readily tuned by changing the amount of GA. In addition, the incorporation of HBPs improved the mechanical performances of the blend comprising iPPO and PMA. A detailed investigation of the influence of the topological structure and weight content of HBPs revealed the structure–performance relationship, indicating their excellent potential for use as compatibilizers.
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