Rapid, Controlled Branching Polymerization of Cyanoacrylate via Pathway‐Enabled, Site‐Specific Branching Initiation

Abstract

Controlled branched structures remain a key synthetic limitation for monomeric tissue adhesives because their on‐site polymerization that enables adhesion formation requires rapid kinetics, high conversion, and straightforward setup. In this context, site‐specific branching initiation by using evolmers is potentially effective for structural control; however, the efficiency and kinetics in current reaction setups persists to be a major challenge. In this paper, an evolmer induces a controlled branching polymerization of cyanoacrylate amid the high monomer reactivity useful in rapid adhesion. The contrasting reactivities between the vinyl and the initiating groups in the evolmer molecule generate a kinetic pathway that favors a control‐enabling branching mechanism. Through density functional theory calculations, the reaction pathway toward branching is shown to kinetically favor site‐specific initiation by six orders of magnitude than the route toward non‐specificity. Reaction monitoring confirms the branching polymerization after the polymerization with the evolmer forms a more compact structure than the linear counterpart. Control of branching density is demonstrated in rapid polymerizations within minutes and in polymerizations completed in an instant. These results provide a template for achieving site‐specific branching initiation during adhesion formation and, broadly, where conditions for kinetic control are necessary.

Site‐specifically initiated branching is expanded to anionic polymerization by using an evolmer that is a reactive vinyl monomer and consists of a weak initiating group. Density functional theory calculations show the kinetic favorability of a reaction sequence that leads to such specificity.Gradual evolution of molecular weight and dispersity and control of branching density are demonstrated in experiments.

1. Introduction

Cyanoacrylates are highly reactive monomers that polymerize into high molecular weight linear chains at extremely high propagation rates.^{[ 1} , ² , ^{3 ]} Their distinct polymerizability, which features an initiation step triggerable even by weakly nucleophilic groups, provides the much‐needed practicality in instant adhesion, their widely known application. Within just a few seconds of contact, cyanoacrylates can rapidly form strong covalent bonds with a wide range of substrates, including glass, rubber, and biological tissues, requiring no complicated setup. This reactivity, however, hinders the synthetic accessibility of structures with non‐linear topologies or defined structural parameters from the polymerization of cyanoacrylates.^{[ 4 ]} Progress toward this objective fails to achieve a compromise between a rapid yet efficient monomer conversion and the simplicity and mildness of reaction conditions, both of which are critical for instant adhesion. Overcoming this long‐standing challenge creates an opportunity to form adhesives that appropriately match various specifications, such as mechanical integrity, flexibility, or in vivo degradability.

Structural features of polymers can be precisely configured for property control beyond combining assorted monomers or additives.^{[ 5} , ^{6 ]} While degree of polymerization can single‐handedly determine the behavior of linear networks,^{[ 7} , ^{8 ]} multiple structural parameters define nonlinear topologies and can be independently tuned toward unprecedented combinations of physical properties.^{[ 8} , ⁹ , ^{10 ]} Of the nonlinear topologies, branched architectures have particularly drawn interest because of their weak entanglements, compact conformations, and abundant terminal groups.^{[ 11 ]} These features lead to promising functional properties^{[ 9} , ^{12 ]} in biology,^{[ 13} , ^{14 ]} interfacial chemistry,^{[ 15 ]} optoelectronics,^{[ 16 ]} and electrochemistry.^{[ 17 ]} Existing strategies to attain precise control over branching, however, are marked by stringent reaction conditions and complex synthetic techniques, hampering their industrial accessibility. In particular, dendrimers and dendrons exhibit the highest structural precision, which is achieved through incremental growth in a synthesis of stepwise alternation of coupling and activation reactions.^{[ 18 ]} By stark contrast, hyperbranched polymers are prepared with minimal control as a trade‐off for the synthetic practicality of a one‐pot setup.

Synthetic approaches to these randomly branched polymers generally include the polycondensation of AB_x monomers or A₂ and B_x monomers,^{[ 6} , ^{19 ]} the copolymerization of vinyl monomers with multivinyl monomers,^{[ 20} , ^{21 ]} and the self‐condensing vinyl polymerization (SCVP) of AB^* monomers.^{[ 22 ]} In this context, branched chains grow through coupling reactions between moieties A and B or between vinyl groups, or through addition reactions of vinyl group A in the propagation initiated by initiator group B^*. In general, these approaches are associated with synthetic challenges, including insoluble networks, low monomer conversion, uncontrolled molecular weight, and high dispersity. Efforts were made accordingly, such as using a high initiator to monomer ratio to avoid intrachain cyclization in multivinyl polymerization,^{[ 20 ]} slow monomer addition in the polycondensation of AB_x monomers or SCVP of AB^* monomers to suppress chain‐to‐chain coupling,^{[ 23 ]} and emulsion polymerization to confine interchain reactions.^{[ 24 ]} Nonetheless, the solutions to overcome these challenges rely on specific reaction conditions and are not inherent to these multifunctional monomers because of their non‐specific reactivity. In particular, non‐specific reactivity enables these monomers to form chains that can undergo chain‐to‐chain coupling, which results in exponential chain growth and thereby complicates the structural control. Alternatively, the reactivity of multifunctional monomers can be modified to disable chain‐to‐chain coupling and induce gradual growth of the branched structure.^{[ 25 ]} This mechanism is similar to linear propagation, wherein chain growth, including the branching reaction, occurs strictly through the addition of monomers to the propagating chain one at a time.

To achieve this, a strategy was developed to induce selectivity in the branching process through a monomer‐directed reaction sequence (Figure 1a). Importantly, the reaction sequence entails that, in each molecule of AB^* monomer 1, branching occurs strictly after its vinyl group undergoes addition reaction with a propagating chain, such as 2.^{[ 26 ]} Specifically, only monomers 1 or 3 can react with the active chain‐end, thereby introducing a site‐specific initiation of branching to inhibit the randomness due to chain combination. Monomers like 1, which are capable of such selective reactivity, are termed “evolmers” because they “evolve” into initiators upon the conversion of their vinyl group. The underlying mechanism arises from the change of the carbon(sp²)‐B^* in the vinyl group to a carbon(sp³)‐B^* upon the addition reaction. This reduces the bond dissociation energy (BDE) of carbon‐B^* and thereby activates the reactivity of the initiator group in 1. So far, however, this elegant approach has been limited to free‐radical polymerizations because the mechanism of initiator activation relies on the lowering of BDE. Furthermore, evolmers are designed to exhibit hierarchical reactivity, that is, the dependence of the reactivity of B^* on the vinyl group.

Figure 1.

Strategies to suppress the chain combination through site‐specific reactivity, wherein the branch formation in each evolmer molecule occurs strictly after the incorporation of the same molecule into a propagating chain. a) Rationally designed evolmer 1 is converted into an active initiator exclusively after the carbon(sp²)–B^* is converted into a carbon(sp³)–B^*, thereby considerably lowering its bond dissociation energy. b) Evolmer 4 initiates the polymerization of monomer 5 slowly relative to its incorporation into propagating chain 6, kinetically favoring the reaction sequence. c) The initiating ability of the evolmer is activated after its incorporation into a polymer chain.

In this work, we expand the strategy to the rapid polymerization of cyanoacrylate by using an evolmer comprising a weak initiator group with a pathway‐dependent activation mechanism. To achieve the reaction sequence for site‐specific initiation, we devised evolmer 4 whose functional groups exhibit dissimilar reactivities, such that the branching process develops slowly relative to its addition reaction with monomer 5 (Figure 1b). To meet this specification, we exploited the high vinyl reactivity of cyanoacrylate and the weak initiating ability induced by the carboxyl group after its deprotonation. On the basis of our calculated mechanism, such disparity in reactivities favors a reaction sequence that suppresses chain combination. Intriguingly, the energy barrier for initiating the branching process is lower after the evolmer is incorporated into a polymer chain (Figure 1c). Such phenomenon replicates the site‐specific branching through a kinetic preferential pathway that is consistent with the reaction sequence toward linear propagation. This differs from previously reported mechanisms of evolmers, wherein the reaction sequence is achieved by using an initially dormant B^*, whose reactivity is activated only during the reaction. More specifically, this study provides proof of principle for designing an evolmer without relying on a polymerization‐induced reduction of BDE to direct the preferred reaction sequence. Interestingly, the branching method can be integrated to the polymerization of cyanoacrylates, where the monomer must efficiently polymerize under mild reaction conditions.

2. Results and Discussion

Due to their high reactivity, the polymerization of cyanoacrylate monomers can be triggered even by weak or uncharged nucleophiles. Such polymerizability can be inhibited by strong acids, which can terminate the propagation by proton transfer to the carbanionic head of propagating chains.^{[ 27 ]} Weak acids can likewise protonate the carbanionic head, but the carboxylate anions produced from the protonation can reinitiate the polymerization.^{[ 1 ]} Building on this knowledge, we selected a carboxyl group as a dormant initiating group in evolmer 4, which can be activated through its deprotonation by a propagating chain 6. An activation step as such is necessary to ensure that, overall, the first initiation step occurs outside the evolmer and that only with nucleophiles converts the evolmer into an initiator. Without this activation step, the evolmer can immediately polymerize 5 upon the addition of these two species together, thereby opposing the reaction pathway toward control.

We hypothesized that 4 can undergo copolymerization with 5 and subsequently induce the branching polymerization of the latter through the preferred reaction sequence. The hypothesis hinges on the following theoretical premises: 1) the kinetic preference for the reaction sequence, which is enabled by the rapid polymerizability of 4 and its relatively sluggish initiation of the branching process (Figure 1b); and 2) the lowering of the energy barrier for initiation after the evolmer is incorporated into a polymer chain, such as indicated by the kinetic difference of intiation between 7 and 8 (Figure 1c). To corroborate these theoretical premises, we performed density functional theory (DFT) calculations for the energy diagram of model reactions. In the simulation, small‐molecule analogs of the reacting species were used for computational efficiency while considering the reproducibility of the calculated energies despite the truncation, as previously shown (see Note S1, Supporting Information).^{[ 28 ]}

First, the propagating chains can react with the carboxyl group first and arguably enable the undesired chain combination. The energy diagrams of the chain combination and the linear propagation mechanisms must be examined to investigate the viability of either mechanism (Figure 2 ). As aforementioned, the initiating ability of the evolmer can be activated upon the deprotonation of the carboxyl group by a propagating chain. Here the reaction of evolmer 9 and propagating chain 10 forms a stable encounter complex 11. This interaction is marked by the elongation of the O─H bond of the carboxyl by 0.05 Å toward the carbanion of 10. If the encounter complex directly generates a deprotonated evolmer, the reaction can undergo chain combination, wherein control is minimal. However, a relatively high energy barrier of 9.5 kcal mol⁻¹ must be overcome for the deprotonation of the carboxyl group. Furthermore, the succeeding steps are kinetically unfavorable due to the confluence of the reversible formation of 13 and the high‐barrier addition reaction toward 14. Conversely, encounter complex 11 can proceed to a low‐barrier protonation of the evolmer to easily regenerate evolmer 9, which can thereafter undergo a non‐reversible addition reaction to form 16. To assess the preference for the two mechanisms, the energy barriers for the first non‐reversible reaction from the stable encounter complex toward either direction were compared. Specifically, the energy difference of 8.6 kcal mol⁻¹ between the relevant transition states (TS), TS_13/14 and TS_9/16 , equates to a selectivity of six orders of magnitude for 16 over 14; hence, the linear propagation route is the more viable pathway as the chain combination route is suppressed by a considerable degree.

Figure 2.

Free energy profile for the linear propagation and chain combination pathways starting from the reaction of evolmer 9 with propagating chain 10. The free energy diagram was determined by DFT calculations at the B3LYP/TZVP level. Energy values are shown in kcal mol⁻¹ and are relative to encounter complex 11, which can form from the reaction between 9 and 10.

Second, the energy barrier for the initiation of the branching process becomes lower after the evolmer is incorporated into a polymer chain (Figure 3 ). For example, in the case of a chain‐incorporated evolmer like 17, the energy barrier for initiation (6.7 kcal mol⁻¹) is lower by 2.8 kcal mol⁻¹ than the barrier for the evolmer molecule 9 (9.5 kcal mol⁻¹). A lower energy barrier for initiation (6.5 kcal mol⁻¹) is likewise observed when the degree of polymerization is increased such as in 21. Such polymerization‐induced reduction in the energy barrier for initiation is different from the previously reported mechanisms of evolmers, which relies on the lowering of the BDE of carbon‐B^*. The usual design of evolmers requires that B^* be covalently linked next to the vinyl group because the BDE decreases when carbon(sp²)‐B^* is converted to carbon(sp³)‐B^*. On the contrary, in this work, the reactivity of B^* in evolmer 9 is likely unaffected directly by the conversion of the distant vinyl group. To test this hypothesis, the DFT calculations were extended to species that represent 9 but with the α‐carbon of its cyanoacrylate moiety changed to a carbon(sp³). The investigation examined three cases where the α‐carbon is substituted by a) two methyl groups, b) two hydrogen atoms, or c) one methyl group and a hydrogen. In each case, the energy barrier for initiation was determined to be 9.9, 10.1, and 9.8 kcal mol⁻¹, respectively (Figure S1, Supporting Information). All these values for carbon(sp³) are comparable to that of evolmer 9 (9.5 kcal mol⁻¹), suggesting that the conversion of the vinyl carbon into a carbon(sp³), that is, the vinyl group conversion, does not precisely cause the activation of B^*. We then attribute this reduced energy barrier to the steric hindrance due to the polymer chain into which the evolmer is incorporated, diminishing the attraction between the carboxyl and the carbanion in the encounter complex. Consequently, this favors the dissociation of the incorporated evolmer from the complex and the subsequent initiation of the branching process.

Figure 3.

Free energy profile for the initiation of the branching process by evolmer 9 and by an evolmer incorporated into a polymer chain, such as in 17 and 21. The free energy diagram was determined by DFT calculations at the B3LYP/TZVP level. Energy values are shown in kcal mol⁻¹ and are relative to encounter complex 11, 18, or 22, which can form from the reaction between propagating chain 10 and the corresponding evolmer 9, 17, or 21, respectively.

To verify our hypothesis, branched structures were synthesized by the copolymerization of 2‐octyl cyanoacrylate monomer (25) and 5‐carboxypentyl cyanoacrylate evolmer (26) (Figure 4a). In this article, polymerization reactions were carried out in bulk monomer medium, except for the reaction monitoring to facilitate sampling. Prior to the addition of the initiator, 25 and 26 can form a stable solution without the occurrence of premature polymerization, denoting the inactivity of the carboxyl group before deprotonation. The relative concentrations of 25, 26, and the initiator used in the polymerization reactions are within the range of concentrations demonstrated in previous studies about evolmers.^{[ 29} , ^{30 ]} The reactions were conducted in open air at room temperature to demonstrate that the practical feature typical of cyanoacrylic monomers, that is, their high polymerizability at ambient reaction conditions, is unaffected by the branching reaction.^{[ 31 ]} Triethylamine (Et₃N), owing to its basicity and solubility in the reaction medium, was used as the initiator primarily to minimize the induction period and, overall, the reaction time.^{[ 32 ]} Using Et₃N was likewise aimed to resemble the initiation of polymerization of cyanoacrylate‐based tissue adhesives by amine groups attached on the surface of biological tissues.^{[ 33 ]}

Figure 4.

Monitoring of the linear and branching polymerization using molar ratios [25]/[26]/[Et₃N] equal to 500/0/1 and 500/10/1, respectively. a,d) Time evolution of the combined consumption of 25 and 26 determined by ¹H NMR spectroscopy. b,e) The number‐average molecular weight and polydispersity index at increasing conversion c,f) Time evolution of the SEC traces.

Monitoring of the polymerization reaction was conducted in tetrahydrofuran solvent using molar ratios of [25]/[26]/[Et₃N] equal to 500/0/1 and 500/10/1, respectively, for the linear and branched polymerization. Proton nuclear magnetic resonance (¹H NMR) spectroscopy and size‐exclusion chromatography (SEC) using linear polystyrene standards were performed after specified reaction times to track the time evolution of monomer conversion and apparent number‐average molecular weight (M _n,app). Monomer conversion was determined by quantifying the unreacted cyanoacrylate and, in particular, by integrating the proton peak of the vinyl group (δ 6.6 or 7.0 ppm) relative to the proton peak at the tertiary carbon (δ 5.0 ppm). Only the combined conversions of 25 and 26 are reported because the proton peaks corresponding to their vinyl groups were indistinguishable from each other. This combined conversion can be mostly assigned to the consumption of 25, with its initial concentration in large excess. Taking this into account, the linearity of the plot of monomer conversion against time suggests that the copolymerization followed a pseudo‐first‐order reaction. The copolymerization with the evolmer likewise showed a significant decline in the polymerization rate (Figure 4a,d), while no polymer gelation was observed.

Significantly lower M _n,app values were obtained in the copolymerization than in the homopolymerization throughout the reaction (Figure 4c,f), denoting relatively small hydrodynamic volumes, which are consistent with branched architectures. In addition, the deviation of M _n,app from the theoretical number‐average molecular weight (M _n,theo), which was determined based on monomer conversion, steadily widened in the copolymerization with increasing monomer conversion. The lower dispersity (Đ), however, is unusual with branched polymers and may have been caused by the slower polymerization, as similarly observed in previous studies.^{[ 2 ]} The evolution of SEC traces with monomer conversion shows a gradual increase in both M _n,app and Đ. This trend is in stark contrast to the exponential growth of M _n,app and Đ, which is attributable to chain‐to‐chain combinations in the polymerization of AB_x monomers, multivinyl monomers, and typical AB^* monomers.^{[ 20} , ^{21 ]} Interestingly, however, the gradual increase observed in this study is consistent with the proposed branching mechanism for evolmers, wherein chain growth, including the formation of branches, occurs strictly by the addition of monomers one at a time at the end of a propagating chain.^{[ 14} , ^{30 ]} Such gradual chain growth is the goal of site‐specific branching initiation to facilitate control over the branched structure.

To assess the quality of control enabled by the strategy, copolymers were prepared by bulk polymerization of various molar ratios of 25 and 26 (Table 1 ). Near‐complete conversions of the vinyl group, that is, values of at least 99% (Figures S4–S10, Supporting Information), were achieved within short reaction periods as measured by ¹H NMR spectroscopy. Although the addition of 26 similarly resulted in lower rates of polymerization, the slowdown was less severe than the above‐mentioned solution polymerization possibly because of the high monomer concentration in the bulk medium. In Entry 1, the absence of 26 verifies the conclusion that linear polymers were generated and that Et₃N could not cause the branching polymerization of 25. The higher M _n,app than M _n,theo is expected due to the high reactivity of 25 and its uncontrolled, rapid linear polymerization.^{[ 4 ]} Such lack of control during linear polymerization is highly evident in the formation of a daughter polymer in Entry 1.^{[ 2 ]} Rapid polymerization occurred even at temperatures close to the melting point of 25 (c.a. −10 °C), which is the lowest temperature possible to conduct this bulk polymerization.

Table 1.

Synthesis of branched polymers of 25 by copolymerization with 26.

Entry a)	[25]/[26]/[Et3N]	time b) [min]	M n,theo c) [ × 104 g mol−1]	M n,app d) [ × 104 g mol−1]	Đ d)	S n
1	250/0/1	5	5.19	11.22	3.11	−
2	250/3/1	120 f)	5.25	2.41	3.65	36
3	250/8/1	120 f)	5.36	1.61	3.54	15
4	250/15/1	120 f)	5.50	1.43	3.43	8.1
5	250/3/10	5	0.53	0.69	3.36	16
6	250/8/10	5	0.54	0.53	2.96	10
7	250/15/10	5	0.56	0.51	2.72	6.3

^a) The reaction occurred at 25 °C in bulk/solventless medium;

^b) Time interval between the addition of initiator (Et₃N) and the solidification of the solution;

^c) Calculated based on complete conversion as estimated through ¹H NMR spectroscopy: M _n,theo = ([25] × MW ₂₅  + [26] × MW ₂₆ )/[Et₃N] + MW _Et3N;

^d) Determined by SEC with a refractive index detector calibrated against polystyrene standards;

^e) S _n = DP ₂₅ /(1 + 2 × DP ₂₆ ), where DP is the degree of polymerization;

^f) Time interval between the addition of initiator (Et₃N) and the measurement of conversion.

Importantly, despite each having an approximately equal M _n,theo (Entry 1–4), considerably lower M _n,app values were obtained from the copolymer samples (Entry 2–4), implying an effective branching method. Control of the branching density was demonstrated by adding more of 26 into the initial reaction solution (Entry 2–4, Figure 5a) to shorten the average distance between two branching junctions in measures of monomer units (S _n). Specifically, the parameter S _n was calculated here as S _n = DP ₂₅ /(1 + 2 × DP ₂₆ ), where DP is the degree of polymerization.^{[ 29 ]} Increasing the relative molar ratio of 26 from 3 to 8 or 15 decreases the S _n value from 36 to 15 or 8.1, respectively. This reduction of S _n led to the formation of more compact conformations as indicated by the declining trend of M _n,app with higher amounts of 26. In efforts to attain reaction times comparable with that of the linear polymerization (Entry 1), the relative amounts of the initiator were increased from 1 to 10 (Entry 5–7, Figure 5b). In addition to the confirmation of the branched structures, a similar decreasing trend of M _n,app suggests control of the branching density even with enhanced rate of polymerization.

Figure 5.

Structural control by increasing the relative amount of the evolmer. a,b) SEC traces of branched polymers with increasing [26]/[Et₃N] using molar ratios of [25]/[Et₃N] equal to 250 (Entry 1–4) and 25 (Entry 5–7). c) M _n,app versus M _n,abs plot of the samples polymerized instantaneously, wherein the branched polymers (Entry 9–10, 12) are found below the M _n,app = M _n,abs line as each demonstrated lower M _n,app than its M _n,abs while the linear polymer (Entry 8) is found above the line. d) M _n,app/M _n,abs plot of the samples polymerized instantaneously, wherein decreasing M _n,app/M _n,abs ratio is observed with increasing amount of the evolmer (26).

To explore the feasibility of integrating 26 in the instantaneous polymerization of cyanoacrylate for fast‐acting liquid adhesives, bulk solutions of 25 with different amounts of 26 (Table 2 ) were prepared. These samples were placed in a closed chamber whose atmosphere was initially saturated by Et₃N vapor. The instantaneous polymerizations could be visibly verified as the solutions instantly solidified, without noticeable discrepancies in the start and end times of the polymerization of different samples. As the effective concentration of the initiator was difficult to measure, the absolute molecular weight (M _n,abs) determined by multi‐angle laser light scattering (MALLS) was determined in lieu of M _n,theo. A linear polymer (Entry 8) and a branched counterpart (Entry 9) were synthesized with the aim of obtaining comparable values of M _n,abs for a precise differentiation of their structural properties. While their values of M _n,abs were close, the M _n,app of the branched structure showed a considerably lower value than that of the linear one, confirming the compact structure of the former. Additional samples with increasing evolmer content were also prepared but showed higher M _n,abs values likely due to the slight concentration differences of the initiator vapor during the instantaneous reaction. Despite having higher M _n,abs values than the linear analogue (Entry 8), the branched polymers obtained lower M _n,app values (Figure 5c). Moreover, with increasing M _n,abs values, the glass transition temperature (T _g) values of the branched polymers were all expected to exceed that of the linear polymer (Entry 8), yet these polymers demonstrated lower values, an indication of a branched structure. The control over branching is then analyzed by evaluating the effect of the evolmer content on the features of the branched structure. While the obtained samples showed increasing M _n,abs values with the evolmer content, a decreasing trend of M _n,app/M _n,abs ratio were observed (Table 2 and Figure 5d), signifying an increasingly compact structure. This result is consistent with the presence of more branching with more evolmer incorporated into the polymer.

Table 2.

Synthesis of branched polymers of 25 by copolymerization with 26 under Et₃N atmosphere.

entry	[26]/[25] [moL%]	M n,app a) [ × 104 g moL−1]	M n,abs b) [ × 104 g moL−1]	M n,app/M n,abs	Đ b)	T g c) [°C]
8	0.0%	5.03	2.18	2.31	1.60	56.1
9	1.0%	0.85	2.07	0.41	2.56	51.4
10	1.3%	1.02	3.24	0.31	2.21	51.2
11	2.0%	1.03	− d)	− d)	− d)	50.7
12	3.0%	1.21	4.24	0.28	1.80	53.1

^a) Determined using SEC calibrated with polystyrene standards;

^b) Determined by MALLS;

^c) Measured by DSC;

^d) No data obtained.

3. Conclusion

We synthesized structurally controlled branched polymers from the highly reactive cyanoacrylate monomer by using an evolmer to direct a pathway‐enabled site‐specific branching initiation. This provides a strategic route toward a controlled on‐site branching polymerization of monomeric tissue adhesives to provide control over the resulting branched structure and therefore its properties. We found that the pathway toward site‐specific initiation is kinetically preferred by an evolmer with contrasting reactivities, that is, an evolmer with a high vinyl reactivity toward addition reaction and a reactive yet relatively weak branching initiator. Such feature of the evolmer plays a significant role in facilitating that the addition reaction occurs prior to the branching initiation in the same evolmer molecule, replicating the reaction sequence in the mechanism of the evolmer strategy. Through DFT calculations, this contrasting reactivity was demonstrated to generate the desired reaction sequence as supported by the significant difference in the reaction barrier between the routes toward site‐specificity and non‐specificity. The formation of branched structures from the copolymerization of cyanoacrylate with the evolmer was corroborated by the compact polymer structures obtained during the reaction monitoring study and the bulk polymerization reactions. Importantly, near‐complete monomer conversions were achieved in reactions that were completed within minutes and, in other occasions, instantaneously. In either case, the molecular weight and the branching density were controlled by adjusting the molar ratios of the monomer, the evolmer, and the initiator. Ultimately, this work provides a strategy for designing a branching mechanism with site‐specific initiation based on the contrasting reactivity in the evolmer, which can be useful where conditions associated with kinetic control is necessary.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Roxas A. P., Yu H., Tamtaji M., Yang Z., Luo Z., Rapid, Controlled Branching Polymerization of Cyanoacrylate via Pathway‐Enabled, Site‐Specific Branching Initiation. Macromol. Rapid Commun. 2024, 46, 2400658. 10.1002/marc.202400658

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.