Controlling Polymer Composition in Organocatalyzed Photoredox Radical Ring-Opening Polymerization of Vinylcyclopropanes

Abstract

Although radical polymerizations are among the most prevalent methodologies for the synthesis of polymers with diverse compositions and properties, the intrinsic reactivity and selectivity of radical addition challenge the ability to impart control over the polymerization propagation and produce polymers with defined microstructure. Vinylcyclopropanes (VCPs) can be polymerized through radical ring-opening polymerization to produce polymers possessing linear (l) or cyclic (c) repeat units, providing the opportunity to control polymer structure and modify the polymer properties. Herein, we report the first organocatalyzed photoredox radical ring-opening polymerization of a variety of functionalized VCP monomers, where high monomer conversions and spatial and temporal control were achieved to produce poly(VCPs) with predictable molecular weight and low dispersity. Through manipulating polymerization concentration and temperature, tunable l or c content was realized, allowing further investigation of thermal and viscoelastic materials properties associated with these two distinct compositions. Unexpectedly, the photoredox catalysis enables a postpolymerization modification that converts l content into the c content. Combined experimental and computational studies suggested an intramolecular radical cyclization pathway, where cyclopentane and cyclohexane repeat units are likely formed.

Graphical Abstract

INTRODUCTION

The merger of photoredox catalysis¹ and controlled polymerizations² has drawn increasing attention as to provide spatial and temporal control over the chain growth and produce polymers with predictable composition, molecular weight (MW), and low dispersity (Ð) under mild conditions.³ A variety of methodologies that implement light to affect diverse reaction mechanisms have been discovered, including photoinduced atom transfer radical polymerization (ATRP),⁴ photoinduced electron/energy transfer reversible addition–fragmentation chain transfer polymerization,⁵ photocontrolled cationic polymerization,⁶ metal-free ring-opening metathesis polymerization,⁷ and others.⁸ Our interest in photoredox-controlled polymerization originated with the motivation to develop an organocatalyzed ATRP (O-ATRP). Our group and others have employed highly reducing organic photoredox catalysts (PCs), such as perylene,^4e phenothiazines,^4f N,N-diaryldihydrophenazines,⁴ⁱ and N-arylphenoxazines,^4j,9 for O-ATRP. To further advance this nascent polymerization methodology, we are motivated to discover new reactivity of these organic photosystems that are otherwise inaccessible.

Vinylcyclopropanes (VCPs) are an intriguing class of monomers in that they can be polymerized through various pathways to yield drastically different polymeric structures, exhibiting low volume shrinkage or even volume expansion—a unique property of modeling and restorative materials.¹⁰ In a radical pathway, the radical ring-opening polymerization (rROP) proceeds to yield a polymer possessing a combination of unsaturated linear (l) and saturated cyclic (c) repeat units.¹¹ These c repeat units have been proposed to be cyclobutanes generated through a 4-endo-trig radical cyclization (Figure 1A).¹² To evaluate l/c selectivity, a linear factor S_L is defined here as the total percentage of l repeat units. Most rROP approaches have utilized traditional free radical polymerization techniques, which achieve low selectivity between l or c content and produce polymers with ill-defined MW characteristics (Figure 1B). To address this limitation, Cu-ATRP was attempted for the rROP (ATrROP) of 1,1-diethyoxycarbonyl-2-vinylcyclopropane (EtVCP) to produce well-defined polymers possessing a high l content (S_L = 98%).¹³ However, presumable bidentate coordination from EtVCP or poly(EtVCP) to the Cu(I) catalyst inhibits polymerization to reach high monomer conversions (<50%)¹⁴ even at elevated temperatures (Figure 1C). Leveraging our recent advances in O-ATRP, we envisaged that the use of an organic PC could eliminate this unfavorable coordination,¹⁵ enabling high monomer conversion and l/c selectivity while affording spatial and temporal control over the chain growth.

Figure 1.

Profile of radical ring-opening polymerizations of 1,1-disubstituted vinylcyclopropanes.

Herein, we demonstrate a photoredox-controlled approach utilizing N,N-diaryldihydrophenazine PCs to address the challenges of polymerization efficiency and l/c selectivity (Figure 1D). Given their strong excited-state reduction potentials, these organic PCs can activate the dormant alkyl bromide polymer chain (Figure 1E, A) to generate a propagating radical B. After monomer additions, the [²PC^•+]⁻[Br⁻] complex recaps the homoallylic radical intermediate D generated from the ring-opening of C to yield the dormant polymer E. Previously proposed backbiting cyclization of D may occur to form c repeat units. To mitigate side-reactivity, low loading of the PC may minimize radical concentrations in solution and minimize bimolecular radical termination pathways. As such, in our system a series of well-defined poly(VCPs) were obtained with controlled MWs, low Ð, and predominately l repeat units (S_L up to 98%). Moreover, through manipulation of polymerization conditions including temperature and concentration, tunable l/c selectivity was achieved (S_L = 1–98%), thus enabling investigation of the polymer properties associated with such distinct chemical compositions. Moreover, a photoredox polymer-chain modification converting l composition into c composition suggests that the cyclic repeat units in poly(VCPs) are likely a mixture of cyclopentanes, cyclohexanes, and previously proposed cyclobutanes.

RESULTS AND DISCUSSION

Synthesis of Poly(EtVCP) with High l Content.

Initial polymerization studies employed EtVCP as the monomer, diethyl 2-bromo-2-methylmalonate (DBMM) as the initiator, and N,N-dimethylacetamide (DMAc) as the solvent to investigate the catalyst performance on the polymerizations using standard O-ATRP conditions of ([EtVCP]:[DBMM]: [PC] = [1000]:[10]:[1] with irradiation by white LEDs and a cooling fan to maintain polymerization temperature at ~28 °C). Three PCs, including N,N-diaryldihydrophenazine 1 and 2 as well as 3,7-di(4-biphenyl)-N-naphthylphenoxazine 3, were found to able to drive the polymerization and produced poly(EtVCP) with high l content (S_L = 97%, Table 1, entries 1–3). Polymers synthesized by using PCs 2 and 3 displayed superior control over polymer molecular weight, with a number-average molecular weight (M_n) close to the theoretical values, as is indicated by initiator efficiencies (I* = M_n,theo/M_n,measured × 100%) close to 100% (entries 2 and 3). Conversely, polymerization with PC 1 afforded lower Ð = 1.21 (entry 1). In a comparison between common ATRP alkyl bromide initiators (Table S1), DBMM proved to be superior, with polymerization reaching the highest monomer conversion and producing polymers with low Ð and high I*. A screen of solvents (entries 4–6) revealed that EtOAc allowed for the best control over the polymerization, achieving 98% monomer conversion in 8 h, producing poly(EtVCP) with Ð = 1.15, high S_L of 97%, and predictable MW as evidenced by an I* of 99% (entry 4). No monomer conversion was observed in the absence of light, PC, or initiator, providing evidence of a photoredox-controlled ATRP mechanism (Table S2).

Table 1.

Optimization of the Synthesis of Poly(EtVCP) with High l Content^a

entry	PC	solvent	[M]/[I]/[PC]	convb (%)	Mnc (kDa)	Đ (Mw/Mn)c	I*d (%)	SLb,e (%)
1	1	DMAc	1000/10/1	72	22.6	1.21	69	97
2	2	DMAc	1000/10/1	70	16.8	1.63	90	97
3	3	DMAc	1000/10/1	77	15.5	1.53	107	97
4	1	EtOAc	1000/10/1	98	21.1	1.15	99	97
5	1	CH2Cl2	1000/10/1	99	29.0	1.20	74	96
6	1	PhCl	1000/10/1	99	22.3	1.17	97	94
7	1	EtOAc	1000/10/2	93	20.4	1.24	98	97
8	1	EtOAc	1000/10/0.5	98	19.8	1.17	106	97
9	1	EtOAc	1000/10/0.1	94	20.1	1.29	99	92
10	1	EtOAc	1000/20/1	99	11.8	1.12	91	97
11	1	EtOAc	1000/5/1	99	41.7	1.21	101	97
12	1	EtOAc	500/10/1	98	11.6	1.10	92	97
13	1	EtOAc	2000/10/1	97	39.7	1.25	104	94
14	1	EtOAc	5000/10/1	95	79.5	1.43	127	90

^aPolymerizations performed using 1.0 mmol of EtVCP, DBMM as the initiator, 1.0 mL of solvent, and irradiated by white LEDs at 28 °C for 8 h.

^bMeasured by crude ¹H NMR. S_L = l/(l + c).

^cMeasured by GPC.

^dInitiator efficiency (I*) = M_n(theo)/M_n(measured) × 100%, where M_n(theo) = MW(initiator) + MW(EtVCP) × conversion × ([EtVCP]/[initiator]).

^eS_L = l/(l + c).

By use of PC 1, catalyst loadings could be decreased to 100 ppm while still maintaining polymerization control [M_n = 20.1 kDa, Ð = 1.29, and I* = 99%, (entry 9)]. In addition, modulation of the monomer or initiator stoichiometry allowed for synthesis of poly(EtVCP) with targeted MWs (from 11.8 to 79.5 kDa) and low to moderate dispersities (Ð = 1.10–1.43) (entries 10–14). First-order kinetics were observed for monomer conversion in the PC 1 catalyzed ATrROP of EtVCP at 28 °C (Figure 2A). Polymerization progress analysis revealed a linear increase in polymer MW as a function of monomer conversion, while low dispersity (Ð = 1.17–1.22) remained during the entire course of polymerization (Figure 2B).

Figure 2.

Synthesis of poly(EtVCP) with high l content: (A) Plots of the natural log of monomer conversion as a function of time. (B) Plots of experimentally measured M_n and dispersity as a function of monomer conversion. (C) Chain-extension polymerization from a poly(EtVCP) macroinitiator with PhVCP and GPC traces before and after polymerization. (D) Plots of monomer conversion and experimentally measured M_n as a function of time for a pulsed irradiation experiment.

To evaluate the control imparted by this photoredox organocatalyzed ATrROP (O-ATrROP), a series of experiments probing chain-end group fidelity were performed to analyze the reversible activation–deactivation equilibrium. First, matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) was performed (Figure S11). In accordance with the proposed reversible activation–deactivation mechanism, the polymer possessed predominately Br chain-end groups as well as minor H chain-end groups, likely due to termination events during the polymerization or loss of the Br chain-end group during the MALDI-TOF MS analysis. The presence of the bromide chain-end groups allows for application of the polymers synthesized to serve as macroinitiators in subsequent polymerizations to access polymers with more complex composition. Thus, poly(EtVCP) (M_n = 11.6 kDa, Ð = 1.10, S_L = 97%) was synthesized, isolated, and employed as a macroinitiator for rROP of a different monomer 1,1-diphenyloxycarbonyl-2-vinylcyclopropane (PhVCP) to synthesize a poly(EtVCP)-b-poly(PhVCP) copolymer. The synthesis of this block copolymer was supported by a combination of NMR characterization (Figure S13) and a shift in the retention time of the gel-permeation chromatography (GPC) polymer trace due to an increase in polymer MW (M_n = 27.8 kDa, Ð = 1.23) (Figure 2C). Moreover, the block copolymer synthesis achieved a high I* of 97%, highlighting the Br chain-end group fidelity of the macroinitiator. We also demonstrated temporal control, a key feature of photoredox-catalyzed processes, using a pulsed-irradiation experiment, where the polymerization proceeded only under light irradiation, paused during dark periods (as long as 12 hours), and could be resumed with continued irradiation, further supporting the light-driven, reversible activation–deactivation mechanism of O-ATrROP (Figure 2D).

Synthesis of Poly(EtVCP) with High c Content.

With the synthesis of poly(EtVCP) with high l content successfully established, we desired to also selectively target high c content. In free radical polymerization of vinylcyclopropane monomers, it has been shown that using solvents (compared to bulk polymerization) or increasing the polymerization temperature promotes intramolecular backbiting cyclization, leading to decreased l composition.¹⁶ Accordingly, we first investigated the effect of concentration on this O-ATrROP of EtVCP at room temperature. Indeed, polymerizations at lower concentration produced polymers with more c content as indicated by decreased S_L (Table 2, entries 1–6). It is noteworthy that polymerizations at low concentrations (0.098–0.192 mol/L) still retained excellent control over the polymer chain-growth as indicated by low Ð (1.12–1.15) and high I* (80–107%) (entries 4–6). The S_L dramatically decreased from 67% to 38% when the polymerization using a concentration of 0.192 mol/L was performed at 60 °C (entry 4 vs 7). Using a high-power blue LED (Figure S27b) instead of the white LEDs continued to promote the formation of c content (S_L = 18%, entry 8). This result illuminates the crucial role of light intensity in photocontrolled polymerizations.¹⁷ PC 2 was proven to be superior to 1, producing poly(EtVCP) with I* of 99%, Ð of 1.09 and S_L of 16% (entry 8 vs 9). Lowering the PC loading resulted in slightly higher Ð and I* (entries 10–12). Control experiments that omitted either light, PC 2, or DBMM from the system led to no monomer conversion (Table S6). Moreover, good control of MW as achieved by manipulating the stoichiometry of monomer to initiator, producing poly(EtVCP) with low Ð of 1.03–1.16 and S_L of 13–16% (entries 13–15).

Table 2.

Optimization and Synthesis of Poly(EtVCP) with High c Content^a

entry	PC	conc (mol/L)	T (°C)	[M]/[I]/[PC]	convb (%)	Mnc (kDa)	Đ (Mw/Mn)c	I*d (%)	SLb (%)
1	1	1.429	28	1000/10/1	98	21.8	1.21	97	97
2	1	0.833	28	1000/10/1	98	21.1	1.15	99	97
3	1	0.455	28	1000/10/1	98	26.1	1.14	81	80
4	1	0.192	28	1000/10/1	95	25.3	1.12	80	67
5	1	0.122	28	1000/10/1	90	17.5	1.13	107	60
6	1	0.098	28	1000/10/1	88	18.6	1.15	102	55
7	1	0.192	60	1000/10/1	95	25.3	1.07	81	38
8e	1	0.192	60	1000/10/1	98	23.8	1.15	86	18
9e	2	0.192	60	1000/10/1	97	21.1	1.09	99	16
10e	2	0.192	60	1000/10/0.5	95	19.7	1.09	104	19
11e	2	0.192	60	1000/10/0.2	91	19.1	1.15	102	15
12e	2	0.192	60	1000/10/0.1	83	16.4	1.21	109	17
13e	2	0.192	60	1000/5/1	98	41.0	1.16	102	14
14e	2	0.192	60	1000/2/1	97	82.8	1.03	124	13
15e	2	0.192	60	1000/1/1	91	105.4	1.03	183	16

^aPolymerizations performed using 1.0 mmol of EtVCP, DBMM as the initiator, in EtOAc and irradiated with white LED for 12 h.

^bMeasured by crude ¹H NMR. S_L = l/(l + c).

^cMeasured by GPC.

^dInitiator efficiency (I*) = M_n(theo)/M_n(measured), where M_n(theo) = MW(initiator) + MW(EtVCP) × conversion × ([EtVCP]/[initiator]).

^e34 W blue LED was used.

Despite using diluted conditions, first-order kinetics (Figure 3A), a linear increase in polymer MW, and low to moderate Ð with respect to monomer conversions (Figure 3B) were observed although MALDI-TOF MS analysis of a poly(EtVCP) sample (Figure S31) showed a relatively higher contribution of H chain-end groups. Temporal control was also achieved using a pulsed irradiation experiment at 60 °C with a high power blue LED (Figure 3C). The polymer MWs before and after a dark period were similar to each other, while Ð slightly increased after a complete “on–off” cycle (Figure 3D). A chain-extension experiment using isolated poly(EtVCP) with high c content (22.9 kDa, Ð = 1.16, S_L = 15%) as the macroinitiator with EtVCP at 28 °C was then performed (Figure 3E). Although good control was obtained (Ð = 1.17, I* = 112%), only 46% monomer conversion was observed after 12 h, which we attributed to less reactive variants of Br chain-end group presented in the macroinitiator. At elevated temperature intramolecular cyclization is greatly promoted during polymerization, leading to increased formation of alkyl radicals (e.g., F in Figure 1E), and subsequent deactivation to form new alkyl bromide chain-end groups (e.g., G in Figure 1E), which are reasonably more difficult than bromomalonate to reduce by *PC (Figures S36 and S37, Table S9).

Figure 3.

Synthesis of poly(EtVCP) with high c content: (A) Plots of the natural log of monomer conversion as a function of time. (B) Plots of experimentally measured M_n and Ð as a function of monomer conversion. Plots of conversion versus time (C) and plots of M_n and Ð as a function of monomer conversion (D) for pulsed irradiation experiment. Filled markers were data directly after irradiations, while open makers were data after the dark periods. (E) Chain-extension polymerization from a poly(EtVCP) macroinitiator with EtVCP and GPC traces before and after polymerization.

Postpolymerization Modification of Polymers with High l Content.

Radical addition, such as thiol–ene “click” reaction,¹⁸ has been used to functionalize polymers containing internal C–C double bonds.¹⁹ The reactive alkene groups periodically spaced along the backbone of poly(EtVCP) with high l content piqued our curiosity about the stability of this polymer-chain under photoredox conditions and the possibility to cross-link the resulting polymers. The investigation began with an isolated poly(EtVCP) (21.3 kDa, Ð = 1.19, S_L = 95%), PC 2 (0.1 mol %), and white LEDs in EtOAc at 28 °C (Table 3, entries 1 and 2). Interestingly, reactions at 0.833 and 0.192 mol/L yielded soluble polymeric products with slightly decreased S_L values (90% and 86%, respectively) after 12 h, suggesting that transformation of the alkene groups in the polymer-chain indeed occurred, albeit not through significant cross-linking. Temperature also impacted this transformation such that S_L decreased from 95% to 80% after 12 h at 60 °C (entry 3). Surprisingly, replacing the light source with a high power blue LED drastically promoted the conversion of l content into c content (S_L = 34%, entry 4). The increase in polymer MW (M_n = 35.2 kDa) when using the blue LED is likely due to radical–radical coupling terminations.

Table 3.

Postpolymerization Modification of Poly(EtVCP) with High l Content^a

entry	conc (mol/L)	T (°C)	Mnb (kDa)	Đ (Mw/Mn)b	SLc (%)
1	0.833	28	23.8	1.22	90
2	0.192	28	24.3	1.23	86
3	0.192	60	27.6	1.15	80
4d	0.192	60	35.2	1.17	34

^aReactions performed using 1.0 mmol of poly(EtVCP) (21.3 kDa, Ð = 1.09, S_L = 95%), in EtOAc and irradiated with white LED for 12 h. The polymer was recovered quantitatively.

^bMeasured by GPC.

^cMeasured by crude ¹H NMR.

^d34 W blue LED was used.

The resulted poly(EtVCP) from polymer-chain modification (Table 3, entry 4) exhibited excellent solubility in common organic solvents (e.g., EtOAc, ether, toluene, and dichloro-methane). As such, we believe it is the intramolecular radical cyclization, other than polymer cross-linking, that contributes to the decrease of S_L. Because of difficult structural analysis based on NMR spectra,¹² we designed a model reaction to mimic the radical cyclization and probe the mechanism. Three-step allylation of diethyl malonate afforded a compound 4 as a 6:1 E/Z mixture at C8 position, which was then placed under similar polymer-chain modification conditions (Figure 4). After 12 h, about 90% of starting material was recovered, and E/Z isomerization at the C3 position (4′) was observed (E/Z = 5.2:1). We attributed this C–C double-bond isomerization to a reversible cyclization/fast ring-opening process, involving two potential pathways forming either cyclopropane or cyclobutane. Moreover, the cyclization product(s) 5 was also observed (in 5% isolated yield), which was likely a mixture of several cyclic bromides formed from distinct tandem cyclization pathways (two of the potential products are outlined in Figure 4; also see Figures S48–S51).

Figure 4.

Results of model reaction of 4.

However, for each tandem cyclization pathway, the resulting product consists of up to eight diastereomers, which plagued the structural analysis of 5 and identification of cyclization mechanism. As such, we turned to density functional theory (DFT) calculations to predict cyclization pathways of a radical intermediate 6, which is generated from two monomer additions of the initiator DBMM (Figure 5). Although 4-endo-trig and 3-exo-trig cyclizations of 6 are both endergonic (by 12.9 and 2.7 kcal/mol, respectively), the latter is more favorable by 10.2 kcal/mol, which matches the classic Baldwin’s rule.²⁰ The subsequent 5-exo-trig and 6-endo-trig cyclization pathways are both thermodynamically exergonic by more than 10 kcal/mol (ΔG = −11.8 and −14.0 kcal/mol, respectively), indicating cyclopentane and cyclohexane are likely the favorable repeat units of chain-modified poly(EtVCP). Moreover, cycloheptane may be accessible as well in this radical cyclization (Figure S52).

Figure 5.

DFT calculations to predict late-stage cyclization pathways.

Mechanism for the Formation of c Content.

In monitoring the polymerization performed at 60 °C (Figure 3A), we found that the S_L decreased over time (Table S7). This implies that considerable intramolecular cyclization is competing with propagation during polymerization at high temperature (60 °C). The c content is either from immediate backbiting (Figure 1E, from intermediate D to F) after the ring-opening process (Figure 1E, from intermediate C to D) or from tandem radical cyclization of an unsaturated linear polymer chain of random length. To gain further insights into the chemical structures of c repeat units, comparison of the proton and carbon NMR spectra of three poly(EtVCP) samples synthesized from distinct methods, benzoyl peroxide (BPO) initiated bulk radical polymerization (M_n = 25.1 kDa, Ð= 3.8, S_L = 30%), photoredox ATrROP of 0.192 mol/L with LEDs at 60 °C (M_n = 21.1 kDa, Ð = 1.23, S_L = 40%), and polymer-chain modification (M_n = 35.2 kDa, Ð = 1.17, S_L = 34%, Table 3, entry 4), was performed. These spectra are extremely similar (Figures S41–S43), suggesting that the aforementioned radical polymerizations (or modification) produce similar c composition, which are likely cyclobutanes, cyclopentanes, and cyclohexanes.

Characterizations of Poly(EtVCP) with Variable l or c Content.

To elucidate the differences that the degree of l or c content has on the polymer’s behavior, the thermal and viscoelastic properties were investigated for four polymer samples of comparable MW (21–23 kDa) with S_L values ranging from 95% to 15% (Figure 6). Polymer I, with an S_L value of 95%, has a glass transition temperature (T_g) of 32.6 °C, as shown by differential scanning calorimetry (DSC) (Figure 6B). As the S_L value decreases from 70% in polymer II, to 35% in polymer III, to 15% in polymer IV, the T_g of the polymer samples also decreases from 31.2 to 30.8 to 28.5 °C, respectively. Therefore, as the c content increases, the chain-end free volume seems to increase. In addition, the melting peaks associated with all four of the polymer samples become less apparent with increasing c content.

Figure 6.

Comparison of ¹H NMR spectra (A), DSC curves (B), viscosity flow curves (C), creep tests (D), and 5% step-strain stress relaxation plots (E) of four poly(EtVCP) samples: (I) 21.3 kDa, Ð = 1.19, S_L = 95%; (II) 21.1 kDa, Ð = 1.17, S S_L = 70%; (III) 22.3 kDa, Ð = 1.18, S_L = 35%; (IV) 22.9 kDa, Ð = 1.17, S_L = 15%.

To investigate the effect that l content of the polymer has on the viscoelastic properties, a series of tests were completed on each polymer sample in the melt at isofrictional conditions (T_g + 30 °C) which included a flow test with a strain rate sweep from 0.001 to 0.3 1/s, a creep test with a constant 100 Pa stress applied over 5 min, and a 5% step-strain stress relaxation experiment over 5 min (Figure 6C–E). In each test performed, it was shown that the lower the S_L value, the more elastic the polymer behavior. For instance, polymer I has the lowest viscosity value (5.58 × 10³ Pa·s at 0.001 1/s), is the most deformed during the creep tests (8.47 × 10² %), and relaxes the fastest after a 5% step-strain (~5 s until a modulus value of 45 Pa was reached). In stark contrast, polymer IV has the highest viscosity value (2.51 × 10⁶ Pa·s at 0.001 1/s), is the least deformed during the creep tests (0.929%), and relaxes the slowest after a 5% step-strain (300 s until a modulus value of 350 Pa was reached).

Monomer Scope.

Subsequent exploration of the monomer scope revealed a variety of vinylcyclopropanes with diverse substituents being amenable to this organocatalyzed photoredox protocol (Table 4). Generally, polymerizations of 0.833 mol/L at 28 °C achieved high conversions (91–98%), near 100% I*, low Ð (1.12–1.24), and predominantly l content (S_L = 92–98%). Low concentration (0.192 mol/L) polymerizations at 60 °C with high power blue LED were also high yielding (95–99% conversions), producing poly(vinylcyclopropanes) with low Ð (1.06–1.20), moderate to excellent I* (65–115%), and cyclic repeat units as the major composition (S_L = 5–40%). The glass transition temperature (T_g) of the resulting poly(vinylcyclopropanes) ranged from −31 to 94 °C, where additional methylene groups to the side chain might increase the free volume of the polymer, thereby significantly decreasing T_g. All of the obtained polymers exhibited decomposition temperatures (T_d) over 310 °C, except poly(^tBuVCP), which started to decompose at 227 °C (entries 7 and 8). Interestingly, the chemical compositions (l or c content) do not significantly impact the T_g and T_d, while most polymers exhibited melting points (T_m) ranging from 47 to 183 °C at high l content and became amorphous at high c content. Regardless of polymer compositions, no T_m was observed for poly(EEVCP) (entries 13 and 14).

Table 4.

Monomer Scope for Organocatalyzed RROP of Vinylcyclopropanes^a

entry	monomer	condition	convb (%)	Mnc (kDa)	Đ (Mw/Mn)c	I*e (%)	SLb (%)	Tg (°C)	Tm (°C)	Td (°C)
1	EtVCP	A	98	21.1	1.15	99	97	32	94	359
2	EtVCP	B	97	21.1	1.09	99	16	30		360
3	PrVCP	A	98	25.7	1.12	93	97	9	93	357
4	PrVCP	B	99	29.3	1.11	83	10	6		362
5	BuVCP	A	98	27.2	1.15	98	93	−20	47	354
6	BuVCP	B	99	31.9	1.10	85	6	−16		364
7e	tBuVCP	A	98	29.3	1.19	91	98	78	169	227
8e	tBuVCP	B	99	30.3	1.18	90	5	87		226
9	BnVCP	A	97	32.6	1.16	101	92	30	84	356
10	BnVCP	B	95	28.1	1.18	115	30	29		356
11e	PhVCP	A	97	30.9	1.24	98	95	81	183	369
12e	PhVCP	B	98	46.8	1.06	65	40	94	180	380
13	EEVCP	A	91	29.0	1.21	95	97	−27		356
14	EEVCP	B	99	35.8	1.03	85	30	−31		351
15	ClEVCP	A	93	26.9	1.23	98	92	47	179	342
16	ClEVCP	B	99	29.3	1.13	97	20	41		349
17	LMVCP	A	95	42.3	1.21	98	75	74	139	313
18	LMVCP	B	99	52.5	1.20	83	1	68		312

^aCondition A: [EtVCP]: [DBMM]: [1] = 1000:10:1, in 1.0 mL of EtOAc (0.833 mol/L), and with homemade white LED beaker at 28 °C for 8 h. Condition B: [EtVCP]: [DBMM]: [1] = 1000:10:1, in 5.0 mL of EtOAc (0.192 mol/L), and with 34 W blue LED at 60 °C for 12 h.

^bMeasured by crude ¹H NMR. S_L = l/(l + c).

^cMeasured by GPC.

^dInitiator efficiency (I*) = M_n(theo)/M_n(measured), where M_n(theo) = MW(initiator) + MW(EtVCP) × conversion × ([EtVCP]/[initiator]).

^ePhCl was used as the solvent.

CONCLUSION

In conclusion, we have established a general photoredox strategy for the radical ring-opening polymerizations of a series of functionalized vinylcyclopropane monomers, producing poly(vinylcyclopropanes) with predictable MW and low Ð. The use of N,N-diaryldihydrophenazines as the photocatalyst enables high monomer conversions (>90%) under mild conditions. By manipulating polymerization concentration and temperature, unprecedented regulation on the linear and cyclic compositions of obtained polymers was achieved, which allowed investigations of composition-associated thermal and viscoelastic properties. Significantly, we have discovered a novel polymer-chain modification that converts l composition into c composition. The combination of a model reaction and DFT computations disclose several tandem radical cyclization pathways to produce cyclopentane and cyclohexane repeat units in the modified polymer. This postpolymerization modification also suggests a novel mechanism, besides previously proposed backbiting cyclization, to form the c composition during polymerization. Combined, these findings provide valuable insights to guide the future design and fabrication of poly(vinylcyclopropane)-based materials.