Radical coupling polymerization (RCP) for synthesis of various polymers

Abstract

A general polymerization method is reported, which involves a direct radical coupling reaction of in situ formed benzyl-type biradicals from dibromide in the presence of a Cu(0)/ligand. The radical coupling polymerization can be employed to synthesize polyarene, polyester, polyether and polysulfone under mild conditions and within a short time.

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Introduction

A radical is an active species employed in many types of organic reactions. It undergoes addition reaction to unsaturated bonds, self-reactions of disproportionation and coupling, and transfer reactions with organic compounds with an abstractable hydrogen atom. Although the radical addition reaction to carbon–carbon double bonds is widely applied in chain polymerization of vinyl monomer,¹ the coupling reaction of a radical, another extremely rapid reaction of radicals, is scarcely used in polymerization. The main reason for this is that the radical simultaneously undergoes redox, disproportionation and transfer reactions, which terminate the chain growth via radical coupling, and make it impossible to produce polymers with high molecular weight.

If the main reactions, such as coupling (k_c), disproportionation (k_d), chain transfer (k_tr) and other reactions (k_x, such as redox, cyclization) of the radical, are considered, the ratio of coupling rate (R_c) to the overall rate of radical reactions (R) is given in eqn (1), where R₂H is the transfer agent, and X is the nonspecific agent related to other reactions. The formation of a carbon–carbon bond via a radical coupling reaction depends on the value of R_c/R. According to eqn (1), a high concentration of radical and large ratios of k_c to other kinetic parameters k_d, k_tr and k_x, leads to a larger value of R_c/R, which is crucial for the radical coupling reaction.

Construction of the polymer chain by a radical coupling reaction needs a continuous generation of radicals. An earlier attempt at this was the formation of radicals by hydrogen atom transfer to alkoxyl radical using the route shown in Scheme 1. Radicals formed in situ undergo a coupling reaction to produce the polymer.² Saturated hydrocarbons, such as diphenyl methane, esters, amines, and organometal compounds can be converted to polymers by this method.^3–6 No precise characterization of that polymer obtained have been provided. A metal catalyzed redox reaction of an organic halide,^7,8 especially using the atom transfer method,^9–15 in which the radical is generated by single electron transfer (SET) to an organic halide and the decomposition of the alkyl halide, provides an efficient pathway to generate a carbon radical under mild conditions. The low active metals used in the atom transfer redox reaction prevent further reduction of a radical to an anion, which make it a promising method for the generation of radicals.

Scheme 1 Typical methods for generation of a radical for its coupling polymerization.

A radical coupling reaction promoted by a transition metal complex, which is described as an atom transfer radical coupling (ATRC) (route b in Scheme 1) was widely applied to prepare block polymers by using a mono- or di-halogenated polymer as a precursor. The coupling efficiency of ATRC does not normally exceed 95%.^16–18 There are only a few reports of the synthesis of polymers from small organic molecules by ATRC. Direct radical coupling of α,α′-dibromo p-xylene¹⁹ and propane-1,3-diyl bis(2-chloro-2-phenylacetate)²⁰ under ATRC conditions have been reported. ATRC of a monomer sequence unit has been used periodically to generate a vinyl polymer with a high molecular weight and broad polydispersity.²¹ The limitation of ATRC is that the polymerization time is very long, e.g., more than 24 hours, because of the low concentration of the radical derived from the equilibrium between RX + Cu(I)X and R˙ + Cu(II)X₂.

Carbon–carbon bond formation by reductive coupling of an alkyl halide catalyzed by various metals or its salts has been extensively investigated for decades. The mechanism of carbon–carbon formation is correlated with the type of metal employed. For example, the Wurtz reaction²² consists of a halogen–metal exchange involving the radical species and then the carbon–carbon bond formation in a nucleophilic substitution reaction between the alkyl anion and the alkyl halide. Although the radical was proved to be an intermediate species, the formation of the carbon–carbon bond is not via a direct radical coupling reaction. Low molecular weight copolymers have been prepared by coupling α,α′-dibromo-m-xylene or α,α′-dibromo-p-xylene catalyzed by chromous chloride for several days and there was evidence for the occurrence of free radicals in the decomposition of the intermediate organochromium complex.²³

Synthesis of the polymer with a high molecular weight by direct radical coupling polymerization (RCP) from small organic molecules has still not been carefully studied. In principle, nearly 100% coupling efficiency and a high concentration of the radical are a prerequisite based on the kinetic considerations. In this paper, RCP of benzyl-type biradicals generated in situ by a redox reaction between a dibromo compound and a Cu(0)/ligand (route c in Scheme 1) is reported as a potential route for the synthesis of some new kinds of polymers under mild conditions and within a short time.

Experimental section

All the dibromides were synthesized by bromination of corresponding substrates with N-bromosuccinimide and characterized by proton nuclear magnetic resonance spectroscopy (¹H-NMR) and elementary analysis.

The typical polymerization procedure is given next. 1,4-Bis(1-bromoethyl)benzene (BBEB; 29.2 mg; 0.1 mmol), tris[(2-pyridyl)methyl]amine (TPMA; 58.0 mg; 0.2 mmol) and copper powder (14.1 mg; 0.22 mmol) were added to an ampule equipped with a stirring bar. The ampule was degassed, backfilled four times with nitrogen (N₂). Deoxygenated tetrahydrofuran (THF; 1 mL) was then added. The ampule was heated at 60 °C for 2 h. The ampule was immersed in liquid N₂ and the mixture was diluted with THF and purified by passing it through a neutral alumina column. The solution was concentrated, precipitated with methanol and dried under vacuum at 40 °C to yield the polymer. The yield was determined gravimetrically and the conversion was calculated by W_polymer/(W_monomer × (weight fraction of carbon and hydrogen of monomer)).

Experimental details can be found in the ESI.†

Results and discussion

Benzyl-type radicals are stable intermediate radicals involved in many organic reactions. The self-reactions of benzyl, α-methylbenzyl and cumyl radicals predominantly give coupling products.²⁴ In radical polymerization of styrene, more than 80% of the chain termination is a radical coupling reaction.¹ Benzyl-type radicals are considered to be a class of suitable candidates for RCP. Two dibromides, such as 1,4-bis(bromomethyl)benzene (BBMB) and (BBEB), were prepared and used for polymerization. The polymerization of the two dibromides was conducted in the presence of Cu(0)/TPMA in THF. No soluble polymer was obtained for BBMB whereas a soluble polymer was obtained for BBEB.

To confirm the radical formation mechanism, both BBMB and BBEB were polymerized with the equivalent amount of spin-trapping agent, 2-methyl-2-nitrosopropane (MNP) in the presence of Cu(0)/TPMA in THF. Polymers with a high molecular weight and alternative unit sequence (BBMB/BBEM-MNP)_n were obtained using both dibromides, which follow the RACP (as shown in Scheme 2) mechanism previously reported by Zhang et al.²⁵ (Fig. S23 and S24†). This reveals the success of in situ generation of radicals via a SET redox reaction promoted by Cu(0)/TPMA for both dibromides. ATRC of BBMB catalyzed by Cu(I)Br/pentamethyldiethylenetriamine (PMDETA) was reported without full characterization because of the insolubility of the poly(p-xylene) (PPX).¹⁹ Although BBMB can undergo RACP, no soluble product was obtained in RCP. After careful removal of the copper complex from the product obtained from BBMB, a pale yellow solid was obtained. The infrared (IR) spectrum (Fig. S25†) of the solid is nearly the same as that reported for PPX.²⁶ Elementary analysis shows it contains 81.02% of carbon and 7.10% of hydrogen, which indicates the molar ratio of C/H is 0.95, which is close to theoretical value of 1.0. The 12% left is probably bromine derived from the end group of the oligomer.

Scheme 2 Mechanisms of radical coupling polymerization (RCP, top) and radical addition coupling polymerization (RACP, bottom).

The variation of molecular weight of the polymer was monitored by gel permeation chromatography (GPC) at different polymerization times. The polymerization was conducted under [BBEB]/[TPMA]/[Cu(0)] = 1/2/2.2, [BBEB] = 0.1 M, at 60 °C in THF. As shown in Fig. 1, a clear shift of the GPC curve towards a high molecular weight was observed. The polydispersity index (PDI) varied between 1.8 and 3.2. The evolution of molecular weight distribution clearly demonstrated that the polymerization follows the step-growth mechanism. The polymer could be obtained within 2 h, which is much faster than normal ATRC reactions.^19–21

Fig. 1 Variation of GPC curves of polymers produced by RCP of BBEB with time.

The ¹H-NMR spectrum of the polymer obtained after 2 h is shown in Fig. 2. No –CH₂Br was found in the spectrum suggesting that there had been a full conversion of the functional group. Three groups of the peaks at 0.75–1.3 ppm, 2.48–3.0 ppm and 6.7–7.2 ppm corresponded to the methyl, methine and benzenic protons of the polymer, respectively.

Fig. 2 ¹H-NMR spectrum of the product of polymerization of BBEB in deuterated chloroform (CDCl₃).

The influence of temperature, concentration of monomer, type of ligand and amount of Cu(0) were studied and the results are summarized in Table 1. In runs 1–4 in Table 1, when the temperature varied from 30 °C to 60 °C, the M_n of the polymer greatly increased. For the three ligands, the M_n of the polymer decreased in the order TPMA > Me₆TREN > PMDETA as shown by runs 4–6. As the concentration of the monomer increased from 0.05 M to 0.5 M (runs 4, 7–9), the highest M_n was obtained when the concentration was 0.1 M, a lower or higher concentration led to slight decrease of M_n. The amount of Cu used was also varied (runs 4, 10–11) and it was found that excess Cu powder has a slight influence on the M_n of the polymer.

Table 1 Radical coupling polymerization of 1,4-bis(1-bromoethyl) benzene (BBEB) promoted by Cu/ligand^a

Run	[BBEB] ([M])	Temp (°C)	[Cu]/[BBEB]	Ligand	Mn^b (g mol^−1)	PDI^b
a General condition: [BBEB] : [ligand] = 1 : 2, THF = 1 mL, 2 h.b Number-averaged molecular weight (Mn in g mol^−1) and PDI were measured using GPC with polystyrene as the standard. Me6TREN = tris[2-(dimethylamino)ethyl]amine.
1	0.1	30	2.2	TPMA	500	1.57
2	0.1	40	2.2	TPMA	1000	2.28
3	0.1	50	2.2	TPMA	1900	2.64
4	0.1	60	2.2	TPMA	6700	1.83
5	0.1	60	2.2	PMDETA	3700	1.70
6	0.1	60	2.2	Me6TREN	4300	1.96
7	0.05	60	2.2	TPMA	4100	1.74
8	0.3	60	2.2	TPMA	4500	1.89
9	0.5	60	2.2	TPMA	3200	2.12
10	0.1	60	4.4	TPMA	6300	1.87
11	0.1	60	11	TPMA	6300	2.13

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As shown in Scheme 2, the mechanism of RCP is not complicated. Radicals are generated in situ by a SET redox reaction of alkyl dibromide and Cu(0)/ligand in the polar solvent, which subsequently undergoes the coupling reaction continuously. As shown by polymerization kinetics, the RCP follows a step-growth polymerization mechanism. The average molecular weight of the polymer prepared by step-growth polymerization depends on the extent of reaction. The extent of the reaction in RCP is determined by two factors. One is the conversion of bromide to radical, and the other is the non-coupling reactions of radicals leading to chain termination.

The SET is a very efficient pathway for producing radicals from an organic halide. The successful RACP of two dibromides in the presence of MNP gave polymers with a high molecular weight. This demonstrates that both dibromides are reduced efficiently to their corresponding radicals by the Cu(0)/ligand. In the reduction reaction of alkyl halide by a metal or its salt, the intermediate radical generated in situ can be further reduced to an anion if the metal is very active, which is proved in the Wurtz reaction.²² Because of the reactivity of the anion, the side reactions are so common that it affects the coupling efficiency. Even for other Wurtz-type reactions catalyzed by less active metals, the yields of the carbon–carbon bond are far from 100%. In the atom transfer radical addition (ATRA), ATRP and atom transfer radical cyclisation (ATRC) reaction, the redox equilibrium between RX + Cu(I)X and R˙ + Cu(II)X₂ results in the low concentration of radicals, which greatly reduces the rate of the coupling reaction. This results in a long reaction time (more than 24 h) and low conversion of the monomer (∼30%) for RCP under ATRC conditions.^19–21

For various benzyl-type small radicals, the values of k_d/k_c are typically in the range of 0.05–0.16.²⁴ The trend of the temperature dependencies of k_d/k_c for small organic radicals is uniformly to decreasing k_d/k_c ratios with increasing temperature.²⁴ Therefore, a high temperature favours a radical coupling reaction.

Values of k_tr and k_x are much smaller than k_c. The concentration of radical species in conventional radical addition polymerization is about 10⁻⁶ to 10⁻⁸ M, and it was much lower in ATRP.¹⁴ A low concentration of radical reduces the chain termination rate through radical self-reaction in radical addition polymerization,¹ and the radical coupling rate as well. In contrast, a relatively high concentration of the radical is required for RCP. In order to achieve high concentration, the monomer concentration (0.05–0.5 M) used in RCP was in the same range as in RACP²⁵ and was much higher than that in ATRC of bromo-polystyrene.¹⁹ SET redox of alkyl halides is a quick way to generate a high concentration of radical under mild conditions. According to eqn (1), the high concentration of radicals leads to the larger value of R_c/R, and results in the high time of reaction required for high average molecular weight, which is the difference between RCP in this work and the reported ATRC.

The activation ability of the three ligands in ATRP decreases in the order Me₆TREN > TPMA > PMDETA,²⁷ whereas the best polymerization result was obtained by using TPMA as ligand. This is because of the quaternization reaction between alkyl bromide/monomer and the ligand. Both PMDETA and Me₆TREN react slowly with BBEB and produce a white solid in THF at room temperature within 4 h, whereas no reaction occurs between TPMA and BBEB overnight.

More dibromides were designed and synthesized by routine methods, which are shown in Scheme 3. The polymerization results are summarized in Table 2. GPC data and ¹H-NMR spectra all confirm the chain structure of the product generated by radical coupling mechanism (see ESI†). Normally, polyether and polysulfone are synthesized under harsh conditions and polyester requires a long time. RCP can be applied to synthesize polyarene, polyester, polyether and polysulfone under mild conditions within a short time. Soluble polymers produced by BBEB and MBMB are modified PPXs with substitution of aliphatic and aromatic carbons, respectively. RCP provides a novel pathway for the synthesis of soluble modified PPX.

Scheme 3 Various dibromides as monomers used in radical coupling polymerization.

Table 2 Radical coupling polymerization of various monomers^a

Run	Monomer	Mn^b (g mol^−1)	PDI^b	Conversion (%)
a General conditions: [monomer] : [TPMA] : [Cu] = 1 : 2 : 2.2, [monomer] = 0.1 M, 4 h, 60 °C, in THF.b Mn and the PDI was measured using GPC.c An insoluble product was obtained.d Reaction time was 2 h.e [Monomer] = 0.025 M.
1	BBMB^c	—	—	83
2	BBEB^d	6700	1.83	93
3	MBMB	4700	1.90	91
4	BMPE	5400	2.15	79
5	BEPS^e	4700	1.74	58
6	EBEB	13 100	9.57	89
7	EBMB	4500	2.76	71

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The ¹H-NMR spectrum of the polymer prepared using MBMB, which is an asymmetrical monomer, is shown in Fig. 3. Only two types of methylene protons are identified in the polymer, which suggests the monomer coupled in head–head and tail–tail mode. This also demonstrates that the RCP is a suitable method to synthesize the periodic polymer as are other step-growth polymerization methods.

Fig. 3 ¹H-NMR spectrum of the product of polymerization of MBMB in CDCl₃.

Compared with ATRC and reductive coupling, the current polymerization reaction is described as radical coupling polymerization, which represents the repeating reaction involved. The generation of radical involves an irreversible “atom transfer” from halide to metal corresponding to the reduction of alkyl halide to alkyl radical. In ATRA⁷ and ATRP,¹⁰ the halogen atom is reversibly transferred between the organic halide and the metal complex.

Conclusions

A radical coupling polymerization of various dibromides promoted by Cu(0)/ligand is reported. A high radical concentration in the polymerization media is obtained via reduction of benzyl bromide by Cu(0)/ligand which favors the radical coupling reaction kinetically. This method provides a fast and efficient approach to the synthesis of the polymer via direct radical coupling polymerization under the mild conditions and is completed within a short time. The monomer can be well designed, which leads to some novel polymers that cannot be prepared by current polymerization methods.

Acknowledgements

Financial support from National Natural Science Foundation of China (21174123) is appreciated.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: GPC and the ¹H-NMR spectra of the polymers obtained. See DOI: 10.1039/c6ra02669a

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