Synthesis and thermal properties of poly(vinylcyclohexane)-b-poly(4-vinylpyridine) diblock copolymers prepared via RAFT polymerization

1 Introduction

Amphiphilic block copolymers are known for their ability to self-assemble into various nanostructures, and have found numerous applications that range from biomedical to electronic technologies (1). Apparently, one of the most widely studied block copolymer system [both in bulk (2), (3) and in thin (4), (5) or ultrathin (6), (7) films] is the one based on polystyrene (PS). This may be ascribed to the ease of PS synthesis (8), its cost-effectiveness and good processability (9). As many practical applications require from nanostructured materials to be stable at elevated temperatures (1), (10), the wider service temperature range of polymer product is a beneficial condition. Particularly, one of the ways to alter the glass transition temperature (T_g) of PS [that is around 100–110°C (11)] is to hydrogenate one to obtain its saturated analogue – poly(vinylcyclohexane) [PVCH, or also known as poly(cyclohexyl ethylene) (PCHE)], whose T_g is up to 65°C higher (12), (13), depending on degree of crystallinity of PVCH (14). Besides, PVCH is much more resistant to oxidative degradation (15). Despite the potential advantage, the study of PVCH-consisting block copolymers is a relatively new field of research (in comparison with PS-based block copolymers), and is currently limited to: poly(methyl methacrylate) (16), poly(ethylene oxide) (17), polylactide (18), poly(ethylene) (19) and other polyolefins (saturated polydienes) (20), (21), (22) as the other block(s). To the best of our knowledge, poly(vinylcyclohexane)-b-poly(4-vinylpyridine) (PVCH-b-P4VP) system has not been reported yet. We believe that a combination of the chemically inert semi-crystalline hydrophobic block (PVCH) and a functional hydrophilic block that can be modified by quaternization (or via supramolecular chemistry approach) (P4VP) will lead to a new family of nanostructured products with promising properties both in solution and in bulk.

In order to obtain block copolymer systems with predictable structure and properties, only controlled polymerization techniques are suitable. Nowadays, atom transfer radical polymerization (ATRP) is one of the most common and versatile approaches towards diverse block copolymer systems (23). However, P4VP prepared by ATRP technique poses challenges to both synthesis and purification (24). The competitive coordination of 4-vinylpyridine (4VP) monomer to transition metal catalyst affects the polymerization, leading to unwanted deceleration of polymerization (25) or cross-linking (26). In our preliminary research, we synthesized low molecular weight PVCH-b-P4VP diblock copolymer using bromo-terminated PVCH as a macroinitiator, CuCl as a catalyst, and considering the solubility of PVCH macroinitiator and PVCH-b-P4VP copolymer, we chose anisole as the solvent (Figure 1, upper path). But this approach is limited as the degree of polymerization of the second, the P4VP block did not exceed 50 units. On top of that, the obtained diblock copolymers remained brownish even after extensive purification such as repeated passing the PVCH-b-P4VP solution through alumina column and reprecipitation. The complexation of P4VP polymer with copper makes it challenging to completely remove the catalyst from the product, yet the presence of metal traces renders the polymer unsuitable for electronic applications.

Figure 1:

Synthesis of PVCH-b-P4VP using ATRP (upper path) and RAFT (lower path) methods.

Reversible addition-fragmentation chain transfer (RAFT) polymerization (27) is a more suitable method for the preparation of vinylpyridine based (block) (co)polymers in terms of polymerization flow and subsequent purification of the obtained polymer as no metal catalysts are utilized. Moreover, depending on the free-radical initiator used, RAFT can be conducted at relatively low temperature, thus minimizing thermal initiation of the monomer(s). For example, Convertine et al. (28) reported the preparation of P2VP and P4VP homopolymers with narrow molecular weight distributions using α,α′-azoisobutyronitrile (AIBN) as free radical initiator and cumyl dithiobenzoate as RAFT chain-transfer agent (CTA).

In the present communication, we have explored utilization of RAFT technique in the preparation of well-defined trace metal-free PVCH-b-P4VP diblock copolymers suitable for electronic applications. For this, a series of novel diblock copolymers comprising of PVCH and P4VP of various molecular weights and block ratios were synthesized by a combination of anionic and RAFT polymerizations with intermediate hydrogenation and functionalization of PS block via coupling reaction with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DTCTMP) (Figure 1, lower path). We chose DTCTMP as the CTA because it was proved to be very effective for a variety of monomers such as acrylates, acrylamide and styrene (29). The coupling reaction yielded a quantitatively functionalized PVCH [as indicated by ¹H nuclear magnetic resonance (NMR) and Fourier transform infra-red (FTIR) spectroscopies] and the excess of DTCTMP can be easily removed by reprecipitation of the polymeric product from acetone or methanol.

2 Experimental part

3 Results and discussion

In our preliminary work, we prepared a bromo-functionalized PVCH (PVCH-ATRP, Figure 1, upper path), which was used as a macroinitiator in ATRP synthesis of PVCH-b-P4VP block copolymers. Considering the solubility of the PVCH macroinitiator and the PVCH-b-P4VP copolymer, anisole and THF were used as the solvents. However, only diblock copolymers with low molecular weight P4VP block were obtained. Figure 2 shows a typical ¹H NMR spectrum of PVCH-b-P4VP copolymers obtained by the ATRP method. The broad peak around 7.0 ppm is assigned to an incompletely hydrogenated PS block (ca. 5%). Based on the comparison of the integration of 4VP protons signals (6.0 and 8.0 ppm) and VCH protons signals (0.5–2.0 ppm), the degree of polymerization of the P4VP block does not exceed 50 units. Consequently, to get PVCH-b-P4VP with the higher molecular weight of the second block, we focused on the utilization of the RAFT technique.

Figure 2:

¹H NMR (CDCl₃) of PVCH-b-P4VP obtained by the ATRP method (experimental M_n: 5500-b-4200; target M_n: 5500-b-25,000).

The key to successful RAFT polymerization is the choice of an effective CTA. To this end, a novel end-functionalized PVCH-RAFT agent was designed and the overall synthesis to PVCH-b-P4VP is shown on Figure 1 (lower path). The mono end-functionalized PVCH-RAFT agent was prepared via a coupling reaction of ω-hydroxy-PVCH and DTCTMP in the presence of triphenyl phosphine and DIAD under an inert atmosphere. The coupling reaction yielded a quantitatively functionalized product and the excess of DTCTMP was completely removed by precipitation into acetone or methanol. The functionality of the resulted PVCH was confirmed by ¹H NMR and FT-IR. The appearance of a new sharp absorption band at 1735.61 cm⁻¹ in the FT-IR spectrum is assigned to the ester carbonyl group of DTCTMP. In the ¹H NMR spectrum of PVCH-RAFT (Figure 3C), disappearance of methylene protons at 3.65 ppm (PVCH-OH, Figure 3A) and appearance of two new resonance peaks at 3.27 ppm and 4.07–4.13 ppm also indicate the formation of ester group, and correspond to methylene group adjacent to the trithiocarbonate moiety and the methylene group adjacent to the end functional group, respectively. Certain broadening of the methylene group adjacent to end group after post functionalization in PVCH-RAFT was also observed in ω-(bromoisobutyl ester)-terminated PVCH (16). The end-group functionalization of all PVCH-RAFT polymers is over 95%, as calculated from integrals corresponding to the above mentioned methylene protons and PVCH protons at 0.5–2 ppm. As expected, PVCH-RAFT exhibited the same solubility as PVCH homopolymers, which are soluble in hexane, THF, chloroform and toluene, and insoluble in methanol, acetone and N,N-dimethylformamide (DMF).

Figure 3:

¹H NMR (CDCl₃) spectra of (A) PVCH-OH (M_n=5500; degree of hydrogenation: 95%), (B) DTCTMP, (C) PVCH-RAFT agent, and (D) PVCH-b-P4VP (M_n=5500-b-14,000).

The polymerization of 4VP on PVCH-RAFT agents with different molecular weights was conducted under an inert atmosphere condition with THF as the solvent and V70 as the initiator. The advantage of V70 is that it is effective at relatively low temperature (25–50°C), thereby any side reactions associated with thermal initiation can be suppressed. The polymerization was monitored by ¹H NMR, and conversion of 4VP monomer increased linearly with the polymerization time during the first 6 h (Figure 4), indicating the polymerization was proceeding in a controlled manner. Over the course of polymerization, the reaction solution became viscous. Usually, the conversion rate of 4VP reached 75–85% within 5–6 h and 95% in 12 h when polymerization was conducted at 30°C. The molecular weights of final diblock copolymers were determined from ¹H NMR spectra and had a good agreement with the reaction feed. In contrast to the block copolymers prepared by the ATRP technique that usually have dark or light brown color due to the presence of metal catalyst, PVCH-b-4VP diblock polymers synthesized by the RAFT method were obtained as a white powder. The characterization details on PVCH-b-P4VP copolymers prepared using PVCH-RAFT agents are presented in Table 1.

Figure 4:

Kinetic plot for the RAFT polymerization of PVCH (M_n=5500) with 4VP in THF at 30°C. [4VP]₀/[PVCH-RAFT]₀/[V70]₀=900/2/1.

After copolymerization, the resulted diblock copolymers exhibit a noticeable difference in solubility as compared to their PVCH homopolymer precursors. The solubility of PVCH-b-P4VP in common organic solvents is very limited due to the distinct solubility behavior of two respective blocks. Generally, they are soluble in hot chloroform, but solubility in THF or DMF depends on the molecular weight and ratio between blocks.

DSC thermograms of the obtained diblock copolymers are depicted in Figure 5, and the analysis summary is presented in the Table 1. As reported in the literature, T_g of amorphous PVCH is ca. 80°C and, depending on its degree of crystallinity, can be as high as 165°C, while the melting point of isotactic-rich PVCH is ca. 370°C (14). The T_g value of bulk P4VP was found to be 142°C, according to the summary of literature data (11).

Figure 5:

DSC thermograms of PVCH-b-P4VP diblock copolymers (2nd heating scans, 10°C): (A) Polymers # 1 (M_n=5500-b-14,000), # 2 (M_n=5500-b-26,000), # 3 (M_n=5500-b-40,000). (B) Polymers # 4 (M_n=12,500-b-9000), # 5 (M_n=12,500-b-18,000), # 6 (M_n=12,500-b-41,000).

First off, we were not able to investigate the melting behavior of PVCH block as a noticeable degradation of the product starts at 250–260°C (DSC data). Nevertheless, DSC traces of the 2nd heating scans of Polymers # 1–3 indicate certain degree of crystallinity of PVCH block that decreases as the size of amorphous P4VP block increases, as explained below. Particularly, Polymer # 3 displays a single T_g at 135°C, which corresponds well to the T_g value of 133°C, predicted using Fox equation for miscible phases:

1Tg=wATgA+wBTgB

where w_A and w_B represent the weight fractions of PVCH and P4VP, respectively; and T_gA and T_gB – their glass transition temperatures: 353 K (80°C) and 415 K (142°C), respectively. This is also in good agreement with ref. (30) reporting that blocks of different nature (i.e. hydrophilic and hydrophobic) are miscible if the molecular weight of either block is ca. 5000 g/mol and thus only one glass transition is observed.

Furthermore, as the P4VP:PVCH molar ratio increases, T_g of the P4VP-rich fraction also increases (129°C and 132°C for Polymers # 1 and # 2, respectively) and there appears the second T_g region that can be assigned to the PVCH-rich phase (Table 1, lower T_g). It is worth noting that with an increase of PVCH content the intensity of the second glass transition increases, indicating increasing phase separation. Such behavior can be explained by partial crystallization of PVCH domains during the first heating/cooling scan, as the rate and degree of crystallization strongly depend on the size of the amorphous block (30), (31), and it is well known that crystallization of one of the blocks leads to phase separation of even miscible phases (31).

Interestingly, the Polymer series # 4–6 also display block miscibility. Thus, the single T_g at 122°C for Polymer # 6 corresponds well to the predicted value of 126°C for miscible phases (calculated using the Fox equation, see above). And as the PVCH:P4VP ratio changes to lower weight fraction of the P4VP block, a noticeable phase separation between P4VP-rich (higher T_g) and PVCH-rich (lower T_g) domains occurs. Besides, one can note that T_g value for P4VP-rich fraction of Polymer # 4 (150°C, Table 1) is higher than the reported T_g value of P4VP homopolymer (142°C) as well as PVCH-rich phase has much higher T_g than the one for the PVCH homopolymer. Both phenomena can be related to the presence of rigid crystalline PVCH domains that confine the amorphous domains and restrict chain movement – thus they need more energy to gain mobility, hence the T_g values increase (14), (31).

4 Conclusions

In summary, the series of novel PVCH-b-P4VP diblock copolymers with well-defined structure were obtained by the ATRP and RAFT polymerization methods, and the advantage of the RAFT approach was shown. Namely, the ability to tailor the polymer composition by varying the 4VP feed ratio and the absence of metal catalyst traces in the final PVCH-b-P4VP make these polymers suitable for electronic applications. Detailed thermal analysis of PVCH-b-P4VP system revealed block miscibility (up to at least 12,500 g/mol of the PVCH block) in the amorphous state and apparent phase separation occurring when the PVCH block is semi-crystalline. The effect of the relative size of the amorphous P4VP block on the relative degree of crystallinity of PVCH block lead to easily tunable thermal properties of these diblock copolymers making them the attractive candidates for dedicated nano-patterning applications.