Synthesis of polystyrene-based Y-shaped asymmetric star by the combination of ATRP/RAFT and its thermal and rheological properties

Abstract

A₂A′-type asymmetric stars and A₂B-type miktoarm star polymers were prepared by the combination of atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer polymerization (RAFT) using the designed initiator. The first step involved the preparation of linear polystyrene with a hydroxyl group (LPSOH) by ATRP using the synthesized initiator 4,4′-di(bromomethyl)benzhydrol. Styrene was polymerized in bulk at 110 °C in the presence of Cu(I)Br and 2,2′-bipyridyl (Bipy) as a catalytic system. Next, the hydroxyl group in the resulting LPSOH chains was esterified to obtain LPS containing thiocarbonylthio (LPSCS₂) chains. The last step consisted of growing the third PS chain or poly-(n-butyl acrylate) chain by RAFT. This methodology enabled us to synthesize A₂A′ triarm PS stars with asymmetry in the molar mass of their branches and A₂B stars with chemically different PS and PBA arms. It also provided us a facile way to synthesize Y-shaped polymers. The effects of the length of the backbone and branched chain on the thermal properties and the rheology of the synthesized asymmetric polystyrene were studied. This method provided a way to obtain well-defined polymers with fixed backbone but different branch length. On using this method grams of sample can be obtained for melt behavior study.

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Introduction

Star polymers have been widely investigated due to their unique physical properties, quite different from their linear counterparts.^1–13 For example, the viscosity of star polystyrene (PS) was different from that of linear PS with the same molecular weight,¹ and the relationship between zero-shear viscosity and molecular weight for both star and linear polymers was also different.² However, most of the studies are focused on symmetric star polymers due to the difficulty of synthesizing asymmetric star polymers. When the chain structure of the polymers changes from linear to star, there is one branch point bringing all branch chains together in each star polymer. Three-armed polymers are the simplest star polymers (“Y-shaped”), which can be a model with one branch chain in a linear backbone. It is a key scientific issue how the physical properties change when the length of the branched chain grows from short to long. So it is urgently needed to synthesize polymers which translate gradually from linear to three-armed stars, that is, polymers with one branch point at the centre of the backbone and a changeable branched chain.

Synthesis strategies for Y-shaped A₂B type polymers include atom transfer radical polymerization (ATRP),^14–17 nitroxide-mediated polymerization (NMP),^17,18 anionic polymerization^19–24 and “click” reaction^10,14,25 as well as versatile combinations of different polymerization methods. Matyjaszewski¹⁴ synthesized symmetric star polymers by a combination of ATRP and the “click” coupling method. Bromo-ended polystyrene was derivatized into azido coupled with multifunctional alkyne-containing coupling agents under a mild condition to produce star polymers. This method was efficient in synthesizing symmetric stars with low molecular weight. They also used both “arm-first” and “core-first” method to synthesize stars and miktoarm star copolymers.²⁶ The products from this method were stars with high arm number and fixed arm length. Gnanou²⁷ used ω-bromo-PS as precursor to synthesize asymmetric and miktoarm polymers using ATRP. The bromo end groups in the resulting PS chains were derivatized into twice as many bromoisobutyrates in order to initiate ATRP polymerization for asymmetric PS synthesis. On using this method polymers with identical branched chain but different backbone were obtained. He²⁸ used the combination of ATRP and azo coupling reaction to synthesize asymmetric polymers. A bi-functional initiator was synthesized with two times of chromatography to initiate ATRP polymerization of polystyrene, then mPEG derivatized into azo functioned with three steps of organic reaction to couple with the main chain. Gao²⁹ synthesized asymmetric AB₂ star polymers using the combination of ATRP and RAFT. They also synthesized a bi-functional initiator with two times of chromatography to initiate RAFT polymerization and ATRP polymerization. They also obtained the asymmetric star polymers for identical branched chain but different main chain.

Due to the development of polymer chemistry, another prevailing methodology for asymmetric and miktoarms star synthesis is anionic polymerization.^19–23 Indeed, both asymmetric and miktoarms stars are traditionally prepared by the “arm-first” approach through deactivation of living carbanions by chlorosilanes or reaction of the same carbanions with divinylbenzene or diphenylethylene derivatives.^6,23,30,31 This synthetic route is well-suited to living carbanionic polystyryl or polydienyl chains, but it cannot avoid fractionation and hardly be applied to engineer polymeric architectures based on other polymers. In addition, it is also time consuming for high molecular weight star polymer synthesis caused by steric hindered and difficult to obtain many grams of sample for melt viscosity measurements.

In this paper, we reports an easy method can afford A₂A′-type asymmetric stars and A₂B-type miktoarm “Y-shaped” stars. The A₂A′ polymers can be seen as a linear chain with only one branch at the center of the main chain. The stars can be derived by combination of atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer (RAFT). On using this method grams of asymmetric polymers with identical backbone but different length in branched chain were obtained without column chromatography during the synthesis of the initiator. The molecular weight of the backbone and branch can be controlled and the location of the branched chain is also identified. The steps include four steps (Scheme 1): (i) synthesizing the ATRP initiator with hydroxyl group for (ii) ATRP polymerization followed by (iii) esterification of hydroxyl groups of these polymers into thiocarbonylthio to (iv) initiating the RAFT polymerization of either styrene or another monomer (e.g., n-butyl acrylate). In addition, no fractionation was needed. This provided the simple and well-defined branched samples for theory study, especially melt behavior study, as the topological structure of polymer chains strongly influences melt behavior of polymers. For example, Frischknecht²¹ and Archer³² studied the rheology of three-arm asymmetric star polyisoprene (PI), they found that the shortest arm of asymmetric stars caused unusually large amount of drag on the backbone. In our previous work,^33,34 we found that the incorporation of C₆₀ in star polymers led to the different change trend in their melt behavior comparing with linear polymers. Therefore, it is urgently called for more convenient method to synthesize asymmetric star polymers with identic main chain length.

Scheme 1 Reaction pathway for the synthesis of asymmetric and miktoarm star polymers.

Experimental

Materials

CuBr (99.999%) were purchased from Aldrich. 2,2′-Bipyridyl and 4,4′-dimethylbenzophenone were purchased from TCI. Styrene (99%), N-bromosuccinimide (NBS), sodium borohydride (NaBH₄), sodium hydroxide (NaOH), benzoyl peroxide (BPO), azodiisobutyronitrile (AIBN) and neutral aluminium oxide were purchased from China National Medicines Corporation Ltd. Toluene, methanol, ethanol, tetrahydrofuran (THF), hydrochloric acid (HCl), hexane, dichloromethane, ethyl acetate, petroleum acetone and triethylamine (Et₃N) were purchased from Beijing Chemical Works. 1-Butanethiol, carbon disulfide (CS₂), 2-bromopropionic acid, N,N′-dicyclohexylcarbodiimide (DCC) and N,N-dimethylpyridin-4-amine (DMAP) were purchased from Acros. Styrene was distilled over CaH₂ prior to use. THF was distilled over K/Na alloy with diphenylmethanone as indicator. BPO, AIBN and NBS were recrystallized from ethanol. DMF, CH₂Cl₂ and toluene were purified by the Braun system. Other reagents were used without further purification.

Synthesis of the 4,4′-di(bromomethyl)-benzophenone

4,4′-Dimethylbenzophenone was dissolved in CCl₄, then NBS and AIBN were added. After stirring for 24 h at 60 °C under nitrogen, CCl₄ was removed under reduced pressure and the product was recrystallized from dichloromethane and n-hexane (yield: 70%). Its structure was verified by ¹H NMR (CDCl₃) δ (ppm): 7.78 (m, 4H), 7.50 (m, 4H), 5.79 (s, 1H), 4.50 (s, 4H).

Synthesis of the 4,4′-di(bromomethyl)-benzhydrol

Sodium borohydride (0.43 g, 11.4 mmol) was added portionwise to a stirred solution of 4,4′-di(bromomethyl)benzophenone (3.8 g, 10.37 mmol) in isopropanol/THF (25 mL) at 0 °C and stirred for 6 h, then diluted with water (10 mL). Isopropanol and THF were removed under reduced pressure and product was extracted with dichloromethane (2 × 25 mL), dried over anhydrous sodium sulphate and concentrated under reduced pressure to afford the product as light yellow solid (yield: 75%). Its structure was verified by ¹H NMR (CDCl₃) δ (ppm): 7.37 (m, 4H), 7.17 (m, 4H), 5.79 (s, 1H), 4.50 (s, 4H), 2.31 (s, 1H).

Synthesis of the linear polystyrene with hydroxyl group (LPSOH)

In a typical experiment, a Schlenk apparatus that was first flamed and dried under vacuum was charged with copper bromide (CuBr) (1.20 mmol) and 2,2-bipyridyl (Bipy) (2.40 mmol) under N₂ atmosphere. Then, 30 mL of distilled styrene was added with a degassed syringe. The mixture turned dark brown, indicating complexation of CuBr and Bipy. At last, 4,4′-di(bromomethyl)benzhydrol (1.20 mmol) was added with a precision syringe. The mixture was heated at 110 °C in an oil bath. After 3 h, the experiment was stopped by opening the flask and exposing the catalyst to air. The final mixture was diluted with CH₂Cl₂/THF. The solution was filtered through a column filled with neutral alumina to remove the copper complex. After that the polymer was precipitated in methanol. Finally, the product was dried at 60 °C under vacuum for 48 h (yield: 25%). Its structure was verified by ¹H NMR δ (CDCl₃ as solvent, ppm): 7.3–6.3 (m, aromatic), 4.6–4.4 (broad, 1H, CHBr), 2.9–2.4 (broad, 1H, OH), 2.1–1.0 (m, aliphatic main chain).

Synthesis of the RAFT agent

RAFT agent was synthesized according to the modified literature³⁵ procedure. 50% aqueous NaOH solution (16 g, 200 mmol) was added to a mixture of 1-butanethiol (18 g, 200 mmol) and water 30 mL, followed by 10 mL acetone and resulted in a colorless solution. The solution was stirred at room temperature for 0.5 h. Carbon disulfide (13.5 mL, 1.27 g mL⁻¹, 225 mmol) was added to the resulting solution to obtain a clear orange solution. The reaction mixture was stirred for 0.5 h and then cooled in an ice bath. The 2-bromopropionic acid (31.14 g, 205 mmol) was added dropwise, followed by 16 g of 50% aqueous NaOH solution at a rate to keep the temperature of the bath below 30 °C. After the completion of the exothermic reaction, water (80 mL) was added and then the reaction was stirred at room temperature for 24 h. Then, 10% aqueous HCl (30 mL) was added slowly at 0 °C until solidification (yield: 90%). Its structure was verified by ¹H NMR spectroscopy (δ, CDCl₃ as solvent, ppm): 4.89 (m, 1H, CH–COO–), 3.38 (m, 2H, CH₂–(C=S)), 1.7 (m, 2H, CH₂–CH₂–CS₂), 1.63 (m, 3H, CH₃–CH–(COOH)), 1.43 (m, 2H, CH₂–(CH₂)₂–(C=S)), 0.95 (m, 3H, CH₃–(CH₂)₃–(C=S)).

Synthesis of the LPSCS₂

A clean, dried round-bottom flask was sealed with a septum and subsequently purged with dry argon for several minutes; 20 mL of CH₂Cl₂ (purified by Braun system) was added and cooled to 0 °C in ice water bath. After that LPS-OH, DMAP and RAFT agent were added sequentially. Under magnetic stirring, DCC dissolved in CH₂Cl₂ was dropped into this mixture within 1 h. Then the reaction was kept in ice water bath for 3 h and then stirred at room temperature for 12 h. After that the polymer was precipitated in methanol. Finally, the product was dried at 40 °C under vacuum for 48 h. Its structure was verified by ¹H NMR spectroscopy (δ, CDCl₃ as solvent, ppm): 7.3–6.3 (m, 5H, aromatic), 4.89 (broad, 1H, CH–CS₂), 4.4 (broad, 1H, CHBr), 3.38 (broad, 2H, CH₂–(C=S)), 2.1–1.0 (m, aliphatic main chain).

Synthesis of (PSx)₂-p-(PSy) by RAFT polymerization

In a typical experiment, LPSCS₂ was dissolved in styrene and placed in a dry glass ampoule, and then BPO was added. The mixture was heated at 110 °C in an oil bath. After 3 h, the experiment was stopped by opening the flask and exposing the catalyst to air. The final mixture was diluted with THF and then precipitated by methanol. Finally, the product was dried at 60 °C under vacuum for 48 h. Its structure was verified by ¹H NMR spectroscopy (δ, CDCl₃ as solvent): 7.3–6.3 (m, 5H, aromatic), 4.89 (broad, 1H, CH–(C–S)), 4.4 (broad, 1H, CHBr), 3.38 (broad, 2H, CH₂–(C–S)′), 2.1–1.0 (m, aliphatic main chain). The constitution of the sample is described as (PSx)₂-p-(PSy) with “x” describing half of the number-average molecular weight of the backbone M_n,bb (g mol⁻¹), “y” the number-average molecular weight of the side chains M_n,br (g mol⁻¹) and p volume fraction of the branched chain.

Synthesis of (PSx)₂–(BAy) miktoarm star by RAFT polymerization

In a typical experiment, LPSCS₂ was dissolved in toluene and placed in a dry glass ampoule, AIBN and n-butylacetate were added sequentially. The mixture was heated to 90 °C in an oil bath. After 3 h, the experiment was stopped by opening the flask and exposing the catalyst to air. The final mixture was diluted with THF and then precipitated by methanol. Finally, the product was dried at 60 °C under vacuum for 48 h. Its structure was verified by ¹H NMR spectroscopy (δ, CDCl₃ as solvent): 7.3–6.3 (m, 5H, aromatic), 4.2–3.8 (broad, 2H, (C=O)–O–CH₂), 2.4–2.1 (broad, 1H, O–(C=O)–CH₂), 2.0–1.8 (broad, 2H, (C=O)–O–CH₂–CH₂), 1.8–1.1 (m, aliphatic main chain), 1.1–0.7 (broad, 3H, –O–CH₂–CH₂–CH₃).

Characterization

¹H NMR spectra were recorded on a Bruker AV 400 MHz spectrometer with CDCl₃ as a solvent. Absolute molecular weights (M_w) and polydispersity index (PDI) of the samples were analyzed by SEC-MALLS (multi angle laser light scattering). The SEC instrument consisted of light scattering detector (DAWN-HELLEOS, λ = 662 nm; Wyatt Technology), the viscometer (ViscoStar; Wyatt Technology), and the RI detector (OptilabrEX, λ = 658 nm; Wyatt Technology). For all SEC analyses, HPLC grade tetrahydrofuran was used as the mobile phase at 25 °C (flow rate: 1 mL min⁻¹). The data were collected by ASTAR@6 (Wyatt Technology). Differential scanning calorimetry (DSC) was performed to determine the glass transition using a Mettler Toledo DSC 1 (Mettler Toledo, Switzerland) operating with version 9.1 of STAR software. The samples (5–10 mg) were encapsulated in aluminium pans having pierced lids to allow escape of volatiles. Each sample was subjected to at least three heating–cooling cycles, where each cycle consisted of heating the sample from 25 to 200 °C at a rate of 10 °C min⁻¹, followed by cooling back to 25 °C at 10 °C min⁻¹, and nitrogen purge at 100 mL min⁻¹ was employed.

The rheological properties of (PSx)₂-p-(PS′y) were performed in a stain-controlled rheometer, ARES-G2 (TA Instruments), using a plate–plate geometry (25 mm diameter) with the plate temperature controlled by a FCO unit. Before measurements, all the samples were pressed into pieces (diameter 25 mm) with the thickness of 1.2 mm at 150 °C. Measurements were done in the dynamic (oscillatory) mode. Frequency sweeps in the range 0.05–200 rad s⁻¹ were performed at 150 °C. A strain amplitude sweep (0.01–100%) at a fixed frequency (ω = 1 rad s⁻¹) was performed to establish the linear viscoelasticity regime. The strain during the dynamic shear test was kept small enough to ensure that all response was in the linear viscoelastic region.

Results and discussion

Synthesis of LPSOH and LPSCS₂

The synthesis route to prepare asymmetric and miktoarm stars is based on a four-step sequence (Scheme 1). 4,4′-Di(bromomethyl)-benzhydrol was synthesized as ATRP initiator, in which the hydroxyl group was used to introduce RAFT initiator sites (thiocarbonylthio) by the esterification of hydroxyl groups with RAFT agent after ATRP polymerization. This method contain a designed initiator that can initiated ATRP polymerization with the bromomethyl group and the left hydroxyl group at the centre of the linear PS chain that can be derivatized for Y-shaped polymer synthesis. The hydroxyl group can also be used to decorate the nanoparticles, such as reaction with the carbonyl group on carbon nanotube or graphene. The ¹H NMR spectra verified the structure of synthesized ATRP initiator (Fig. S1†).

To accurately monitor the subsequent chemical modification of hydroxyl group by ¹H NMR, LPSOH samples with low molecular weight were targeted. The polymerization of styrene was performed in bulk at 110 °C with CuBr/Bipy (1/2) as the catalytic system (Scheme 1). Fig. 1a shows ¹H NMR spectra of LPSOH-1. There was a hydroxyl group in the center of the polymer chain, which could easily be used to introduce RAFT initiating sites after the esterification of hydroxyl groups with RAFT agent. Its derivative (LPSCS₂-1) presented the new peaks at 4.89 and 3.38 ppm (Fig. 1b). This confirmed the successful introduction of thiocarbonylthio. The number-average molecular weight (M_n) and polydispersity index of the obtained LPSOH and LPSCS₂ determined by SEC-MALLS were summarized in Table 1.

Fig. 1 ¹H NMR spectra of precursors: (a) LPSOH-1 and (b) LPSCS₂-1. LPSOH was synthesized by ATRP using 4,4′-di(bromomethyl)benzhydrol as initiator.

Table 1 Molecular weight of the synthesized LPSOH and LPSCS₂^a

Run	Sample	Mn (g mol^−1)	Mw/Mn
a LPSCS2-1 was prepared from LPSOH-1.
1	LPSOH-1	7100	1.2
2	LPSCS2-1	7500	1.2
3	LPSOH-2	8900	1.2
4	LPSCS2-2	9300	1.2

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Synthesis of (PSx)₂-p-(PSy)

The synthesis of asymmetric star PS was achieved upon using LPSCS₂ as a macroinitiator for RAFT polymerization of styrene. BPO was employed in the polymerization system, and all the polymerization experiments were conducted in bulk at 110 °C as previous reported.³⁵ To study the kinetic of RAFT polymerization, the polymerization was stopped at different reaction time with low conversion because no solvent was used. Molecular weight of all the samples was tabulated in Table S1,† and the elution curve was shown in Fig. 2. Fig. 3 shows that number average molecular weight of the asymmetric PS increases linearly with polymerization time, meaning that this polymerization is controlled polymerization. The observed curves for SEC analysis of the asymmetric PS with high molecular weight (sample 12, 13, 14) presented a tail in the low molecular mass region (Fig. 2), indicating that irreversible termination reactions may have occurred by chain transfer.³⁶

Fig. 2 SEC traces of the products from St polymerization (LPSCS₂, Table 1): [LPSCS₂]₀ = 1.15 × 10⁻⁴ M, [St]₀ = 0.17 M, [BPO]₀ = 1.2 × 10⁻⁵ M, T = 110 °C (here molecular weights of the samples corresponding to the SEC traces of 5 to 14 were tabulated in Table S1†).

Fig. 3 M_n vs. monomer conversion during RAFT polymerization of St initiated by LPSCS₂ (LPSCS₂, Table 1): [LPSCS₂]₀ = 1.15 × 10⁻⁴ M, [St]₀ = 0.17 M, [BPO]₀ = 1.2 × 10⁻⁵ M, T = 110 °C.

Furthermore, a linear plot of ln([M]₀/[M]_t) vs. polymerization time is observed (Fig. 4). As previously reported by Rizzardo^37,38 that neglected the reverse reaction the kinetic of RAFT polymerization can be described as: R_p/[M] = k_p[P˙]. So ln([M]₀/[M]_t) changed linearly with polymerization time (t). These results indicated that LPSCS₂ successfully initiated the controlled polymerization (RAFT) of St.³⁹ The kinetic plot of the monomer conversion vs. polymerization time was linear up to 14% monomer conversion, which indicated that a constant number of radicals exist in the polymerization system. The molecular weight of asymmetric PS stars increased linearly with the conversion of monomer in spite of the star-like structure (Fig. 4). In addition, polydispersity index kept at fixed value about 1.2 (Table S1†). All these results demonstrated that the polymerization of St was controlled polymerization.

Fig. 4 Plot of ln([M]₀/[M]) vs. polymerization time for RAFT of St (LPSCS₂, Table 1): [LPSCS₂]₀ = 1.15 × 10⁻⁴ M, [St]₀ = 0.17 M, [BPO]₀ = 1.2 × 10⁻⁵ M, T = 110 °C.

Synthesis of (PSx)₂-p-(BAy) miktoarms stars

Besides styrene-type monomer, this method also can be used to synthesize acrylate-type polymers to synthesize A₂B type miktoarm star polymers. This samples can used to study the compatibility of the main chain with the branched chain. So poly(n-butyl acrylate) (PBA) arms were synthesized by RAFT polymerization using n-butyl acrylate as the monomer and LPSCS₂ as the precursors. Similar conditions for synthesizing those (PS)₂(PS′) were applied for the synthesis of (PS)₂(PBA) except for polymerization temperature, which was 90 °C.

The structure of (PS)₂(PBA) was verified by ¹H NMR (Fig. 5). Fig. S2† shows GPC traces of the samples of miktoarm star (PS)₂(PBA) polymerization. All the samples were obtained at different polymerization time with the same LPSCS₂ macro-initiator. The data pertaining to these experiments and the molecular features of the samples determined by SEC using linear PS standards are presented in Table S1.† The samples in Table 2 were obtained at different polymerization time. The GPC results confirmed the growth of the PBA arms from the LPSCS₂ precursor. Moreover, the low polydispersity index (<1.3) indicated a controlled growth of PBA chains.

Fig. 5 ¹H NMR spectra of (PS)₂-(PBA).

Table 2 Asymmetric polystyrene stars (PSx)₂-p-(PS′y) and (PSx)₂-p-(BAz) prepared by RAFT polymerization in the presence of LPSCS₂ macroinitiator

Run	Sample	LPSCS2 precursor			A2B or A2A′		Tg
		Mn,b^a	Mw/Mn	Mn,s^b	Mw/Mn	ϕbr^c
a Molecular weight of the backbone determined by SEC-MALLS.b Molecular weight of the asymmetric polystyrene.c The volume fraction of branched PS and PBA chain. The density of PS and PBA was 1.05 g cm^−3 and 1.08 g cm^−3, respectively.
15	LPSOH-9.3	8900	1.2	8900	1.2	—	96.8
16	LPSCS2-9.3	9300	1.2	9300	1.2	0.00	88.3
17	(PS4.6)2-0.13-(PS1.8)	9300	1.2	11 100	1.2	0.13	91.2
18	(PS4.6)2-0.25-(PS4.0)	9300	1.2	13 300	1.2	0.25	92.1
19	(PS4.6)2-0.35-(PS7.1)	9300	1.2	16 400	1.2	0.35	96.3
20	(PS4.6)2-0.55-(PS23.8)	9300	1.2	33 100	1.3	0.55	97.8
21	LPSCS2-7.5	7500	1.2	7500	1.2	0.0	87.5
22	(PS3.6)2-0.14-(BA1.4)	7500	1.2	8900	1.2	0.14	76.8
23	(PS3.6)2-0.30-(BA4.0)	7500	1.2	11 500	1.2	0.34	69.7
24	(PS3.6)2-0.40-(BA5.2)	7500	1.2	12 700	1.2	0.40	Two Tgs
25	(PS3.6)2-0.92-(BA68.1)	7500	1.2	75 600	1.5	0.92	−44.1

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Properties of asymmetric polymers

To study the properties of the asymmetric polystyrene, more A₂A′ samples were synthesized and the information of the sample were tabulated in Table 2. The symbol “x”, “y”, and “p” in (PSx)₂-p-(PSy) represents half of the molecular weight of the backbone, molecular weight of the branched chain and the volume fraction of the branched chain respectively. Glass transition temperatures (T_g) of the samples were obtained by DSC (Fig. 6a). The higher molecular weight of (PS)₂(PS′), the higher value of T_g as the Flory–Fox equation⁴⁰ depicted. The Flory–Fox equation relates the number-average molecular weight, M_n, to the glass transition temperature, T_g, as shown below:where T_g,∞ is the maximum glass transition temperature that can be achieved at a theoretical infinite molecular weight, and K is an empirical parameter that is related to the free volume in the polymer sample. The plot of T_g versus (−1/M_n) is shown in Fig. 6b. It can be seen that T_g changed linearly with (−1/M_n). However, T_g of LPSCS₂-9.3 was lower than that of LPSOH-9.3, which resulted from the presence of hydrogen bond between polymer chains.⁴¹

Fig. 6 (a) DSC measurement results for (1) LPSOH-9.3, (2) LPSCS₂-9.3, (3) (PS4.6)₂-0.13-(PS1.8), (4) (PS4.6)₂-0.25-(PS4.0) (5) (PS4.6)₂-0.35-(PS7.1) (6) (PS4.6)₂-0.55-(PS23.8). (b) Plot of glass transition temperature (T_g) vs. (−1/M_n) for asymmetric polystyrene except for LPSOH-9.3.

Glass transition temperatures (T_g) of (PS)₂(PBA) were obtained by DSC (Fig. 7). The symbol “x”, “z”, and “p” in (PSx)₂-p-(BAz) represent half of the molecular weight of the backbone, molecular weight of the branched chain and the volume fraction of the branched poly(n-butyl acrylate) with the density of PS and PBA 1.05 g cm⁻³ and 1.08 g cm⁻³, respectively. Different from (PS)₂(PS′), T_g of (PS)₂(PBA) depend more on the content of the polymers. To our best knowledge, T_g values of PS and PBA were identified. When the content of PBA increased, T_g of the copolymer decreased. Low the content of PBA did not lead to large size phase separation due to the chemical bond between PS and PBA. But for (PS3.6)₂-0.40-(BA5.2) there were two T_gs. The two distinct T_g values at lower and higher temperatures can be attributed to macro-phase separation of the two components. When the content of PBA increased to 92%, there was only one T_g for the block polymer.

Fig. 7 DSC measurement results for (a) LPSCS₂-7.5, (b) (PS3.6)₂-0.14-(BA1.4), (c) (PS3.6)₂-0.30-(BA4.0), (d) (PS3.6)₂-0.40-(BA5.2) and (e) (PS3.6)₂-0.92-(BA68.1).

Fig. 8 shows the plots of complex viscosity (η*) versus angular frequency of (PSx)₂-p-(PSy). All the samples showed a typical Newton fluid behavior in the low frequency range. Melt viscosity LPSOH-9.3 was higher than that of LPSCS₂-9.3, which was caused by the formed hydrogen bond between LPSOH.⁴² Molecular weight of the third PS arm (PS′y) in (PSx)₂-p-(PSy) increased from (PS4.6)₂-0.13-(PS1.8) to (PS4.6)₂-0.55-(PS23.8), so the complex viscosity of (PSx)₂-p-(PSy) increased also. Comparing the complex viscosity of LPSCS₂-9.3 and (PS4.6)₂-0.13-(PS1.8), an obvious increasing was observed. When the molecular weight of branched chain increased from 1.8 kDa to 4.0 kDa less increasing in complex viscosity was observed. But when the molecular weight of the branched chain ((PS4.6)₂-0.55-(PS23.8)) increased to higher than that of the backbone, complex viscosity of which increased significantly. These resulted from that the branched chain increased to higher than the backbone, the real backbone changed, so the viscosity increased exponentially. This means the melt viscosity of the branched polymer depend more on the length of the backbone. The complex viscosity of (PS4.6)₂-0.55-(PS23.8) showed an obvious shear-thinning behavior in the frequency range larger than 1 rad s⁻¹ due to the presence of long PS branch chain.

Fig. 8 Complex viscosity (η*) vs. angular frequency (ω) for (a) LPSOH-9.3, (b) LPSCS₂-9.3, (c) (PS4.6)₂-0.13-(PS1.8), (d) (PS4.6)₂-0.25-(PS4.0) (e) (PS4.6)₂-0.35-(PS7.1) (f) (PS4.6)₂-0.55-(PS23.8) at 150 °C.

Conclusions

This paper demonstrated an easy method to synthesize “Y-shaped” polymers based on the combination of ATRP and RAFT, including A₂A′-type asymmetric and A₂B-type miktoarm stars. Using this method, “Y-shaped” polymers were synthesized to obtain (PS)₂PS′ and (PS)₂PBA stars in a controlled manner. This provided the simplest and well-defined branched samples for property studying of star polymers with different length of the branched chains, especially melt behaviour. The DSC results showed that the glass transition temperature increased with the length of branched chains. The rheology measurement showed that the melt behaviour depended on the molecular weight of branched chains in star polymers, and the complex viscosity increased with the length of branched chains.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China for the Projects (51233005 and 21004060).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20541k

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