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Article Preparation and Properties of Branched through Radical Suspension Polymerization

Wenyan Huang, Weikai Gu, Hongjun Yang, Xiaoqiang Xue, Bibiao Jiang *, Dongliang Zhang, Jianbo Fang, Jianhai Chen, Yang Yang and Jinlong Guo

School of Materials Science and Engineering, Changzhou University, Changzhou 213164, China; [email protected] (W.H.); [email protected] (W.G.); [email protected] (H.Y.); [email protected] (X.X.); [email protected] (D.Z.); [email protected] (J.F.); [email protected] (J.C.); [email protected] (Y.Y.); [email protected] (J.G.) * Correspondence: [email protected]; Tel.: +86-519-8633-0006; Fax: +86-519-8633-0047a

Academic Editor: Shin-ichi Yusa Received: 15 November 2016; Accepted: 28 December 2016; Published: 6 January 2017

Abstract: Radical solvent-free suspension polymerization of styrene with 3-mercapto hexyl-methacrylate (MHM) as the branching has been carried out using 2,20-azobisisobutyronitrile (AIBN) as the initiator to prepare branched beads of high purity. The molecular weight and branching structure of the polymers have been characterized by triple detection size exclusion chromatography (TD-SEC), proton nuclear magnetic resonance spectroscopy (1H-NMR), and Fourier transform infrared spectroscopy (FTIR). The glass transition temperature and rheological properties have been measured by using differential scanning calorimetry (DSC) and rotational rheometry. At mole ratios of MHM to AIBN less than 1.0, gelation was successfully avoided and branched polystyrene beads were prepared in the absence of any solvent. Branched polystyrene has a relatively −1 higher molecular weight and narrower polydispersity (Mw.MALLS = 1,036,000 g·mol , Mw/Mn = 7.76) than those obtained in solution polymerization. Compared with their linear analogues, lower glass transition temperature and decreased chain entanglement were observed in the presently obtained branched polystyrene because of the effects of branching.

Keywords: solvent-free; radical polymerization; suspension polymerization; branched polymer

1. Introduction Branched polymers have some unique properties, such as lower solution and melt viscosities, increased solubility, and many more terminal groups. It is expected that these branched polymers will be used as polymer rheological modifiers [1–4]. It was not until 1995 that Fréchet et al. reported the preparation of branched vinyl polymer through self-condensing vinyl polymerization of an inimer (SCVP) [5]. Moreover, star polymers were prepared in one-pot by atom transfer radical polymerization (ATRP) of maleimide inimer and styrene [6]. The SCVP of inimer strategy was also used to prepare branched vinyl polymers through living radical polymerization [7–13]. To eliminate the tedious preparation process of the inimer, Baskaran and Sherrington’s group prepared branched polymers using commercially available divinyl monomer as the branching agent through anionic polymerization and ATRP, respectively [14,15]. To date, a series of branched vinyl polymers were prepared using divinyl monomer as the branching agent [16–21], and star polystyrene was prepared by ATRP of bismaleimide and an excess of styrene [22]. Several groups studied the development of branching [23–26], and concluded that the limited molecular weight and broad polydispersity of the obtained branched polymers resulted from primary chain residue and intramolecular cyclization [25–27]. Moreover, the primary chain residue contained no branching agent unit [28]. It is well known that radical polymerization can be carried out under much milder

Polymers 2017, 9, 14; doi:10.3390/polym9010014 www.mdpi.com/journal/polymers Polymers 2017, 9, 14 2 of 11 reaction conditions, so the preparation of branched polymers through radical polymerization is much more attractive and valuable. To date, a series of research works have reported on the preparation of branched polymers through radical polymerization, involving strategies of copolymerization of a vinyl monomer polymerized with a divinyl monomer in the presence of a radical transfer agent to avoid gelation [29–34], and polymerization in the presence of a polymerizable chain transfer or chain transfer monomer (CTM) [35,36]. With regard to the copolymerization of vinyl and divinyl monomer, the prepared branched polymers show limited molecular weights and much broader polydispersities [32,34]. Additionally, this polymerization must be performed in diluted media, or gelation will occur (except in aqueous or suspension reaction). In our experience, 4-vinyl benzyl thiol—the reported CTM—exhibited poor storage stability [36]. We designed a methacrylic CTM—3-mercapto hexyl methacrylate (MHM)—and prepared branched polymers through radical polymerization [37,38]. In order to inhibit gelation, organic solvent should also be introduced. A branched polymer with high molecular weight and relatively narrow polydispersity can be prepared through emulsion polymerization in the presence of MHM [39]. It is considered that the particular chain termination mechanism in emulsion reaction plays the determinant role in inhibiting gelation. Additionally, water serves well as a heat exchanging agent. However, the polymers obtained after coagulation of the latex are contaminated by impurities such as the emulsifier and coagulating agent residues, since the emulsifier concentration used in emulsion polymerization is typically as high as 1%–5% in weight of the aqueous phase. Polymerization in aqueous suspension offers many advantages that have led to its wide use in commercial production. Suspension polymerization gives polymers of better purity which are isolated easily; i.e., directly by centrifuging or filtering compared with emulsion polymerization, since the levels of suspension stabilizer are typically less than 1.0% in weight of the aqueous phase, which is much lower than the emulsifier concentrations in emulsion polymerization. Suspension polymerization also offers some unique advantages compared to bulk and solution polymerization, since polymer beads of high purity can be obtained directly and water provides an ideal heat-transfer medium during the reaction. Since suspension polymerization has so many marked advantages, it is postulated here that beads of branched vinyl polymers with high purity can be prepared directly through radical solvent-free suspension polymerization and simple filtration, and this is the objective of our research here. In addition, we also investigated the effects of branching on glass transition temperature and the rheological properties of branched vinyl polymers.

2. Experimental Section

2.1. Materials Styrene (St, analytical grade, Shanghai Chemical Co., Shanghai, China) was distilled under reduced pressure. 2,20-Azobisisobutyronitrile (AIBN, analytical grade, Shanghai Chemical Co.) was recrystallized in ethanol. Methacrylic acid and iso-butyric acid (analytical grade, Shanghai Chemical Co.) were used as received. 3-Mercapto-hexanol (MHA, analytical grade, Shijiazhuang Dali Chemical Co., Shijiazhuang, China) was used without further purification. Poly(vinyl alcohol) with alcoholysis degree of 88% and polymerization degree of 1700 (PVA-1788, Shanghai Chemical Co.) was used as received. Other reagents and solvents made from Shanghai Chemical Co. were directly used as received. MHM and 3-mercapto hexyl-isobutyrate (MHIB) were synthesized according to the literature and obtained purities of 99.3% and 98.5% (HPLC), respectively [37,39]. The 1H-NMR spectra of MHM and MHIB with all of the resonances assigned are shown in the Supporting Information (Figures S1 and S2).

2.2. Suspension Polymerization Preparation of branched polystyrene: the aqueous phase containing PVA-1788 (0.75% in weight to water) and the organic phase containing styrene, MHM, and AIBN were mixed in a 100 mL Polymers 2017, 9, 14 3 of 11 round-bottom flask equipped with a stir bar, and degassed in freeze–pump–thaw cycles five times. The weight ratio of the aqueous phase to the organic phase was 3:1. Polymerization was allowed to proceed at 80 ◦C for 8 h under an argon atmosphere. The polymer was collected by filtration using a Buchner funnel and dried under vacuum to a constant mass. For preparation of linear analogue, the polymerization procedure was similar to that of branched polystyrene with or without MHIB as the chain transfer agent. These polymerizations and the related polymers are designated using the form Stx-MHM(MHIB)y-AIBNz, where the subscripts x, y, and z denote the reagent dosages in mmol.

2.3. Fourier Transform Infrared Spectroscopy FT-IR spectra were recorded on a Thermo Nicolet 6700 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with NaCl plates.

2.4. Proton Nuclear Magnetic Resonance Spectroscopy 1H-NMR spectra were recorded on a Bruker ARX-500 type NMR spectrometer (Bruker, Karlsruhe, ◦ Germany) at 25 C with CDCl3 as the solvent and tetramethylsilane as the internal standard.

2.5. Triple Detection Size Exclusion Chromatography (TD-SEC) The molecular weight, polydispersity(PDI), root-mean-square (rms) radius of gyration, and intrinsic viscosity were obtained by TD-SEC detection at 25 ◦C. The instrumentation consisted of the following: a Waters 1515 isocratic HPLC pump with 5 µm Waters styragel columns (Guard, 0.5 HR, 1 HR, 3 HR, 4 HR, and 5 HR columns, with molecular weight ranges of 100–5000 g·mol−1, 500–30,000 g·mol−1, 5000–500,000 g·mol−1, and 50,000–4,000,000 g·mol−1), a Waters 717 PLUS auto-sampler, a Waters 2414 differential refractive index (DRI) detector with a wavelength of 880 nm, an 18-angle laser light scattering (MALLS) detector (Wyatt mini-DAWN HELEOS-II, Wyatt, Cal., Goleta, CA, USA) (from 22.4◦ to 151.4◦) with a wavelength of 690 nm and power of 220 W, a Wyatt Visco Star viscometer detector, and a Waters Empower data manager. The eluent was HPLC-grade tetrahydrofuran (THF) delivered at 1.0 mL·min−1.

2.6. Differential Scanning Calorimetry (DSC) Sample preparation: polymer sheets of 85 mm × 85 mm × 0.3 mm were formed at 200 ◦C under pressure. Rectangular films of 25 mm × 5 mm were tailored from the formed sheets. Glass transition temperature (Tg) was measured using differential scanning calorimetry (PerkinElmer DSC 6000, Waltham, MA, USA) under nitrogen atmosphere. The temperature range was ambient temperature to 200 ◦C. The heating rate of 10 ◦C·min−1, kept for 3 min to eliminate the thermal history, and then cooled down to ambient temperature at a rate of 10 ◦C·min−1. Under the same conditions, a second heating run was applied in order to determine Tg, taken at the inflection point of heat capacity change.

2.7. Rheological Measurements Rheological properties were measured by means of an MCR 301 rheometer (Anton Paar, Graz, Austria) using parallel plate oscillation mode in air atmosphere (25 mm diameter, 2 mm gap). Dynamic frequency sweeps from 0.1 to 100 rad/s were performed under a strain of 1% at 190 ◦C.

3. Results and Discussion In our previous reports, branched polymers were successfully prepared through radical polymerization in the presence of the chain transfer monomer MHM [37,38]. However, it is essential to introduce enough solvent to dilute the reaction system for soluble branched polymers, or gelation will occur. We hypothesize that it is difficult for heat transfer to occur in bulk polymerization and therefore should be an induction for gelation. Suspension polymerization is actually bulk polymerization in Polymers 2017, 9, 14 4 of 11

Polymers 2017,, 9,, 1414 4 of 11 branched polymers with high purity are expected to be prepared directly through solvent-free suspensionbranched polymers polymerization. with high purity are expected to be prepared directly through solvent-free monomersuspension droplet, polymerization. so the heat transfer should be much more convenient, and branched polymers with 3.1.high Preparation purity are of expected Branched to Polymers be prepared directly through solvent-free suspension polymerization. 3.1. PreparationFigures 1 and of Branched 2 show the Polymers typical FTIR and 1H-NMR spectra of the polystyrene prepared through 3.1. Preparation of Branched Polymers suspensionFigures polymerization 1 and 2 show the with typical and FTIRwithout and the 1H-NMR chain transferspectra ofmonomer the polystyrene MHM. Theprepared signal through at 1724 −1 Figures1 and2 show the typical FTIR and 1H-NMR spectra of the polystyrene prepared through cmsuspension in Figure polymerization 1 (relating to withthe carbonyl and without group, the >C=O chain in transfer MHM) monomer and the signal MHM. at The around signal 4.2 at to 1724 4.5 ppmsuspension−1 in Figure polymerization 2 (see inset) (relating with and to without–COO–CH the2– chainin MHM) transfer prove monomer the incorporation MHM. The of the signal MHM at cm in Figure− 1 (relating to the carbonyl group, >C=O in MHM) and the signal at around 4.2 to 4.5 1724unit into cm 1thein polystyrene; Figure1 (relating these toresults the carbonyl are the group,same as >C=O those inobserved MHM) andin solution the signal and at emulsion around ppm in Figure 2 (see inset) (relating to –COO–CH2– in MHM) prove the incorporation of the MHM 4.2 to 4.5 ppm in Figure2 (see inset) (relating to1 –COO– CH – in MHM) prove the incorporation of the polymerizationunit into the polystyrene; [37,39]. Figure these S3 results shows arethe theH-NMR same spectrumas 2those observed of the linear in solution polystyrene and preparedemulsion MHM unit into the polystyrene; these results are the same as those observed in solution and emulsion withoutpolymerization MHM and [37,39]. MHIB, Figure but noS3 relatedshows signthe 1alsH-NMR were observedspectrum from of the 4.0 linear to 4.5 polystyrene ppm. prepared polymerization [37,39]. Figure S3 shows the 1H-NMR spectrum of the linear polystyrene prepared without MHM and MHIB, but no related signals were observed from 4.0 to 4.5 ppm. without MHM and MHIB, but no related signals were observed from 4.0 to 4.5 ppm.

St -AIBN 100 0.5 St -AIBN 100 0.5

1724 cm-1

1724 cm-1 St -MHM -AIBN 100 2.0 3.0 St -MHM -AIBN 100 2.0 3.0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers (cm-1) 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers (cm-1) Figure 1. Typical Fourier transform infrared (FT-IR) spectra of the . Figure 1. Typical Fourier transform infrared (FT-IR) spectra of the polystyrenes. Figure 1. Typical Fourier transform infrared (FT-IR) spectra of the polystyrenes.

CH3 a CH2 CH CH2 C a COCH3 O a CH2 CH CH2 C a CHCO2 b

CHO 2

H3C H2C H2C CH2 Sb CH2 CH

CH2 a

H3C H2C H2C CH S CH2 CH a

b

4.5 4.4b 4.3 4.2

4.5 4.4 4.3 4.2 76543210 ppm 76543210 1 ppm 0 Figure 2. H-NMR spectrum of the polystyrene St100-MHM2.0-AIBN3.0. AIBN: 2,2 -Azobisisobutyronitrile ; Figure 2. 1H-NMR spectrum of the polystyrene St100-MHM2.0-AIBN3.0. AIBN: 2,2′-Azobisisobutyronitrile; MHM: 3-mercapto hexyl methacrylate; St: Styrene. MHM: 3-mercapto hexyl methacrylate; St: Styrene. Figure 2. 1H-NMR spectrum of the polystyrene St100-MHM2.0-AIBN3.0. AIBN: 2,2′-Azobisisobutyronitrile; MHM: 3-mercapto hexyl methacrylate; St: Styrene. Table1 1 illustrates illustrates thethe TD-SECTD-SEC resultsresults ofof polystyrenepolystyrene preparedprepared usingusing MHMMHM asas thethe branchingbranching monomer. When the mole ratio of MHM to the initiator AIBN was less than 1.0, soluble polystyrene monomer.Table When1 illustrates the mole the ratio TD-SEC of MHM results to theof poinitialystyrenetor AIBN prepared was less using than MHM1.0, soluble as the polystyrene branching could be prepared without gelation. However, when the above mole ratio was greater than 1—as in the couldmonomer. be prepared When the without mole ratio gelation. of MHM However, to the wheninitiator the AIBN above was mole less ratio than was 1.0, greater soluble than polystyrene 1—as in cases of branched polystyrene (BPS)-3 and BPS-4—gelation occurred. For selected styrene and MHM thecould cases be preparedof branched without polystyrene gelation. (BPS)-3 However, and whenBPS-4—gelation the above mole occurred. ratio wasFor greaterselected than styrene 1—as and in concentrations, polymers prepared at higher initiator concentration exhibited lower molecular weights the cases of branched polystyrene (BPS)-3 and BPS-4—gelation occurred. For selected styrene and

Polymers 2017, 9, 14 5 of 11

MHM concentrations, polymers prepared at higher initiator concentration exhibited lower molecular Polymers 2017, 9, 14 5 of 11 weights (BPS-5 vs. BPS-8; BPS-9 vs. BPS-10, Table 1). These results should be accounted for by the shorter primary chain length at high initiator concentration. For selected styrene and initiator concentrations,(BPS-5 vs. BPS-8; polymers BPS-9 vs.prepared BPS-10, at Tablehigher1). M TheseHM concentration results should showed be accounted higher molecular for by the weights shorter (compareprimarychain BPS-7 length vs. BPS-9 at high and initiator BPS-10 concentration. vs. BPS-11), Forwhich selected are similar styrene andresults initiator to those concentrations, that were observedpolymers in prepared solution at and higher emulsion MHM reaction concentration becaus showede of the availability higher molecular of some weights more MHM (compare units BPS-7 per primaryvs. BPS-9 chain and BPS-10[37,39]. vs. BPS-11), which are similar results to those that were observed in solution and emulsion reaction because of the availability of some more MHM units per primary chain [37,39]. Table 1. Triple Detection Size Exclusion Chromatography (TD-SEC) results of the obtained polymers.

Table 1. TripleSample Detection No. Size Feed Exclusion ratio ChromatographyMn.SEC a (g·mol (TD-SEC)−1) Mw.MALLS resultsb (g·mol of the−1) obtainedPDI polymers. LPS-1 St100-AIBN0.5 50,700 191,400 3.53 a −1 b −1 Sample No.LPS-2 FeedSt100 ratio-MHIB0.5-AIBN0.5M n.SEC (g12,900·mol ) Mw.MALLS 123,100 (g·mol 6.42) PDI BPS-1 St100-MHM0.25-AIBN0.5 44,300 389,200 5.29 LPS-1 St100-AIBN0.5 50,700 191,400 3.53 LPS-2 BPS-2 St100-MHIBSt100-MHM0.5-AIBN0.5-AIBN0.5 0.5 12,90049,500 795,600 123,100 6.96 6.42 BPS-1 BPS-3 St100-MHMSt100-MHM0.25-AIBN0.75-AIBN0.5 0.5 44,300gelation 389,200 5.29 BPS-2 BPS-4 St100-MHMSt100-MHM0.5-AIBN1.0-AIBN0.5 0.75 49,500gelation 795,600 6.96 BPS-3 BPS-5 St100-MHMSt100-MHM0.75-AIBN1.0-AIBN0.5 1.0 47,800 gelation 723,000 6.04 BPS-4 St -MHM -AIBN gelation BPS-6 100 St100-MHM1.0 1.0-AIBN0.75 1.5 22,600 289,700 6.19 BPS-5 St100-MHM1.0-AIBN1.0 47,800 723,000 6.04 BPS-7 St100-MHM1.0-AIBN2.0 18,700 256,400 6.50 BPS-6 St100-MHM1.0-AIBN1.5 22,600 289,700 6.19 BPS-8 St100-MHM1.0-AIBN2.5 15,000 96,800 4.29 BPS-7 St100-MHM1.0-AIBN2.0 18,700 256,400 6.50 BPS-9 St100-MHM1.5-AIBN2.0 33,600 1,036,000 7.76 BPS-8 St100-MHM1.0-AIBN2.5 15,000 96,800 4.29 BPS-10 St100-MHM1.5-AIBN3.0 15,700 219,300 6.33 BPS-9 St100-MHM1.5-AIBN2.0 33,600 1,036,000 7.76 BPS-10 BPS-11St 100-MHMSt100-MHM1.5-AIBN2.0-AIBN3.0 3.0 15,70028,100 731,900 219,300 7.87 6.33 c BPS-11BPS-12 St 100-MHMSt100-MHM2.0-AIBN2.0-AIBN3.0 0.5 28,10032,900 985,400 731,900 10.26 7.87 c BPS-12 BPS-13 dSt 100-MHMSt100-MHM2.0-AIBN2.0-PPDS0.5 0.5 32,900144,900 2,184,000 985,400 4.42 10.26 d BPS-13 St100-MHM2.0-PPDS0.5 144,900 2,184,000 4.42 a Mn.SEC is the number-averaged molecular weight determined by the differential refraction detector a M is the number-averaged molecular weight determined by the differential refraction detector size size exclusionn.SEC chromatography. b Mn.MALLS is the weight-averaged molecular weight determined by exclusion chromatography. b M is the weight-averaged molecular weight determined by the multi n.MALLS c theangle multi laser angle light laser scattering light detectorscattering size detector exclusion size chromatography. exclusion chromatography.c Polymer prepared Polymer through prepared solution throughpolymerization solution [ 37polymerization]. d Polymer prepared [37]. d through Polymer emulsion prepared polymerization through emulsion [39]. BPS: branchedpolymerization polystyrene; [39]. BPS:LPS: branched linear polystyrene; polystyrene; PPDS: LPS: potassium linear polystyrene; peroxodisulfate. PPDS: potassium peroxodisulfate. Figure 3 illustrates the dependence of molecular weight on elution volume for the polymers Figure3 illustrates the dependence of molecular weight on elution volume for the polymers prepared with or without MHM. BPS-1 consistently had a larger MW.SEC than that of LPS-1 at any prepared with or without MHM. BPS-1 consistently had a larger M than that of LPS-1 at any selected elution volume, regardless of its very low MHM concentrationW.SEC (Figure 3A), confirming the selected elution volume, regardless of its very low MHM concentration (Figure3A), confirming the branching structures of BPS-1 [1–5,40]. For a fixed initiator concentration, polymers prepared at branching structures of BPS-1 [1–5,40]. For a fixed initiator concentration, polymers prepared at higher MHM concentration exhibited higher molecular weight at given elution volume (Figure 3B,C), higher MHM concentration exhibited higher molecular weight at given elution volume (Figure3B,C), indicating that polymers obtained at higher MHM concentration showed some higher degree of indicating that polymers obtained at higher MHM concentration showed some higher degree branching. of branching.

107

6 A 10 BPS-1 ) -1 5 10 LPS-1

104 107 B 106 BPS-2

5 10 BPS-1

104 7 10

6 C 10 BPS-11 Molecular Weight (g mol 5 10 BPS-10

104 24 26 28 30 32 34 36 38 40 42 Elution Volume (mL)

FigureFigure 3. 3. PlotsPlots of of molecular molecular weight weight versus versus elution elution volume. volume.

PolymersPolymers2017 2017,,9 9,, 14 14 66 ofof 1111

The Mark–Houwink exponents of α were determined to be 0.71, 0.55, and 0.41 for LPS-1, BPS-2, The Mark–Houwink exponents of α were determined to be 0.71, 0.55, and 0.41 for LPS-1, BPS-2, and BPS-9, respectively (Figure 4). In addition, LPS-1 and BPS-9 exhibited the highest and the lowest and BPS-9, respectively (Figure4). In addition, LPS-1 and BPS-9 exhibited the highest and the lowest viscosities at any given molecular weight, respectively. These results coincide well with those shown viscosities at any given molecular weight, respectively. These results coincide well with those shown in Figure 3, and furthermore confirm the branching structure of the polymers prepared through in Figure3, and furthermore confirm the branching structure of the polymers prepared through radical radical suspension polymerization in the presence of MHM [5]. suspension polymerization in the presence of MHM [5].

103 )

-1 BPS-2: α = 0.55 mL g

( LPS-1: α = 0.71

102

BPS-9, α = 0.41 Intrinsic Viscosity

105 106 107 -1 Molecular Weight (g mol )

FigureFigure 4.4. TheThe Mark–HouwinkMark–Houwink plotsplots ofof somesome typicaltypical polymers.polymers.

WithWith regard to to polymer polymer branching branching structure, structure, we weobtained obtained the intrinsic the intrinsic viscosity viscosity (IV) and (IV) mean- and mean-squaresquare radius radius of gyration of gyration (Rg) through (Rg) through TD-SEC TD-SECanalysis analysisof the polymers. of the polymers. It is noted that It is the noted samples that thefor samplesbranching for structure branching information structure information were fraction weres with fractions relatively with relativelynarrow polydispersity narrow polydispersity through throughpolymer polymerfractionation. fractionation. The Zimm The branching Zimm branching factors g and factors g′ asg theand qualitativeg0 as the qualitative indicator, as indicator, well as asthe well relationship as the relationship between g between and g′ areg and giveng0 arein Equations given in Equations (1)–(3) [40,41]. (1)–(3) [40,41].

0 g′ = IVBranched/IVLinear (1) g = IVBranched/IVLinear (1) g = Branched/Linear (2) 2 2 g = Branched/Linear (2) g′ = gb (3) g0 = gb (3) For regular star polymers, the exponent b is less than 1, and it is equal to 0.5 in θ condition and 0.6–0.8For in regular good starsolvent polymers, [26,40,41]. the exponentFor randomlyb is less branched than 1, polymers, and it is equal the exponent to 0.5 in θ bcondition has a starting and 0.6–0.8value larger in good than solvent 1 and [26decreases,40,41]. Forwith randomly molecular branched weight [26,40]. polymers, Figure the 5 exponent illustratesb thehas changes a starting in valuethe exponent larger than b with 1 and molecular decreases weight with of molecular typical branched weight [ 26polystyrene,40]. Figure prepared5 illustrates here the (BPS-11, changes as inan theexample), exponent theb referencewith molecular star, and weight randomly of typical branched branched polystyrene polystyrene in the prepared literature here [26], (BPS-11, which proves as an example),that polymers the reference prepared star, in the and presence randomly of MHM branched through polystyrene radical suspension in the literature polymerization [26], which were proves of thatrandomly polymers branched prepared structure. in the presence of MHM through radical suspension polymerization were of randomly branched structure.

PolymersPolymers 20172017,, 99,, 14 14 77 of of 11 11

5 4 Randomly Branched Polymer 3 b 2 1 0 4.0x105 6.0x105 8.0x105 1.0x106 1.2x106 5 4 Star Polymer b 3 2 1 0 4.0x105 8.0x105 1.2x106 5 4 3 BPS-11 b 2 1 0 106 107 -1 Molecular Weight (g mol )

FigureFigure 5. 5. ChangesChanges in in the the exponent exponent bb withwith molecular molecular weight weight for for branched branched polymers. polymers.

3.2.3.2. Properties Properties of of the the Branched Branched Polymers Polymers FigureFigure 6 shows the DSC curves of the polymerspolymers prepared with and without MHM at different concentrations.concentrations. Similar Similar to to the the results results in in the the liter literatureature [42], [42], linear linear polystyrene polystyrene (LPS-1) (LPS-1) showed showed the the ◦ highesthighest glass glass transition transition temperature temperature (Tg ( T=g 106.6= 106.6 °C) comparedC) compared to allto three all threebranched branched polystyrenes polystyrenes (BPS- 7,(BPS-7, BPS-9, BPS-9, and BPS-10) and BPS-10) prepared prepared here, because here, because branched branched polymers polymers had more had morechain chainends and ends their and polymertheir polymer segments segments were weremore more mobile mobile [43]. [ 43Additionally,]. Additionally, the theMHM MHM used used in inthis this study study is is a a soft soft monomermonomer compared compared to to styrene. styrene. At a At given a given initiato initiatorr concentration, concentration, polymer polymer prepared prepared at higher at MHM higher concentrationMHM concentration showed showed lower lowerglass glasstransition transition temperature temperature than than that that prepared prepared at atlower lower MHM MHM ◦ ◦ concentrationconcentration ( (TTgg == 92.8 92.8 °CC for for BPS-9 BPS-9 vs. vs. TTgg == 100.1 100.1 °CC for for BPS-7). BPS-7). Besides, Besides, the the higher higher degree degree of of branchingbranching of of BPS-9 BPS-9 illustrated illustrated some some higher higher soft soft monomer monomer concentration concentration present present in inBPS-9 BPS-9 than than that that of BPS-7.of BPS-7. Thus, Thus, the higher the higher degree degree of branching of branching should should be the be determining the determining factor factorthat accounts that accounts for lower for glasslower transition glass transition temperature temperature of BPS-9 of BPS-9 compared compared to BPS-7, to BPS-7, because because polymers polymers with with higher higher degree degree of branchingof branching would would have have more more polymer polymer chain chain ends ends at atthe the same same molecular molecular weight. weight. At At a a given given MHM MHM concentration,concentration, polymer polymer prepared prepared at at lower lower initiato initiatorr concentration concentration exhibited exhibited higher higher glass glass transition transition ◦ temperaturetemperature compared compared with with that that obtained obtained at at higher higher initiator initiator concentration concentration ( Tg == 97.5 97.5 °CC for for BPS-9 BPS-9 vs. vs. ◦ TTgg == 92.8 92.8 °CC for for BPS-10). BPS-10). The The reason reason for the aboveabove isis thatthat thetheprimary primary chain chain length length would would be be shorter shorter at athigher higher initiator initiator concentration, concentration, and and therefore therefore the th polymere polymer segments segments would would be be more more mobile, mobile, leading leading to tolower lower glass glass transition transition temperature temperature with with higher higher initiator initiator concentration. concentration.

PolymersPolymersPolymers 20172017 2017, ,9,9 ,9 ,14, 14 14 88 8of of of 11 11 11

° LPS-1: Tg= 106.6 C° LPS-1: Tg= 106.6 C ° BPS-7: Tg= 100.1 C° BPS-7: Tg= 100.1 C

° BPS-9: Tg= 97.5 C° BPS-9: Tg= 97.5 C

° BPS-10: Tg= 92.8 C° BPS-10: Tg= 92.8 C

7070 80 80 90 90 100 100 110 110 120 120 130 130 Temperature (°C° ) Temperature ( C) Figure 6. Differential scanning calorimetry (DSC) curves of the linear and branched polystyrenes. FigureFigure 6. 6. Differential Differential scanning scanning calorimetry calorimetry (DSC) (DSC) curves curves of of the the linear linear and and branched branched polystyrenes. polystyrenes.

FigureFigureFigure 7 7shows shows a a typical typical variation variation of of the the complex complex viscosity viscosity with with angular angular frequency frequency for for LPS-1, LPS-1, BPS-9,BPS-9,BPS-9, and andand BPS-10 BPS-10BPS-10 samples. samples. Although Although Although BPS-9 BPS-9 BPS-9 had had had much much much higher higher higher molecular molecular molecular weight weight weight compared compared compared to to that to that that of of LPS-1ofLPS-1 LPS-1 (Table (Table (Table 1), 1), 1the ),the the latter latter latter showed showed showed much much much higher higher higher complex complex complex viscosity viscosity viscosity at at any any at anygiven given given angular angular angular frequency frequency frequency and and evidentandevident evident shear shear shear thinning thinning thinning compared compared compared to to BPS-9, toBPS-9, BPS-9, demo demo demonstratingnstratingnstrating that that that the the the chain chain chain entanglement entanglement in inin the thethe branchedbranchedbranched polymer polymerpolymer was waswas much muchmuch lower lowerlower than thanthan that thatthat in inin linear linearlinear polymer. polymer.polymer. For ForFor BPS-9, BPS-9,BPS-9, the thethe decrease decreasedecrease in inin complex complexcomplex viscosityviscosityviscosity with with with angular angularangular frequency frequencyfrequency confirmed confirmedconfirmed the the the exis existenceexistencetence ofof of chain chainchain entanglement entanglemententanglement and andand disentanglement disentanglementdisentanglement withwithwith increased increasedincreased angular angularangular frequency. frequency.frequency. However, However,However, the thethe complex complexcomplex viscosity viscosityviscosity of ofof BPS-10 BPS-10BPS-10 was waswas much muchmuch lower lowerlower than thanthan thosethosethose of ofof BPS-9 BPS-9BPS-9 and andand LPS-1, LPS-1,LPS-1, but butbut almost almostalmost no nono decrease decrease decrease in in in complex complex complex viscosity viscosity viscosity with withwith angular angular angular frequency frequencyfrequency was waswas observedobservedobserved in inin the thethe former, former,former, implying implyingimplying there therethere was waswas almost almostalmost nono no chainchain chain entanglemententanglement entanglement in inin thethe the branchedbranched branched polymer polymerpolymer ofofof low lowlow molecular molecularmolecular weight. weight.weight. Further FurtherFurther investigation investigationinvestigation also alsoalso provided providedprovided evidence evidenceevidence for forfor the thethe dependency dependencydependency of ofof chain chainchain entanglemententanglemententanglement on onon molecular molecularmolecular weight weight weight in in in branched branched branched polymers polymers polymers (Figures (Figures (Figures S4 S4 S4 and and and S5). S5). S5).

LPS-1 LPS-1

3 10103 BPS-9 BPS-9

2 BPS-10 10102 BPS-10 Complex Viscosity (Pa s) (Pa Complex Viscosity Complex Viscosity (Pa s) (Pa Complex Viscosity

0.10.1 1 1 10 10 100 100 -1 AngularAngular Frequency Frequency (rad (rad s s)-1)

Figure 7. Variation of the complex viscosity with angular frequency of the polymers. FigureFigure 7. 7. Variation Variation of of the the complex complex viscosity viscosity with with angular angular frequency frequency of of the the polymers. polymers.

4.4.4. Conclusions ConclusionsConclusions BranchedBranchedBranched polystyrene polystyrenepolystyrene beads beadsbeads were werewere successfully successfullsuccessfull preparedy y preparedprepared directly directly throughdirectly solvent-free throughthrough solvent-freesolvent-free suspension suspensionpolymerizationsuspension polymerization polymerization using 3-mercapto-hexyl using using 3-mercapto-hexyl 3-mercapto-hexyl methacrylate methacrylate methacrylate (MHM) (MHM) (MHM) as the as branchingas the the branching branching monomer monomer monomer and 0 and2,2and -azobisisobutyronitrile2,2 2,2′-azobisisobutyronitrile′-azobisisobutyronitrile (AIBN) (AIBN) (AIBN) as the as as initiator. the the initiator. initiator. Compared Compared Compared with solution with with solution solution polymerization, polymerization, polymerization, soluble solublesoluble branched branched polymer polymer beads beads were were prepared prepared at at MHM/AIBN MHM/AIBN feed feed ratios ratios less less than than 1 1without without any any

Polymers 2017, 9, 14 9 of 11 branched polymer beads were prepared at MHM/AIBN feed ratios less than 1 without any introduction of solvent, because water effectively transfers the polymerization heat. The incorporation of MHM into the polymer was confirmed by FTIR and NMR measurements, and TD-SEC analysis provided evidence of the branching structure of the obtained polymers. Furthermore, polymers prepared at higher MHM concentration exhibited a higher degree of branching. Compared with their linear analogues, lower glass transition temperature was observed in branched polymers because of the introduction of branching. The complex viscosity of branched polymers was much lower than those of linear polymers, suggesting decreased chain entanglement in branched molecules. Furthermore, almost no decrease in complex viscosity with angular frequency was observed in branched polymers of low molecular weight, implying that there was almost no chain entanglement in branched polymers of low molecular weight. Thus, there was also a dependency of chain entanglement on molecular weight in branched polymers.

Supplementary Materials: The supplementary materials are available online at www.mdpi.com/2073-4360/9/ 1/14/s1. Acknowledgments: This work was supported by the Natural Science Foundation of China (21174020, 21474010) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Author Contributions: Wenyan Huang and Bibiao Jiang conceived and designed the experiments; Weikai Gu and Jinlong Guo performed the experiments; Dongliang Zhang, Jianbo Fang, Jianhai Chen and Yang Yang analyzed the data; Hongjun Yang and Xiaoqiang Xue contributed reagents/materials/analysis tools; Wenyan Huang and Bibiao Jiang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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