Columbia International Publishing Topological and Supramolecular Polymer Science (2014) Vol. 1 No. 1 pp. 31-41
Research Article
______________________________________________________________________________________________________________________________ *Corresponding e-mail: [email protected] 1* Key Laboratory of Nonferrous Materials and New Processing Technology, Ministry of Education; College
of Material Science and Engineering, Guilin University of Technology, Guilin 541004, China; 2 Department of Chemistry and Chemical Engineering, Zhongkai University of Agriculture Engineering,
Guangzhou 510225, China
31
Synthesis of Branched Poly(butyl acrylate) with Controlled Molecular Weight and Distribution Using
Abstract Branched poly(butyl acrylate) was prepared successfully via semi-continuous emulsion polymerization of butyl acrylate copolymerized with a branching divinyl monomer, divinylbenzene (DVB), together with a chain transfer agent, 1-dodecanethiol (C12SH) to inhibit cross-linking. The particle size and size distribution of the polymer emulsion, molecular weight and molecular weight distribution of the branched polymer changed regularly as the variation of both DVB and C12SH or separately. In optimal condition (molar ratio: BA/DVB/C12SH=100/10/10-15), the branched polymer with relatively large molecular weight (Mn=10,000-15,000 g/mol) and very narrow molecular weight distribution (Mw/Mn=1.5-1.9) was obtained. Keywords: Branched polymer; Poly(butyl acrylate); Semi-continuous emulsion polymerization; Brancher; Chain transfer agent
1. Introduction Last two decades have seen the rapid progress in the synthesis of hyperbranched polymers due to their unique chemical and physical properties as well as their potential applications in drug (Rosenholm et al., 2008), additives (Voit, 2005), coatings (Choi et al., 2013; Wang et al., 2014), nanotechnology and supramolecular science (Gao and Yan, 2004), chemical engineering (Seiler, 2006), templates for the generation of nanoporous organosilicates (Mecerreyes et al., 2001) and so on (Rosenholm and Linden, 2007). The most common methodology in preparing of hyperbranched polymer is the polycondensation of an ABn monomer where A and B react with each other but not with themselves (Costello et al., 2002; Slark et al., 2003). The method requires at least a two-step
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process in which a precursor is made first and then a branch is formed in a second step. More facile, versatile and cost effective routes to synthesize branched vinyl polymers are continuously investigating (O'Brien et al., 2000). Vinyl hyperbranched polymers have attracted attention since the discovery of self-condensing vinyl polymerization (SCVP) by Frechet (1995). This method involved a vinyl monomer with a second functional group capable of initiating the polymerization of other vinyl monomer. In later years, the methodology of synthesis hyperbranched polymers by SCVP of double-monomer or multi-monomer has undergone a fast evolution (Zhou et al., 2011). Different from the traditional methodology as polycondensation, ring-opening polymerization, and proton-transfer polymerization, the SCVP methodology via facile copolymerization of vinyl monomers could be used to prepare hyperbranched polymers with unique structures and properties. Whereas the functional monomers with multi-vinyl groups were introduced to fabricate hyperbranched vinyl polymers, the polymers had the tendency to form cross-linking network and gelation. Sherrington et al. (2000) first discovered that the cross-linking and gelation could be inhibited by use of appropriate level of a chain transfer agent, such as mercaptan. This approach provided a practical and highly convenient route for preparing large quantities of branched, vinyl polymer with high conversion via using readily available raw materials. Later, Sherrington et al. (2003) prepared branched poly(methyl methacrylate) (PMMA) applying free radical solution polymerization with ethylene glycol dimethacrylate (EGDMA) as branching comonomer and 1-dodecanethiol (C12SH) as the chain transfer agent, the effect of branching comonomer structure on the hyperbranched PMMA was investigated as well (Slark, et al., 2003; Isaure et al., 2004). Essential to this strategy, the whole reactions were carried out in an organic solvent such as toluene to ensure that the polymer was formed in a highly expanded state without cross-linking. In this case, solvent might have some chain transfer capability and was uneconomic in commercial production. It was pointed out that bulk or dispersion polymerization might encourage cross-linking (Isaure et al., 2003). Although Campbell et al. (2005) prepared hyperbranched polystyrene with controlled high cross-link density and molecular weight without gelation by utilization of large amount of cross-linker, high reaction temperature (>300 oC) was needed. In order to facilitate this reaction in a more mild condition, Sherrington and Baudry (2006) took the leading in carrying out the copolymerization of methyl methacrylate (MMA) and divinylbenzene (DVB) in aqueous emulsion using batch emulsion polymerization. However, the hyperbranched polymer obtained by this methodology had relatively low molecular weight (Mn < 3,000 g/mol) and broad molecular weight distribution (Mw/Mn > 10). In batch emulsion polymerization, all the reactants are added to the system at the beginning of the reaction. The polymerization is not easy to be controlled after the initiating of monomers, in addition, monomer and comonomer (branching agent) may have different reactivity ratios. These two reasons are responsible for broad molecular weight distribution. Yoo et al. (2002) prepared a hyperbranched polyacrylate in emulsion by atom transfer radical polymerization (ATRP). This method produced polymer with high molecular weight (Mn 3,500~22,100 g/mol), but still broad molecular weight distribution (Mw/Mn > 5.0). Recently, Liu et al (2008) synthesized branched poly(butyl methacrylate) (PBMA) via semi-continuous emulsion polymerization by applying bisphenol A dimethacrylate (BPDM) as a
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branching agent and C12SH as a chain transfer agent. They obtained branched PBMA with relatively high molecular weight (Mn 40,000~160,000 g/mol), and narrow molecular weight distribution (Mw/Mn 1.6~5.3). With a purpose of extending preparation of branched polymer with controlled molecular weight and molecular weight distribution via semi-continuous emulsion polymerization methodology, in this paper, we focused on synthesis of a branched poly(butyl acrylate) (PBA) from using divinylbenzene (DVB) as a branching agent and C12SH as a chain transfer agent. The semi-continuous emulsion polymerization of butyl acrylate and DVB in the presence of C12SH was investigated and particle size and its distribution of the emulsion, the solubility, branched degree, molecular weight and molecular weight distribution of the branched PBA were explored.
2. Experimental 2.1 Materials Butyl acrylate (BA), sodium dodecyl sulfate (SDS), and potassium persulfate (KPS) were purchased from Shantou Xilong Chemical Factory. Sodium hydrogen carbonate was obtained from Tianjin Damao Reagent Co. Dodecanethiol (C12SH) was supplied by China National Medicines Co.. Divinylbenzene (DVB, 80 wt. % divinylbenzene isomers) was bought from Alfa Aesar China Co., Ltd, and washed with 5 wt. % of aqueous sodium hydroxide and dried over calcium chloride anhydrous prior to use. β-Cyclodextrin (β-CD) was purchased from Sinopharm Chemical Reagent Co., Ltd.. BA was distilled under reduced pressure and stored at 4 oC until use. Distilled water was used in all the experiments. Except for said above, other materials were applied as received.
2.2 Preparation of branched PBA The semi-continuous emulsion polymerization was carried out in a four-neck jacketed glass reactor (100 mL) fitted with a reflux condenser, an overhead PTFE screw propeller stirrer, a N2 inlet and a feeding inlet. The typical experimental procedure was as follows: in the first stage, BA (12.42 g, 0.1 mol), DVB (1.59 g, 0.01 mol), C12SH (3.93 g, 0.02 mol), KPS (1 wt. %, relative to BA and DVB), 60 wt. % of SDS (4 wt. %, based on BA, DVB and C12SH), and water (27.0 g) was pre-emulsified under stirring for 0.5 h. In the second stage, phase transfer agent β-CD (0.06 g), which kept the monomer and C12SH at reaction loci simultaneously, 40 wt. % of SDS and water (21.0 g) were added into the reactor. The system was thoroughly purged with nitrogen while the reaction mixture was heated to 80 °C. After the reactor temperature stabilized at 80 °C, a solution of NaHCO3 (0.15 g in 3.0 g of water) as a pH buffer and 3 wt. % of monomer pre-emulsion was added into the reactor. The seed latex was obtained within 0.5 h. In the third stage, the rest of the monomer pre-emulsion was fed into the seed latex evenly for about 4 h by a fluid metering pump in order to maintain the polymerization system in monomer-starved condition. When the feeding was completed, the polymerization was kept at 80 °C in the batch for another 2 h. Then the reaction was cooled to room temperature, and the latex was broken down by adding a aqueous solution of Al2(SO4)3 (10 wt. %). The polymer was rinsed thoroughly with distilled water and dried to constant mass in a vacuum oven at 40 °C. 2.3 Measurements and Characterization The coagulum, solid content and monomer conversion were determined gravimetrically. Particle size and particle size distribution index (PDI) were measured by a Zetasizer Nano-ZS90 (Malvern
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Co). 1H-NMR spectra were recorded on an AVNCE 500 MHz NMR spectrometer (Bruker) using CDCl3 as solvent in 5 ml NMR tubes. Polymer molecular weight and molecular weight distribution were measured by a LC-20AD gel permeation chromatography (GPC) equipped with differential refractive detector using tetrahydrofuran (THF) as the elution solvent at a flow rate of 1.0 mL/min, polystyrene as calibration standard and the column oven temperature was 30 °C.
3. Results and discussions 3.1 Latex properties The properties of the branched PBA latex with different monomer ratios were determined and given in Table 1. All the semi-continuous emulsion polymerization reactions have relatively high monomer conversion and low coagulum. Some reactions do not produce any coagulum. The solid contents of the latex are close to the feed values. We may note that the emulsion polymerization with both a branching agent and a chain transfer agent achieves a little higher monomer conversion, compared with that without these one or two agents. This indicates that the emulsion polymerization of butyl acrylate monomer goes completely upon the introducing of both a branching agent and a chain transfer agent into the system. The monomer BA had more chance to polymerize as a branching agent (divinyl monomer DVB) was used, while short polymer chains were formed that were easy to move and copolymerize as a chain transfer agent was used, these two situations contributed for a little higher monomer conversion. Table 1 Properties of the branched PBA latex with different monomer ratios
It is very interesting to see the change of the average particle size of the emulsions with different amount of DVB and C12SH. The emulsion of pure PBA has nearly monodispersed particle (sample BD0C0, mean particle size 67 nm, PDI 0.120) (Fig. 1a). Upon introducing of a 2 mol % (relative to BA, same hereafter) chain transfer agent C12SH to the polymerization system (sample BD0C2), it produced a smaller particle size (57 nm) and a same narrow size distribution (PDI 0.099) in comparison to that without C12SH. This may be caused by the chain termination of C12SH, which increased the hydrophobicity of the polymer chain leading to the particle compressed in the water phase hence small particle size. When adding the branching agent DVB into the BA emulsion polymerization system (sample BD10C0), it produced a broad particle size distribution (PDI 0.223) and a narrow particle size.
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Fig. 1a shows that there are two peaks in the curve of particle size distribution of sample BD10C0 , a very small peak is centered at 4544 nm and a large peak is centered at 53 nm, the average particle size is 54 nm. This suggests that BA molecules homopolymerized with DVB resulting in more dense structure hence smaller particle size (54 nm), furthermore, the double bonds on the particle surface were tended to polymerize each other after the emulsion polymerization where a sufficient amount of crosslinker existed (Tobita et al., 2000), which formed very large polymer particles thus broad PDI (based on two peaks).
10 100 1000 10000
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nsity, %
Particle size, nm
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56
b
10 100 1000 10000
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nsity, %
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Fig 1. Particle size distribution curves of the branched poly(butyl acrylate) samples BD0C0(1),
When both a small amount of DVB (1 mol %) and C12SH (2 mol %) was added at a molar ratio of 1:2 (Fig. 1b, sample BD1C2), the resultant emulsion exhibits a small particle size (57 nm) with a broad size distribution (PDI 0.253). As an increase of DVB and C12SH at a fixed molar ratio of 1:2 (sample BD5C10 and BD10C20), the average particle sizes of the emulsions increase gradually (78 nm and 81 nm, respectively) and particle size distributions broaden (PDI 0.162 and 0.241, respectively). The two samples again display the bimodal distribution. The presence of both DVB and C12SH in the small amount level cause the decrease of particle size, just as when they are used separately. However, the mean particle sizes of the emulsions increase when more amount of DVB and C12SH are used. More amount of the branching agent and chain transfer agent might bring about randomly branching effect on the molecular structure of PBA, some of the polymer chains were not
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terminated by chain transfer agents, and therefore, bimodal distribution of particle size was formed. At this point, we believe that crosslinking has taken place and some of the particles have connected together. High coagulum for the sample (BD10C20) also provides further evidence for crosslinking. In these cases, we fixed the molar ratio of DVB and C12SH at 1:2.
From theoretical point, each DVB will add two more chain ends to the backbone, at most two C12SH molecules are needed to cap the chain ends to prevent cross-linking (Campbell et al., 2005). In fact, not all of the branching agent will produce the chain end to the backbone, thus less amount of C12SH may be needed. In order to prepare the branched polymer with high branching degree, we kept the level of DVB at 10 mol % and changed the level of C12SH to investigate how the chain transfer agent influenced the particle size and size distribution. It is noteworthy that decreasing the level of the C12SH decreases the mean particle size accordingly but the size distribution varies. When 15 and 10 mol % C12SH were applied (Fig. 1c, samples BD10C15 and BD10C10), the emulsions showed smaller particle sizes (80 and 63 nm, respectively) and the narrow size distributions, further, the sample BD10C10 exhibits a little narrowed size distribution (PDI 0.210). However, if the level of the C12SH is below 10 mol %, it cannot prevent polymer chain from crosslinking, resulting in particle aggregation and bimodal size distribution (sample BD10C5 and BD10C0). It seems that 10-15 mol % of C12SH is needed when 10 mol % of DVB is applied. In PBMA/BPDM/C12SH emulsion polymerization system, Campbell et al. (2005) measured the particle size and size distribution of the branched poly(butyl methacrylate) latex, but they did not notice this variation. Therefore, this work reports firstly the branching agent and the chain transfer agent on the influence of particle size and size distribution of hyperbranched polymer latex. 3.2 Solubility Due to their unique structures and a large number of functional groups, branched polymers show a remarkable solubility in organic solvents. Toluene, tetrahydrofuran, acetone, chloroform and pyridine are poor solvents for PBA at room temperature (sample BD0C0); however, the branched PBAs with different amount of DVB and C12SH show very good solubilities in these solvents. Branched structures within the polymers increase their solubilities obviously. The solubility increased with the increase of DVB and C12SH levels when the molar feed ratio of the branching agent and the chain transfer agent was maintained constantly. It is attributed to the more branched structures within a molecule caused by increasing of DVB and C12SH levels. The decrease of DVB level had a slight influence on solubility when the molar feed ratio of the branching agent was maintained constantly. However, the solubility of the polymer was very poor if the polymer was made in the absence of C12SH (sample BD10C0). Remarkably, it was extremely difficult to inhibit cross-link formation when no any chain transfer agent was used, and the polymer with cross-link and network structure is always poorly dissolved in solvent. 3.3 NMR Spectrum The 1H-NMR spectra of the branched PBAs made by semi-continuous emulsion polymerization are presented in Fig. 2. 1H-NMR (CDCl3, 298K), δ (ppm): 7.28 (residual H in CDCl3), 6.99 (4H, C6H4 from DVB), 4.06 (2H, -OCH2- from BA), 2.30 (H, -CH- from BA), 2.47 (2H, -CH2S- from C12SH). Fig. 2a shows that the different amount of DVB and C12SH at a fixed molar ratio of 1:2. The relative peak strengths increase with the increase of the amount of DVB and C12SH. Fig. 2b shows that the different amount of C12SH at fixed level of DVB. The relative peak strengths decrease with the
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decrease of the amount of C12SH. The result indicates that obtained copolymers include the structure of BA, DVB and C12SH components.
CH CH2 CH CH2 S CH2 (CH2)10
CH CH2
CH3
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COOCH2CH2CH2CH3ab
cd
8 7 6 5 4 3 2
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a
b
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8 7 6 5 4 3 2
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bc
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Fig 2. 1H-NMR of the branched PBA: BD1C2(1), BD5C10(2), BD10C20(3),BD10C15(4), BD10C10(5)
and BD10C5(6) As presented in Table 2, six molar feed ratios of BA/DVB/C12SH were investigated, in which the DVB contents ranged from 0 to 10 mol % (relative to BA) and C12SH contents from 0 to 20 mol %. After polymerization, the DVB and C12SH contents in each polymer were determined from the 1H NMR spectrum (Fig. 2). The molar ratio from 1H-NMR was influenced by the level of molar feed ratio. There is a good correlation between the BA/DVB/C12SH molar feed ratios and the corresponding molar ratios found by 1H-NMR, although the levels of DVB and C12SH in the products are somewhat lower than the feed value, but obviously, both the DVB and C12SH are incorporated very efficiently. This data suggest that both the DVB and C12SH have high reactivity in the semi-continuous emulsion polymerization. Almost all of the DVB and C12SH have taken part in the polymerization. This result is in good agreement with MMA/DVB/benzylthiol polymer system via batch emulsion polymerization (Baudry and Sherrington, 2006).
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3.4 Molecular weight and molecular weight distribution Molecular weight and molecular weight distribution are important indexes to evaluate the property of the branched polymers. In contrast to linear polymers, hyperbranched polymers have a low molecular weight and broad molecular weight distribution. Compared with batch emulsion polymerization, semi-continuous emulsion polymerization methodology results in polymers with relatively narrower molecular weight distribution (Liu et al., 2008). The molecular weight and molecular weight distribution of the samples prepared via semi-continuous emulsion polymerization were measured by conventional GPC, and the results are given in Table 2.
Table 2 Molecular weight and molecular weight distribution of the branched PBA with different
monomer ratios
Sample Molar feed ratio BA/DVB/C12SH
Molar ratio BA/DVB/C12SH*
Mn (×103g/mol)
Mw (×103g/mol)
Mw/Mn
SBD1C2 100:1:2 100:1.0:1.9 27.5 58.1 2.1
SBD5C10 100:5:10 100:4.6:7.9 10.6 16.1 1.5
SBD10C20 100:10:20 100:9.2:18.3 8.0 11.2 1.4
SBD10C15 100:10:15 100:9.7:14.9 10.3 16.4 1.6
SBD10C10 100:10:10 100:9.9:8.4 15.2 28.7 1.9
SBD10C5 100:10:5 100:8.5:5.4 31.4 168.1 5.3
*by 1H-NMR
9 10 11 12
3
2
1
Inte
nsity
Retention Volume(mL)
a
7 8 9 10 11 12
6
5
4
3
Inte
nsity
Retention Volume (mL)
b
Fig 3. Molecular weight distribution curves of the branched poly(butyl acrylate) BD1C2(1),
BD5C10(2), BD10C20(3), BD10C15(4), BD10C10(5), and BD10C5(6) Table 2 shows that the molecular weight and molecular weight distribution are strongly influenced by the level of DVB and C12SH. We did not obtain the molecular weight and molecular weight distribution of the linear PBA because of its insolubility in THF. The molecular weight distribution is relatively narrow when appropriate levels of DVB and C12SH were applied (as shown in Fig. 3). The molecular weight decreased and the molecular weight distribution narrowed with the increase of DVB and C12SH levels when the molar feed ratios of the branching agent and the chain transfer agent were maintained at 1:2. Fig. 3a shows this trend clearly. The main cause of the result is that
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double amount of C12SH molecules were offered to cap the chain ends to prevent cross-linking, which inhibit the propagation of polymer chain. As mentioned above, not all the DVB will produce two molecular ends, so it is necessary to determine the least amount of chain transfer agent. When the molar feed ratio of the branching agent was maintained at 10 mol % (relative BA), the level of the chain transfer agent was varied. In this case, the molecular weight increased and the molecular weight distribution broadened with the decreases of C12SH. Fig. 3b presents this result more detail. The sample BD10C5 (10 mol % DVB and 5 mol % C12SH) exhibits the broadest molecular weight distribution. The least value (in mole) of C12SH should be the same as the DVB. The more level of branching and/or cross-linking may be formed when a small amount of C12SH is applied, which attributes for large molecular weight and broad molecular weight distribution. We apply Mn values and BA/DVB/C12SH ratios from the experiments to calculate the average number of BA units (NBA), DVB units (NDVB) and C12SH units (NC12SH) per polymer chain, as described by literature (Liu et al., 2008). As large amount of C12SH was used, we should not ignore its contribution to the molecular weight. The branching agent DVB divides the polymer chain into many “parts”. Each DVB unit adds two more “parts” to the polymer chain. Thus, the average number of “parts” per chain (X) is calculated with the equation
X = 1 + 3× NDVB (1)
and the average number of BA units per “parts” is expressed by
nBA = NBA/X (2)
The calculated results are list in Table 3. In the case, the samples have different average number of BA units per chain because of the different molecular weight of the polymer. X increases with NDVB, however, nBA decreases greatly with the increase of DVB level, in contrast, the C12SH level has little influence on nBA. As an example, the sample BD1C2 has average 29 of BA units in each “parts” of a polymer chain when only 1 mol % of DVB was used (low branched PBA was formed), while only average 3 of BA units was estimated for the sample BD10C20 where 10 mol % of DVB was applied (high branched PBA was formed). The decrease of C12SH level increases the Mn, NBA, NDVB, and X, but nBA does not change much. The results of former three samples (BD1C2, BD5C10 and BD10C20) are in good agreement with the literature (Liu et al., 2008) while the later three results have not reported.
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a from 1H-NMR. bNBA, NDVB and NC12SH indicated the average number of BA units, DVB units and C12SH units per polymer chain, respectively. c The average number of “part” divided by DVB per chain. d The average number of BA units between two branching points.
4. Conclusions Branched PBAs were prepared successfully via semi-continuous emulsion polymerization of BA as well as a branching agent DVB and a chain transfer agent C12SH with relatively high monomer conversion and low coagulation. In contrast to linear polymers, the branched polymers exhibit a remarkable solubility in organic solvents. As an increase of DVB and C12SH at a fixed molar ratio of 1:2, the average particle sizes of the emulsions increase gradually and the size distributions broaden, the molecular weight decreased and molecular weight distribution narrowed. At fixed DVB level, increasing the C12SH level also increases the average particle size accordingly, the molecular weight decreased and molecular weight distribution narrowed. In the optimal condition, we obtain branched polymer with relatively large molecular weight (Mn=10,000-15,000 g/mol) and very narrow molecular weight distribution (Mw/Mn=1.5-1.9).
Acknowledgements We gratefully acknowledge the financial supports from National Natural Science Foundation of China (51263004), Innovation team of Guangxi universities' talent highland, Guangxi Funds for Specially-appointed Expert, Key Laboratory of New Processing Technology for Nonferrous Metal and Materials, Ministry of Education (12AA-10).
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