Molecular Simulation of Hyperbranched Polyester

563 | P a g e O c t o b e r 2 5 , 2 0 1 3 Molecular Simulation of Hyperbranched Polyester Liu Yanmei, Li Haihua, Tai Yulei, Chao Guoku, Zhao Yajuan* a,b,c,d College of Chemistry and Materials Science,Wenzhou University,Wenzhou, Zhejiang,325027,China *E-mail: zyj@wzu.edu.cn E-mail: LiuYanMei198711@yeah.net E-mail: haizhifeng123@126.com E-mail: taiyulei@163.com E-mail: 46008906@qq.com ABSTRACT A new types of hyperbranched polyester was synthesized by the 2,2-bis(hydroxymethyl) propionic acid as an AB2-type monomer and glycerol as the core moiety. Molecular weights were confirmed by Gel Permeation Chromatography. Acid values were titrated by KOH. The hydroxy value was obtained by titration. Furthermore, we calculate logarithmic value of acid value, hydroxy value, and molecular weight, respectively, and the simulation model curves were obtained. Based on the simulation model curves, we establish the empirical equation of the relationship of molecular weight, acid value and hydroxy value.


INTRODUCTION
Recently, dendritic polymer including dendrimers and hyperbranched polymer have received increasing attention due to their unique chemical and physical properties. Compared to traditional liner polymer, they have very large numbers of branch points and end groups, low viscosity and excellent solubility [1][2][3][4][5][6], such as polyesters [7][8], polyamids [9], polysioxysilanes[10] which have been report in past decades.Dendritic polymers mainly include dendrimers and hyperbranched polymers [11]. Dendritic polymer generally synthesized using a multi-step procedure [12][13], however, it is difficult to separate and purity the products. Hyperbranched polymers less regular structure and have more random branches relative to dendrimers, which can be synthesized by simple one-pot polymerization strategies such as polycondensation [14][15][16][17], living polymerization (atom transfer radical polymerization [18][19], reversible additionfragmentation-chain transfer [20]. However, many physical properties of hyperbranched macromolecules have not yet been investigated completely [21].Studies on molecular modeling through atomistic simulations would allow us to know some of their properties before we find their applications. In this paper, a one-pot synthesis of hyperbranched polyesters by polycondensation using 2,2-bis(hydroxymethyl) propionic acid as an AB2-type monomerand and glycerol as the core moiety, is presented. Then acid values were titrated by KOH. The hydroxy values were determined by acetylation of hydroxide groups with phthalic anhydride, which was hydroysed. This paper mainly study on the relationship between acid value, hydroxy value and molecular weight.

Synthesis of hyperbranched polyester
The reaction was conducted in a 1000mL reaction kettle equipped with a nitrogen inlet tube, a reflux condenser, heating device and a vacuum device. The mixture of glycerol and 2,2-bis(hydroxymethyl) propionic acid (DMPA) was add to the reaction kettle, The reaction mixture was slowly heated to190℃. After the reactants were completely melted, The temperature was maintained between190℃ and 200℃ with the continuous nitrogen flow for about 36h. We monitored the reaction periodically by determining the acid value and stopped reaction until the acid was stabilization. Then keep in vacuum for a certain time.

Acid value
Acid value was determined by titrated by KOH using phenolphthalein as the indicator. In a typical titration procedure, milligrams KOH were required to neutralize free acids in 1g hyperbranched polyester. sample was dissolved in ethanol and ether and neutralized with KOH.The acid value, i.e., the total concentration of the carboxylic groups, was measured by diluting about 1g of the sample to 60 cm 3 neutralized distilled water, then the sample was titrated with 0.8185 mol/L KOH.

Hydroxy value
The hydroxy value is usually determined by titration methods. The hydroxy value was determined by acetylation of the hydroxide groups with phthalic anhydride, which was hydrolysed. The excess acetic acid and the free acid groups in the reaction media were titrated with KOH. The hydroxy value was determined with the acid value according to the ISO standard[23] .

Gel Permeation Chromatography(GPC)
Weight-average molecular weight (Mw), number average molecular weight (Mn) and dispersity (D= Mw/Mn) were obtained using GPC instrument. GPC measurements were carried out using Waters 2414 Series (Japan) with a refractive index detector to determine the molecular weight of the polymer solutions. Samples were prepared in tetrahydrofuran (THF) solvent at a 3 mg/mL concentration. The columns were eluted using THF and calibrated with poly(methyl methacrylate) standards. All calibrations and analysis were performed at 40℃ and the flow rate of the mobile phase was kept at 0.4 mL/min.

NMR
The 1 H and 13 C NMR spectra of the hyperbranched polyesters were recorded on Bruker UXNMR 300 MHz spectrometers in dimethyl sulfoxide d6 (DMSO-d6) at ambient temperature.

The acid value, hydroxy value and the number average molecular weight
Different hypherbranched polyesters were synthesis in different conditions, then acid value and hydroxy value was titrated by KOH and number average molecular weight (Mn) was measured by GPC .The dates were shown in the  Table 2. According to the data of Table 2 we deduced equations and model curves. Figure 1 showed the model curve of Mn and Logarithm of acid value. The model curve of acid value and Logarithm of Mn was showed on Figure 2. Both the equation of Figure 1 and Figure 2 showed their related coefficient was low, so the two equation are unreasonable.