# Poly-(3-ethyl-3-hydroxymethyl)oxetanes--Synthesis and Adhesive Interactions with Polar Substrates
## 3 Results and Discussion  
Poly(3-ethyl-3-hydroxymethyloxetane)s can be synthesized according to cationic, anionic or activated monomer mechanisms [1; 2; 13; 15; 38; 39]. In the case of cationic polymerization, the catalyst concentration and temperature have fundamental impacts on the degree of the branching of the obtained product. Figure 1 shows the mechanism of the reaction and the possible substructures of the polymer. In the structure that was synthesized with the use of trimethylopropane (TMP) as aninitiator and EHO as a monomer, the starting (S) and terminal (T) units were identical, which simplified the NMR spectra (Figure 2). Other units present in the polymer were dendritic ones (D) in which all hydroxyl groups reacted; linear ones (L), with one free OH group; and terminal cyclic ones (T'). In case of the polymerization of 3-ethyl-3-hydroxymethylacetate at a constant concentration of catalyst, the number of branching points depended mainly on the reaction temperature. When the reaction was conducted at \(-\)30, \(-\)2, 30 and 70 \({}^{\circ}\)C, the degrees of branching calculated from \({}^{13}\)C NMR spectra by using the Frey equation [38] were equal to 0.21, 0.36, 0.43 and 0.50, respectively. Figure 2 shows a typical \({}^{1}\)H NMR spectrum of hyperbranched poly(3-ethyl-3-hydroxymethylacetate) measured in DMSO-d\({}_{6}\). All the signals can be assigned to the structural elements of the polymer: -OH (4.7 ppm), -CH\({}_{2}\)-O-(T') (3.7-3.6 ppm), -CH\({}_{2}\)-O-(3.3 ppm), -CH\({}_{2}\)-OH (3.1 ppm), -CH\({}_{2}\)-CH\({}_{3}\) (1.3 ppm), and -CH\({}_{2}\)-CH\({}_{3}\) (0.8 ppm). The doublet of doublets (e) at 3.7 ppm indicates the presence of terminal units T' (Figure 1) and was visible only for the polymers synthesized at low temperatures. The degree of branching of the hyperbranched polyoxetanes was determined with the use of b signals (Figure 3) in the \({}^{13}\)C NMR spectrum. The signal of the methylene group carbon of the ethyl substituent was split according to the chemical surrounding into signals of dendritic (b\"), linear (b')  
Figure 1: Cationic polymerization of 3-ethyl-3-hydroxymethylacetate. Figure 2: \({}^{1}\)H NMR (400 MHz, DMSO-d\({}_{6}\)) spectrum of poly(3-ethyl-3-hydroxymethylacetate). and terminal (b) units. It was deconvoluted and integrated by using MestReNova 10.0.2-15465 software (Mestrelab Research, S.L., Escondido, CA, USA). For the purpose of adhesive tests, four polyoxetanes of different molar masses were synthesized. Reagent amounts and theoretical molar masses are shown in Table 1. A high abundancy of polar hydroxyl groups is a prerequisite for an adhesive material to form strong interactions with polar substrates. Additionally, thermoplastic character of POXs makes them considered to be potential hot-melt adhesives. The reactions were performed at 70 \({}^{\circ}\)C to assure high (0.5) degrees of branching and high (>95%) reaction yields. The \({}^{1}\)H NMR spectra shown in Figure 4 indicate the efficient polymerization of the EHO monomer with the TMP core molecule. The signals of TMP units have the same chemical shifts as signals coming from EHO units which make the spectra quite simple. For the same reason, the calculation of molar mass of the polymers from the NMR spectra was not possible. The T' units signals in these experiments were not visible. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Theoretical Molar Mass [g/mol]** & **TMP/EHO Molar Ratio** & **EHO [g]** & **TMP [g]** & **BF\({}_{3}\)Et\({}_{2}\)O [g]** \\ \hline
714 & 1:5 & 10.25 & 2.36 & 0.13 \\
1295 & 1:10 & 18.00 & 2.36 & 0.25 \\
2457 & 1:20 & 20.50 & 1.18 & 0.25 \\
5942 & 1:50 & 51.58 & 1.18 & 0.63 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Molar mass and amounts of reagents used in synthesis of polyoxetanes (POXs). Figure 4: \({}^{1}\)H NMR (400 MHz, DMSO-d\({}_{6}\)) spectra of the studied polyoxetanes. Figure 3: \({}^{13}\)C NMR (400 MHz, DMSO-d\({}_{6}\)) spectrum of poly(3-ethyl-3-hydroxymethylloxetane) of degree of branching equal to 0.36.