Polycaprolactone (PCL) is a biodegradable polyester with a low melting point (~60 °C) typically prep — Organic Chemistry Chemistry Question

Polycaprolactone (PCL) is a biodegradable polyester with a low melting point (~60 °C) typically prepared by ring opening polymerization (ROP) of ε–caprolactone (ε–CL) using a catalyst such as tin(II) 2–ethylhexanoate (stannous octanoate).

[VISUAL]

PCL is fully biodegradable. Furthermore its low melting point makes PCL a useful component of a composite biodegradable material. For example, PCL mixed with starch is used to make cheap biodegradable trash bags.
PCL is degraded by hydrolysis of its ester linkages under physiological conditions and, therefore, has also received a great deal of attention for use as an implantable biomaterial.
PCL has been approved in certain countries for use in the human body, and may be potentially used in drug delivery, sutures, adhesion barriers and scaffolds for tissue repair. So far, a variety of drugs have been encapsulated within PCL beads for controlled release and targeted drug delivery.
Recently, it has been reported that the ROP of ε–CL can proceed with a heat in the presence of natural amino acids. Therefore, the biocompatibility and in vivo safety of PCL thus–obtained could be satisfying for medical and pharmaceutical purposes.

[VISUAL]

In this experiment, four ROP reactions will be carried out for different time intervals to prepare polymer samples with varying molecular masses. Since the degree of polymerization (DP) of these samples is relatively low and each polymer molecule contains an end–group suitable for simple acid–base titration, the average molecular mass of the polymers can be determined by end–group analysis. A main problem in such an approach for molecular mass determination is finding a solvent for the polymer that is compatible with the titration. Fortunately, an appropriate solvent system is available for PCL. PCL can be titrated with KOH in isopropyl alcohol/1,4–dioxane solvent (v/v=1/4) using 1% phenolphthalein solution in pyridine as an indicator. The average molar mass, Mn, of the polymer can be calculated as follows from the sample mass and the number of moles of the end group:

Mn = mass of polymer sample in g / amount of substance (in moles) from end group analysis

The degree of polymerization (DP) at specific reaction time can be obtained from Mn for each polymer sample.

DP = Mt / M₀,
Mt – molar mass at time t
M₀ – molar mass of one monomeric unit

Chemicals and materials
• L–alanine,
• ε–caprolactone
• KOH
• Tetrahydrofuran
• Methanol
• isopropyl alcohol
• 1,4–dioxane
• 1% phenolphthalein solution in pyridine

Apparatus and glassware
• Balance with at least 0.01 g accuracy,
• four 50 cm₃ one–neck round bottom flasks,
• four 250 cm₃ beakers,
• test tubes,
• 50 cm₃ burrette,
• Pasteur pipette,
• oil bath,
• hot plate stirrer,
• vacuum oven,
• mg–scale balance

Procedure 1: Neat ring–opening polymerization
(1) In each of four 50 cm₃ one–neck round bottom flasks (RBF), mix 0.13 g L–alanine (1.5 mmol) and 5.13 g ε–caprolactone (45 mmol) and stir the mixture at 160 °C in an oil bath. Connect the flasks to nitrogen line to release any pressure developed during the heating.
(2) After 1, 5, 12, and 24 h, remove one of the flasks from the bath and cool it down to room temperature. Dissolve the mixture in 5 cm₃ tetrahydrofuran (THF) and precipitate the polymer product by pouring the solution into 80 cm₃ methanol/H₂O (v / v = 4 / 1) solution.
(3) Filter the precipitated polymer products and dry in a vacuum oven for several hours. Measure the mass of dried polymer products.

Procedure 2: Titration with KOH
(1) Prepare a standardized solution of KOH with a concentration of about 0.008 mol dm–3 in isopropyl alcohol / 1,4–dioxane (v / v = 1 / 4).
(2) Dissolve each polymer sample obtained above in 5.0 cm₃ of isopropyl alcohol / 1,4–dioxane (v / v = 1 / 4). Add 4 drops of 1% phenolphthalein / pyridine solution to 1.0 cm₃ aliquot of each polymer solution and titrate with the standardized KOH solution. Repeat this titration.
(3) Calculate the average experimental molar mass (g mol–1) of the sample from the average volume of the titrant.
(4) Repeat steps 2 and 3 for other polymer samples.

Questions
Assume that 100% conversion of monomer is obtained after 24 h, and all the amino acid (alanine) is incorporated into the polymer.