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Chapter 8

Synthesis and Study of Sustainable Polymers in the Organic Chemistry Laboratory: An Inquiry-Based Experiment Exploring the Effects of Size and Composition on the Properties of Renewable Block Polymers Grant W. Fahnhorst,1,† Zachary J. Swingen,2,† Deborah K. Schneiderman,1 Christa S. Blaquiere,3 Michael T. Wentzel,2 and Jane E. Wissinger*,1 1Department

of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States 2Department of Chemistry, Augsburg College, Minneapolis, Minnesota 55454, United States 3Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5C9, Canada †Both of these authors contributed equally to this work. *E-mail: [email protected]

A discovery-based experiment highlighting the use of renewable starting materials to synthesize triblock polymers was developed and successfully implemented in our introductory organic chemistry laboratory course. Green chemistry principles applied in this experiment include the use of renewable feedstocks, minimal/green solvents, and design for degradation. Groups of students are tasked with synthesizing polymers of different compositions so that comparisons can be made of their mechanical properties. 1H NMR analysis allows estimation of the overall molecular weight and quantification of the polymer blocks incorporated. The experiment serves to share modern state-of-the art technologies in sustainable polymers and addresses the ACS Committee on Professional Training’s accreditation guidelines to include synthetic polymers and macromolecules in the chemistry curriculum.

© 2016 American Chemical Society Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Introduction


Background Historically, polymer experiments have been underrepresented in the organic chemistry laboratory curriculum. One reason for this omission is the fact that the topic of polymers is generally relegated to the final chapters of organic chemistry textbooks and therefore not covered in the lecture courses that are partnered with the laboratories. Additionally, many of the traditional polymer-themed experiments found in laboratory textbooks, such as the synthesis of polystyrene (1, 2) and nylon (3, 4), use toxic and difficult to handle reagents. Finally, many of the techniques commonly used to analyze and characterize polymeric materials are not typically taught in introductory organic chemistry laboratory courses. Considering the large number of chemists employed by the polymer industry and the importance of synthetic polymers (plastics) in the everyday lives of students, the introduction of a polymer experiment into the University of Minnesota’s (U of MN) sophomore level organic chemistry laboratory course was deemed essential. Annually, this course is taken by 1200 students, approximately 20 percent of whom are chemistry and chemical engineering majors, with the balance representing a wide distribution of other disciplines. Curricular design is focused on choosing experiments representing a breadth of topics relevant to chemistry in society and, for the last fourteen years, those that exemplify modern applications of green chemistry principles (5). Initially, an experiment based on Williamson’s recycling of a PETE bottle into a fiber-glass reinforced polyester was employed as an introduction to polymer chemistry (6). Though this experiment provided a forum to discuss recycling of today’s plastics, the resulting thermoset product could not be recycled and was not degradable, and in this way was emblematic of the end of life issues that plague many of today’s commercial polymeric materials. Moreover, the insolubility of the crosslinked thermoset prevented any molecular weight analysis or characterization instructive in teaching polymer science. Therefore, a new experiment was sought which would reflect modern scientific advances in the field of sustainable polymers. In 2012, new research published by Professor Marc A. Hillmyer with support from the NSF Center for Sustainable Polymers at the University of Minnesota demonstrated the bulk room temperature polymerization of a readily available natural cyclic lactone, δ-decalactone (DDL) (7, 8). Poly(δ-decalactone) (PDDL), a soft polymer with a low softening temperature could be further reacted with L-lactide in a well-controlled process to obtain an ABA triblock polymer (8). Inspired by these results from the research laboratory, an experiment amenable to the non-stringent environment and instrumentation available in the organic chemistry teaching laboratory was pursued. Significant exploration into a more suitable air-stable catalyst and the replacement of hazardous solvents with greener protocols were investigated. As a result, a synthesis of an ABA triblock polymer, summarized in Figure 1, prepared from δ-decalactone and L-lactide was developed and published in the Journal of Chemical Education (9).

124 Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 1. Overview of the Triblock Polymer Experiment Published. This new experiment, performed over two laboratory periods, involves the solvent-free ring-opening transesterification polymerization of δ-decalactone catalyzed with 1.0 M hydrochloric acid in diethyl ether (HCl/Et2O) using a diol initiator. Chain extension of the resulting homopolymer with the renewable L-lactide monomer is catalyzed using tin(II) 2-ethylhexanoate (Sn(Oct)2). The synthesized poly(L-lactide)-block-poly(δ-decalactone)-block-poly(L-lactide) polymer is isolated as a white Styrofoam-like solid, then annealed in an aluminum pan to a transparent thin film disk (typically the annealed film is a soft semi-solid or a tough plastic). Surveys indicate that students enjoy isolating the polymeric material and the inclusion of plastics as a relevant topic in the course. The students appreciate discussions of synthetic polymers and their effects on human health and the environment, learning about renewable feedstocks replacing petroleum-based starting materials, and the connection between the polylactide (PLA) synthesized in lab and the compostable dining ware found on campus. Experimental Design Given the inherent tunablity of block polymer thermoplastic elastomers, this renewable polymer experiment was an ideal candidate for further development into an inquiry or discovery-based experiment. That is, by varying the total molar mass of the block polymer as well as the relative ratios of the soft and rubbery poly(δ-decalactone) midblock and the hard and brittle poly(L-lactide) end blocks, it seemed possible to access a wide range of physical and mechanical properties. Therefore, multiple parameters were explored to evaluate the limitations of the chemistry with the goals of maintaining: cost effective and green syntheses; use of renewable starting materials, products with tangible and measurable properties; reproducible results; and flexibility for adoption by other institutions. A first modification of the original experiment involved investigating the potential of using δ-dodecalactone (DDDL) as an alternative to δ-decalactone as the midblock. As shown in Figure 2, both DDDL and DDL are alkyl-substituted six-membered ring lactones, with DDDL having a side chain that is two carbons longer. Like δ-decalactone (DDL), δ-dodecalactone (DDDL) is used as a food flavoring, is available commercially in kilogram quantities (10), and can be found in nature (11). A practical advantage of δ-dodecalactone with respect to use in a teaching lab is that it is less fragrant and slightly less volatile than δ-decalactone, therefore it can be used on the benchtop without ventilation. In contrast, it is 125 Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


recommended that manipulations involving δ-decalactone (DDL) be performed in the hood. Not surprisingly, δ-dodecalactone polymerized under similar acidic conditions as δ-decalactone to form a viscous homopolymer. It should be noted that other lactone monomers, such as δ-valerolactone and ε-caprolactone form block polymers, however their semi-crystalline properties would produce much stiffer materials with less tunable properties than those containing soft amorphous midblock segments as provided by DDL and DDDL (12, 13). The question left to explore was whether the additional two carbons would provide a measureable difference in the resulting triblock polymers with poly(L-lactide) (PLLA).

Figure 2. Structures of monomers δ-decalactone (DDL) and δ-dodecalactone (DDDL). Both monomers, DDL and DDDL, polymerize under acidic conditions at room temperature to the corresponding homopolymers poly(δ-decalactone) (PDDL) and δ-(dodecalactone) (PDDDL), respectively, in approximately 85-87% conversion. Because of their low ceiling temperatures, heating of the reactions is counterproductive and results in lower conversions (12, 14). As illustrated in Scheme 1, the two acidic catalysts which work well for both polymerizations under teaching lab conditions are the 1.0 M HCl/Et2O, used in the original experiment, and diphenyl phosphate (DPP) (12, 14, 15). Each catalyst has distinct advantages and disadvantages. The HCl/Et2O is a more efficient catalyst reaching 97% of full conversion after 40 hours at 1.8 mole % loading relative to starting lactone; under similar conditions, the DPP catalyzed polymerization requires a week to reach the same conversion. HCl/Et2O is also less expensive than the DPP and can be easily removed from the homopolymer by blowing air over the sample or by concentration under reduced pressure. However, in the large university setting with laboratory sessions running all day and night, the HCl/Et2O catalyst was difficult to manage as a reagent from the stockroom’s perspective because it is both volatile and hydroscopic. In contrast, DPP is comparatively easier to handle as it is a solid, relatively stable, and non-toxic. DPP’s drawback is that separation from the homopolymer is more challenging and requires diluting the crude sample with solvent and either precipitating the polymer or washing the solution with water. These steps, if needed, would add significant time and reduce the green characteristics of the synthesis. Because neither DPP nor HCl is an efficient catalyst for the polymerization of lactide, a different transesterification catalyst, namely Sn(Oct)2, is added for the chain extension step. Importantly, residual acid from the midblock polymerization can deactivate Sn(Oct)2, and either slow or entirely prevent the polymerization of lactide. In the original procedure, HCl is used as a catalyst 126 Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


and removed prior to addition of lactide. In early versions of the experiment, the quantity of Sn(Oct)2 required for the chain extension step was 0.6 mole % relative to L-lactide monomer. However, after three semesters of running the experiment in the organic chemistry laboratory, it appeared that using 1.0 mole % of 0.2 M Sn(Oct)2 in toluene to L-lactide provided more consistent results; most likely due to variability in the students’ abilities to effectively remove the HCl.

Scheme 1. Ring-opening transesterification polymerization of δ-decalactone or δ-dodecalactone. Alternatively, a protocol was developed using diphenyl phosphate (DPP). In contrast to the HCl/Et2O catalyst which is readily soluble in neat lactone, the DPP catalyst dissolves more slowly at room temperature and, depending on the target triblock polymer, requires up to an hour for dissolution. Yet, the ease of handling the DPP offsets the extra half hour needed for this short first day of the experiment. As mentioned above, separation of the DPP catalyst from the homopolymer before lactide polymerization did not appear a good alternative for the experiment, therefore a series of catalysts alone and in combination with DPP were screened based on literature precedence. Examples of catalysts explored for the chain extension included 1,8-diazabicycl[5.4.0]undec-7-ene (DBU) (16, 17) and 4-dimethylaminopyridine (DMAP) (18). However, Sn(Oct)2 was more active and yielded more consistent results under the teaching lab conditions than any of the other catalysts studied. A strategy was then taken to add an equivalent of Sn catalyst specifically to react with the DPP used in the homopolymerization in addition to the 1% required for the lactide polymerization. The total amount of Sn(Oct)2 used for the DPP procedure was therefore approximately 3% relative to lactide monomer, and higher than the original procedure using HCl. The concentration of the tin catalyst solution was increased to 0.4 M Sn(Oct)2 in toluene, in order to keep the quantity of solvent added to the reaction to a minimum, with no effect on the efficiency of the reaction compared to the 0.2 M Sn(Oct)2 catalyst solution. To adapt the lab to a directed-inquiry teaching approach, the impact of using different alcohol initiators for the polymerization was explored. In addition to 1,4-benzenedimethanol, used in the original lab experiment to make a triblock, mono-functional alcohols (e.g. 1- octanol and benzyl alcohol) leading to diblocks, and a triol (glycerol) and tetraol (pentaerythritol) leading to highly branched block polymers, were investigated as initiators. Though both linear and branched block polymers were capable of forming mechanically robust polymer films, the physical properties of the resulting materials were much more influenced 127 Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


by the relative ratios of the poly(δ-(do)decalactone) and PLLA blocks than the initiator used. Comparisons of the 1H NMR spectra of the different initiator-based block polymers indicated that the 1,4-benzenedimethanol initiator, with the distinct aromatic protons, gave the most straightforward analysis for students. Additionally, 1,4-benzene dimethanol is a fine, white solid that students can accurately weigh, crucial for synthesizing copolymers of targeted molecular masses, and was thus selected as the initiator for the experiment moving forward. The first iterations of the guided-inquiry experiment involved dividing laboratory sections of approximately eighteen students into groups of two, with each pair preparing a different polymer, resulting in the synthesis of ten different copolymers. HCl/Et2O was used as the catalyst for the homopolymerization of either δ-decalactone or δ-dodecalactone for the day 1 midblock synthesis using 1,4-benzenedimethanol as the initiator. Following removal of the HCl/Et2O catalyst, 0.6 mole % Sn(Oct)2 to L-lactide was used for PLLA end block addition to prepare triblock polymers of 35%, 40%, 45%, 50%, and 55% weight PLLA of total polymer mass. Students were responsible for calculating the mass of L-lactide and milliliters of 0.2 M Sn(Oct)2 solution to use. In initial class trials, it was evident that the compositions measured by 1H NMR were in some cases significantly different than those targeted synthetically. For example, NMR analysis showed that some products were largely PLLA, with very little of the lactone midblock. This was most likely a result of poor mixing during day 2. Other samples were observed to have lower than expected end block addition, presumably because residual HCl catalyst destroyed the tin catalyst required for polymerization of the L-lactide. Therefore, in all future trials, 1H NMR analysis was performed and considered before any comparisons of physical and mechanical properties. The 5% increment in weight % L-lactide did not provide significant observable trends, whereas 10% or greater difference in end block composition resulted in notable increases in rigidity. Also, if the poly(lactide) end block was 50% by weight or greater, the annealing process required higher heating and took significantly longer. Finally, a mechanical testing method was sought to help students quantify the different block polymer characteristics and add interest to the experiment. Further optimization resulted in the experiment described in this chapter which has been successfully performed by over 900 students to date. Students, either individually or in pairs, are assigned one of four linear ABA triblock polymers to synthesize comprised of a midblock of either poly(δ-decalactone) or poly(δ-dodecalactone) and end blocks of poly(L-lactide) with a total PLLA content of 35% or 45% by weight. The quantity of reagents needed to prepare approximately 3.5 g of each triblock polymer is provided so that polymers of similar mass and thickness can be analyzed side-by-side. Optionally, instructors could challenge the students to perform these calculations themselves given the ratio of the reagents required for each step but allowing student independence in designing their composition. DPP is used as the catalyst for the midblock polymerization and a 3% mole ratio of a 0.4 M solution of Sn(Oct)2 in toluene is used for the PLLA end block formation. These conditions increased the success rate of students forming solid thin film disks to greater than 70% with excellent correlation between target % L-lactide content and the composition determined 128 Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


by 1H NMR. It is made clear to students that obtaining a thin film disk is not a criterion for success and that the balance of the students who obtain thick stickier films can evaluate their adhesive properties or ability to mold their materials, for example into a ball (see Figure 3). More specifics of students’ results will be shared later in the chapter.

Figure 3. Representative photographs of triblock polymer products. Following synthesis, a 1H NMR sample is prepared in CDCl3 of each triblock and submitted for analysis. The triblock polymers are allowed to age for a lab period at room temperature to allow time for the PLLA blocks to crystallize and for students to receive and interpret their 1H NMR spectra. Group discussions ensue where students compare the number average molecular mass (Mn) and poly(L-lactide) content calculated from their NMRs to the target composition and to the observed mechanical properties of their products. A suggested mechanical test is provided to students, which involves clamping their disk to a ring stand and hanging a weight, such as a cork ring, to the polymer using a paper clip as illustrated in Figure 4. The bend of the disk is then measured using a protractor. Alternatively, students within a section (groups of approximately seventeen students per teaching assistant or instructor) collectively evaluated the properties of their materials and designed their own qualitative or quantitative testing best suited to the polymers synthesized. Examples of creative mechanical tests designed by students within a section/group included measuring how far different polymers could stretch or how long they could hold an item stuck to a wall.

Figure 4. Example of a simple mechanical test to evaluate the effect of size and composition for four different triblock polymers synthesized. 129 Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Green Chemistry Principles and Pedagogy This experiment addresses many green chemistry principles. All reactants used are theoretically renewable with the exception of the 1,4-benzenedimethanol initiator and catalysts. The midblock polymerization is run without solvent. The end block addition polymerization utilizes a minimal amount of toluene introduced only to facilitate transfer of the Sn(Oct)2 catalyst, which, because of its high viscosity, is difficult to weigh and deliver. Though methylene chloride would more effectively dissolve the triblock polymer formed, warm ethyl acetate, a greener solvent, is used for transfer to methanol before precipitation. It is expected that the triblock polymer product will be degradable based on the compostability of PLA and literature precedence (19). Additionally, initial degradation trials in the research lab indicated that PDDL/PLLA and PDDDL/PLLA copolymers degrade relatively quickly in refluxing 3 M NaOH, with 30% mass reduction in 5 hours, whereas very slowly in 3 M HCl. This aspect of the experiment is one that would be interesting for students to further explore as a special project. Inquiry-based experiments are proven to promote a higher order learning experience and be more engaging for students in the laboratory curriculum (20, 21). This experiment provides an opportunity for students to learn how fundamental research would evaluate the relationship between polymer composition and physical and mechanical properties. Students are also encouraged to keep an open-mind and think creatively about potential end use of the new materials. Successful implementation of this directed-inquiry version has been achieved in both the small liberal arts school environment of Augsburg College as well as the large university laboratory setting at the University of Minnesota. The level of student independence can be varied from a directed-inquiry version to a guided-inquiry approach where students design the composition of the triblock polymers themselves and/or substitute new lactone monomers for the δ-decalactone or δ-dodecalactone in the procedure given below (22). Additionally, the guided-inquiry approach could be utilized in concert with an initial directed-inquiry approach by repeating the experiment later in the organic lab sequence or in an advanced lab, but restructured with a greater level of independence of monomer and polymer composition choice for students based on the primary literature. In addition to conforming to many of the principles of green chemistry, this experiment introduces students to one of the major challenges of contemporary polymer science − the design of sustainable alternatives to petroleum-derived non-degradable plastics. This experiment also teaches students about the relationship between polymer structure at a molecular level and bulk physical properties, adding to a relatively short list on the subject currently found in the education literature (9, 23–26). Synthesis of triblock polymers of molecular masses in the 20-30 kg/mole range is an interesting contrast to the small molecule chemistry found in a typical introductory level organic chemistry laboratory curriculum. In particular, the students can compare the synthesis and characterization of discrete small molecules (where the structure and molar mass of the desired products are known) to the typically less familiar synthesis and 130 Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


characterization of macromolecules (where the structure is expressed in terms of a probable average composition and molecular weight). Students find the 1H NMR spectra interpretation of the triblock polymers to be challenging but, with careful instruction they gain pride in connecting the theory they have learned in using NMR integration for small molecule mixtures to determination of triblock polymer composition. As an instructor, a motivating factor for incorporating a polymer experiment into the curriculum can be found in the new 2015 American Chemical Society (ACS) Guidelines and Evaluation Procedures for Bachelor’s Degree Programs (27). The Committee on Professional Training (CPR) requires that, “At least two of the following four types of systems must be covered: synthetic polymers, biological macromolecules, supramolecular aggregates, meso- or nanoscale materials,” for ACS major accreditation. We have found incorporation of this experiment into our organic laboratory curriculum helps to satisfy these criteria in an engaging and environmentally conscious learning experience.

Laboratory Methods and Instruction Student Handout Below is the handout provided to students using DPP as the catalyst. Note that one week is required between day 1 and day 2 of the experiment using this catalyst. If a laboratory course meets twice a week and thus a shorter time frame is required between the two days, or if using smaller volumes of the Sn(Oct)2 is preferred, the protocol found in the original paper using 1.0 M HCl/Et2O as the catalyst works well (9).

EFFECT OF SIZE AND COMPOSITION ON THE PROPERTIES OF RENEWABLE POLYMERS

Question: How can the physical and mechanical properties of a triblock polymer, prepared from all renewable starting materials, be tuned using different monomers for the “soft” midblock and varying proportions of the “hard” end block?

Green Concepts: Catalytic reagents, renewable feedstocks, biodegradable materials, minimal solvents



Introduction Polymers, the giant chain-like molecules which comprise plastics and rubbers, are amazing and versatile materials. From the shoes we wear, to the personal care products we use, and the cars we drive, these macromolecules permeate nearly every facet of our daily lives. However, some of the characteristics of these materials that make them so desirable for commercial applications - their stability to temperature changes and resistance to corrosion and degradation – are responsible for their continued buildup and harmful effects to our health and the environment. Additionally, these polymers are derived from non-renewable petrochemicals. To address the future needs of our society, scientists and engineers are designing and developing renewable and degradable alternatives to conventional synthetic polymers. These sustainable polymers must meet the needs of consumers without damaging the environment, human health, or the economy. In this experiment, two naturally occurring lactones, δ-decalactone (1) and δ-dodecalactone (2), will be used as renewable monomers for the synthesis of polyesters. They are both commercially available from flavors and fragrances suppliers because of their tropical odor and taste (δ-decalactone is characterized by a creamy coconut smell (28), δ-dodecalactone has a creamy peach fragrance (29)). These lactones have been shown to undergo ring-opening transesterification reactions under acidic catalytic conditions as illustrated in Scheme 1 (8, 9). As is common in these types of polymerizations, an alcohol initiator is used to begin the ring-opening process. In this procedure, the diol, 1,4-benzenedimethanol, serves as the initiator to grow linear polymer chains outwards from both sides. Recognize that the initiator contributes very little to the overall polymer characteristics, because the number of repeating units, n, is large, resulting in polymers with molecular weights (described as number average molecular weight, Mn) in the kilogram per mole range. Note that the difference between δ-decalactone (1) and δ-dodecalactone (2) is the length of the side chain which can change the properties of the corresponding copolymer; for example consider the different properties and uses for polyethylene and polypropylene. The resulting homopolymers, (3) and (4), are both viscous liquids with a low softening temperature. These polymers have alcohol functional groups on the end of the polymer chain, meaning they can act as initiators for the polymerization of a second monomer. This results in a linear chain with distinct regions or blocks derived from the sequential polymerizations of each of the two different monomers. In this work we use L-lactide as the monomer in the second reaction. Because the second block grows out from the ends, the resulting polymer chain contains poly(L-lactide) end blocks, as illustrated in Scheme 2. The L-Lactide monomer is derived from plants such as corn and sugarcane, and poly(L-lactide) (PLLA) is the most prevalent eco-friendly biopolymer on the market today. Because the PLLA homopolymer is very brittle and stiff, incorporation of PLLA segments in the block polymer adds strength and rigidity to the material.



Scheme 2. Addition of L-lactide as the end block for the triblock polymer.

The PLLA-block-poly(δ-decalactone)-block-PLLA, (5), and PLLAblock-poly(δ-dodecalactone)-block-PLLA (6) triblock polymers, are both thermoplastics, that is, they can be molded by heating above the melting temperature of the hard poly(L-lactide) blocks. A similar triblock architecture is used for a number of commercial applications. One example, comprised of polystyrene and polybutadiene blocks, is called Kraton® and is used in shoe soles and adhesives (30). The unique elastomeric properties of many block polymers such as Kraton® is a result of the different contributions of each homopolymer unit to the overall block polymer structure. In this experiment, a combination of four different triblock polymers will be synthesized by combining poly(δ-decalactone) or poly(δ-dodecalactone) with varying weight percent of poly(L-lactide) incorporation as presented in Table 1. These triblock polymers, represented by the ABA depiction in Figure 5, will be compared to determine how each combination of lactone derived block and polylactide block influences properties such as flexibility, clarity, tackiness, or other observable properties.

Figure 5. Generic representation of an ABA triblock polymer.


Table 1. Triblock Polymer Reagent Quantities Polymer

Day 2 Reagents

bPLLA

cDDL

dDDDL

eBDM

fDPP

(wt.%)

(mL)

(mL)

(g)

(g)

L-Lactide (g)

g

Catalyst (mL)

A

PDDL

35

2.40

-

0.015

0.063

1.23

0.76

B

PDDL

45

2.05

-

0.013

0.054

1.59

0.71

C

PDDDL

35

-

2.45

0.013

0.055

1.24

0.68

D

PDDDL

45

-

2.05

0.011

0.046

1.58

0.63

PDDL = Poly(δ-decalactone), PDDDL = poly(δ-dodecalactone), b PLLA = poly(L-lactide) c DDL = δ-decalactone, d DDDL = δ-dodecalactone, BDM = 1,4-benzenedimethanol initiator, f DPP = diphenyl phosphate, g 0.4 M tin(II) 2-ethylhexanoate (Sn(Oct)2) in toluene

a

134


Day 1 Reagents aMidblock

Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

e

Experimental Procedure


Hazards: Ethyl acetate, methanol, and toluene are flammable. LLactide, δ-decalactone, δ-dodecalactone, 1,4-benzenedimethanol, and tin(II) 2-ethylhexanoate are potential irritants. Chloroform-d is an inhalation hazard and suspected carcinogen. δ-Decalactone is reported to have an odor threshold detection limit in water of 100 parts per billion and has a persistent aroma. Therefore, it should be handled in a hood with gloves at all times. Diphenyl phosphate has no known hazards.

Day 1. Synthesis of Homopolymer − Poly(δ-decalactone) or Poly(δ-dodecalactone) Place a glass vial (6 dram with cap, 23 x 85 mm) with a micro-stirbar in a beaker on a balance. Close all of the doors and tare the balance to zero. Open the door and add precisely the quantity, ± 1 mg, of 1,4-benzenedimethanol of your assigned triblock polymer. Close the door and record the exact mass you used since the amount of initiator has a large effect on the number of repeat units of the polymer. Clamp the vial in the hood on a stirrer hotplate with cap nearby. Carefully, add either the δ-decalactone or δ-dodecalactone monomer via syringe and cap quickly. When ready to add the catalyst, remove the cap and add the designated mass (± 5 mg) of diphenyl phosphate. The exact quantity of the catalyst is less crucial so it is not necessary to spend too much time trying to get the target mass. Cap the vial tightly and stir until the solids are dissolved (typically 30 minutes to one hour). Place the vial in a beaker in your drawer until the next laboratory period. Remember to record all observations before and after the polymerization. Calculate the theoretical molecular weight of the polymer based on quantities of the initiator and monomer used.

Day 2. Addition of L-Lactide End Blocks Record the appearance of the polymer from Day 1. Prepare a sandbath in a 50 mL heating mantle and place it on a stirrer hotplate resting on a ringstand (note, the hotplate will be used only for stirring, NOT heating). Attach a thermometer to the ringstand and place it in the sandbath. Plug the heating mantle into a variac (be sure not to plug it into a regular outlet) and set the variac to ca. 30-40 to begin heating. When the temperature reaches approximately 92-94 °C (the melting point of L-lactide), dig a well in the sandbath using a metal scoopula large enough for the vial and submerge the vial so that the level of the contents is below the level of the sand. Add the designated quantity of L-lactide through a glass funnel and stir. An opaque, white gel should form in 1-2 minutes as the L-lactide melts. Once the L-lactide is melted and stirred in, add the 0.4 M tin(II) 2-ethylhexanoate (Sn(Oct)2) in toluene and stir. Continue to heat the sand bath until it reaches 130 °C and begin timing the reaction. Monitor 135 Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


the temperature throughout the experiment so the sand does not become too hot (maintain the temperature between 130 and 140 °C). Either use mechanical stirring or the metal spatula to be sure mixing continues and two layers do not form indicating separate polymerization of polylactide. Lightly place the cap on the vial and heat at 130 °C for 50 minutes. As the new polymer is formed, the solution will thicken. When 10 minutes remain, begin to warm 20 mL of ethyl acetate on a hot plate (setting 4). Record the reaction time and remove the vial from the heat. Allow to cool for NO MORE than 1-2 minutes since the polymer will begin to solidify to an opaque solid and will make the next step difficult. Slowly add the warmed ethyl acetate in small portions while stirring vigorously with a glass stir rod to dissolve the solid pieces of polymer. The solution will remain opaque during this process. Use the minimal amount of ethyl acetate necessary. Pour the dissolved polymer slowly into 40 mL of methanol, which has been cooled in an ice bath, while stirring. The polymer most likely will be a flocculent white powder material. If a solid forms, continue with workup A. If no precipitate forms, go to workup B.

Workup A for Solid Allow the polymer to settle. Either the solution can be decanted from the polymer or a quick gravity filtration can be applied to isolate the polymeric material before vacuum filtration. Note, that direct vacuum filtration of the poly(δ-decalactone) is not recommended without a filtration trap to avoid the odor from remaining monomer from entering the house vacuum line. Vacuum filter the triblock polymer on a Buchner funnel, breaking up any chunks with the thin metal spatula, and dry for 10 minutes. Continue below.*

Workup B if No Solid Formed If no precipitate formed, leave the polymer solution in an ice bath for 10 minutes undisturbed. Decant off the methanol and scrape the viscous polymer into the pre-weighed aluminum tin. Continue below.* *Weigh a small aluminum weigh boat. Transfer the white polymer material to the aluminum boat on a hot plate and heat (setting of 3.5-4.0 and increase if necessary) to remove excess solvent, and melt. Use your wooden or metal test tube holder to handle the aluminum tin (not your fingers). Samples with a large amount of L-lactide incorporation and high yield may be slow to anneal. In these cases, place an aluminum foil tent over the top of the aluminum boat to heat more uniformly and decrease the annealing time required. Record any change in appearance of the polymer during this process. If the polymer begins to evolve smoke at any time, remove it from the heat. When it appears that all or most of the solvent bubbles have been removed and a uniform polymer is formed, remove from the hotplate and cool to room temperature. Determine the mass of the polymer isolated and calculate the % yield. Prepare a 1H NMR sample by dissolving 20 mg of the triblock polymer in the appropriate 136 Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

amount of CDCl3. Alternatively or in addition, an IR spectrum may be requested by your instructor. Place the annealed polymer in a beaker in your drawer for further curing.


Mechanical Property Testing On the day that the polymers will be tested, attempt to remove the polymer from the aluminum tin using a spatula to loosen the sides and/or by peeling away the aluminum boat. If a disk did not form, see if the material can be rolled into a ball or gathered on a wooden stick. Each lab section should design a method appropriate for testing the polymers isolated. Be creative and consider the properties observed. An example of a flexibility test would be to clamp the polymer to a ring stand. Use a paper clip to hang a weight, such as the cork ring. Use a protractor to measure the degree of bending and record. Compare the physical and mechanical properties of all of the synthesized polymers within your lab section (e.g. odor, color, flexible, stiff, sticky, brittle, opaque, etc.) and carefully record all data for discussion in your worksheet or laboratory report. Was a difference observed between the triblock polymers prepared from poly(δ-decalactone) versus poly(δ-dodecalactone)? Was a trend observed for the different target poly(L-lactide) content in one or both series? Did the calculated average number molecular weights (more accurately molecular mass) and poly(L-lactide) composition from the 1H NMRs agree with the theoretical values expected?

Waste Disposal The polymer filtrate (containing mostly methanol and ethyl acetate) should be disposed of in the ORGANIC SOLVENT HAZARDOUS WASTE. The vial and all glassware that come in contact with the polymers should be rinsed immediately with acetone into the ORGANIC SOLVENT HAZARDOUS WASTE. The aluminum tins can be placed in the green trash cans. Please save your products until directions are received from your TA. Calculations of Theoretical Mn (Number Average Molar Mass) During day 1, time is spent in the laboratory while the reaction is stirring to teach the key concepts necessary for the students to calculate the theoretical Mn (number average molar mass) of both their homopolymer and triblock polymer. The Mn is described as the total molecular weight of all the polymer chains in a sample divided by the number of polymer chains. Additionally, it is assumed that all of the monomer used is divided evenly between the moles of initiator so that the number of repeat units, n, can be calculated as follows:


Therefore, for homopolymer A:


The Mn can then be calculated using the molecular mass of the initiator, 1,4-benzenedimethanol, plus the number of repeat units of monomer times the molecular mass of the monomer as follows:

This calculation is useful in helping students understand how polymer structures are represented using the brackets and what the values of the repeat units, n, mean outside the brackets. Since a diol initiator was used, an approximation is that each side will be n/2 repeat units long, or approximately 65 repeating monomers of δ-decalactone per side in the case of homopolymer A. The overall Mn for the homopolymer can also be calculated by adding the sum of the masses of the initiator plus the monomer and dividing by the moles of initiator used. Though simpler, this method is not as instructive as that above:

Similarly, the Mn for the triblock polymer can be calculated by assuming that the moles of L-lactide used in the synthesis are divided evenly per mole of initiator used for each polymer chain. Students would find that for polymer A comprised of polyDDL and 35 weight % poly(L-lactide), the total Mn would be estimated to be 34.8 kg/mol. An interesting exercise for students to perform is to compare the theoretical Mn’s of triblock polymers synthesized using 2 mg more of the initiator than suggested and 2 mg less and observe how the size of the polymer substantially decreases and increases, respectively. 1H

NMR Spectrum Interpretation

As noted in our previous publication (9), the homopolymer PDDL and PDDDL midblocks and PLLA end blocks have distinct chemical shifts in the 1H NMR spectrum as illustrated in Figure 6. Student spectra were obtained using a 300 MHz NMR instrument and a macro was applied that set the aromatic initiator protons integration to 4. The spectra were printed from 4.0 ppm to 7.5 ppm in order to exclude the large alkyl peaks present. Students were then able to use integration values of the methine peaks of the δ-lactones and the methine peak of the lactide block to determine the mass contribution of each (31). Though use of the end group for determination of molecular mass was introduced, it was explained that impurities present in this region gave unreliable integration values, therefore the aromatic protons would be used. Table 2 illustrates how the 1H NMR data can be used to determine an approximate molecular mass, Mn, for the triblock polymer as well as the weight percent of poly(L-lactide) in the sample for comparison to the theoretical values. 138 Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Table 2. Sample Student Data for a 300 MHz 1H NMR Spectrum of a Triblock Polymer B Integration from NMR

Relative # H per repeat unit, n

Integration per repeat unit

Relative mass per unit based on BDM

Wt. % of each unit in triblock

7.34 (BDM)

4

4

1

138

0.4

5.16 (PLLA block)

196

2

98.1

14,125

44.6

4.86 (PDDL block)

101

1

101

17,170

54.5

4.36 (end group)

2.5

2

1.25

144

0.45

Sum to get total Mn

31,433 g/mol 31.2 kg/mol

---

139


Chemical Shift, δ

Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 6. 1H NMR Chemical shifts of Poly(L-lactide)-block-poly(δdodecalactone)-block-poly(L-lactide) for Polymer D.

Results and Discussion Results Typical student yields of the final triblock polymer products were 55-75%, not taking into account the fact that the homopolymers’ maximum conversion is ~87%. Comparison of student 1H NMR calculation of % wt. L-lactide with the target polymer composition demonstrated that ~70% of the student samples were within ~5% of the target weight % L-lactide planned. Students with 35% or greater L-lactide incorporation generally obtained solid products that were clear to opaque, whereas samples with low L-lactide addition were varying degrees of sticky semi-solids. A direct correlation was observed between increasing weight % L-lactide incorporation and increasing solidity of the thin films after annealing. They also observed that generally polymers derived from the δ-dodecalactone monomer had less strength in stretch and flexibility testing than those synthesized with δ-decalactone as the midblock as illustrated in Figure 4. Data from the spring 2016 semester’s run of this chapter’s experiment at the U of MN with 30 sections and 450 students, revealed that over 70% of the students working in pairs obtained solid products – in several sections 100% of the products were solids – suitable for mechanical testing with the paper clip, cork ring, and protractor, as illustrated in Figure 4. Most sections also took the initiative to design other types of stretch and strength tests. Sections with a higher percent of semisolid materials developed methods to evaluate the adhesive properties of their products. For example several groups timed how long their polymers could hold an object of varying weights to a wall. All students seemed to enjoy the time and freedom allotted to investigate the properties of their synthesized polymeric products.



Student Feedback This directed-inquiry experiment has been successfully performed in the University of Minnesota’s introductory organic chemistry laboratory course six semesters (including two summer sessions), as well as in a second semester organic chemistry course at Augsburg College. The Augsburg trial involved 41 students from their CHM 352 Organic II Laboratory course during the spring semester of 2015. A survey questionnaire was sent to these students and 29 responded (71% response rate) with representative comments below and summarized feedback shown in Table 3. It is worth noting that students found the 1H NMR interpretation challenging with this first trial at Augsburg College, as was observed with U of MN students. However, repeated implementations at the U of MN have demonstrated that a semester of experience teaching the experiment was extremely helpful in preempting students’ problems in interpreting the NMR spectra. Representative student comments included: -

It was a great experiment!

-

Interesting lab experiment!

-

I thought the main ideas on polymers and sustainability were very applicable to many "real world" examples.

-

Because polymer chemistry is so prevalent in today’s society, more materials labs should be performed.

-

I thought the experiment was easy to follow and fun to learn about.

-

Even though my polymer personally did not turn out correctly it was interesting to see how we found the error and how (to) think backwards about how we could fix it.

-

The experiment was worth doing and I recommend that the other organic labs do it.

-

I thought this was an enjoyable lab because we made something that we can directly see the real world application.


Table 3. Student Response Data from Augsburg College Organic II Laboratory Course, Spring 2015 (N=29 with 100% response on all questions) Strongly Agree (5)

Agree (4)

Neutral (3)

Disagree (2)

Strongly Disagree (1)

Average

1) I enjoyed learning about how polymers are made in contrast to the small molecule chemistry typical of the organic chemistry lab.

45%

45%

10%

0%

0%

4.35

2) The 1H NMR spectrum interpretation was manageable and instructive.

24%

31%

21%

24%

0%

3.55

3) I enjoyed learning about and performing an experiment representative of new technologies in the area of sustainable polymers.

59%

35%

6%

0%

0%

4.53

4) This new experiment is suitable and would be a valuable addition to the organic chemistry lab curriculum.

59%

38%

3%

0%

0%

4.56

5) I felt that the polymer synthesis reaction was a good teaching application of the carbonyl chemistry taught in organic lecture.

38%

52%

10%

0%

0%

4.28


Statement

Tips and Adaptations Reagents and Materials It is important that the δ-lactones (1) and (2) be of sufficient purity for the experiment. The most common impurity is thought to be the ring-opened carboxylic acid/alcohol. Both monomers can be purchased from Sigma-Aldrich chemical company, however on one occasion, the δ-dodecalactone was yellow in color instead of the expected colorless liquid and student results were less satisfactory. We have had the most success with Alfa Aesar DDL and DDDL purchased from Fisher Scientific (32, 33). Alternatively, the lactone monomers can be purified by vacuum distillation before use. The L-lactide should also be of high purity and can be purchased from Frinton Laboratories in sufficient quality 142 Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

to be used without recrystallization (34). L-Lactide can be recrystallized from ethyl acetate or toluene or sublimed. The 1.0 M HCl/Et2O and diphenyl phosphate catalysts can be purchased from Sigma-Aldrich and used directly (35, 36). The HCl/Et2O should be stored in the refrigerator and sealed well, or more preferably, a fresh bottle should be purchased each semester. Aluminum boats, with dimensions of 4.4 x 1.3 cm which hold a volume of 20 mL, work best for the annealing and can be purchased from VWR: disposable Aluminum Weighing Dishes Catalog #: 25433-016.


Versatility of the Experiment This triblock polymer experiment could be used as an introduction to sustainable polymers with no accompanying instrumental analysis. Simple mechanical testing or degradability testing of the polymeric materials can serve to model how chemists are tuning the properties of polymers through varying the size and composition of the thermoplastic elastomeric materials. For those without easy access to high-field NMR, IR can be used to characterize the triblock polymers. The carbonyl C=O stretch found in all of the starting monomers (DDL, DDDL, L-lactide) are distinguishable and consistent both before and after polymerization (see Table 4).

Table 4. Infrared Carbonyl Stretches for Triblock Polymers of Poly(δ-decalactone) or Poly(δ-dodecalactone) and Poly(L-Lactide) aMonomer/Block

bVibrational

Polymer

Frequency of C=O stretch, cm-1

DDL/PDDL

1727-1730

DDDL/PDDDL

1730-1733

LL/PLLA

1754-1758

DDL/PDDL = δ-decalactone/poly(δ-decalactone), DDDL/PDDDL = δ-dodecalactone)/ poly(δ-dodecalactone), LL/PLLA = L-lactide/poly(L-lactide) b As determined from a PerkinElmer Spectrophotometer 100 with resolution ±0.5 cm-1 a

This experiment could also be used in an upper level organic chemistry or polymer laboratory course. Students could be asked to design their own target products and calculate the quantity of reagents needed for their synthesis. Analysis of the products could include size exclusion chromatography (SEC) to analyze the molecular weight distributions and dispersities (37) of the midblock and triblock samples. Representative data from an advanced undergraduate student is illustrated in Table 5. If a differential scanning calorimeter (DSC) is available, the glass transition temperatures and melting points of the polymer samples can be analyzed. Another interesting variation of this experiment is to prepare polymers with poly(D,L-Lactide) end blocks. These materials have different thermal properties and mechanical performance than triblocks with 143 Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

poly(L-Lactide) end blocks. Finally, comparison of 13C NMR spectra of the polymers would also be instructive since distinct carbonyl carbons are observable for each block.


Table 5. Comparison of Four ABA Triblock Polymers Comprised of Poly(δ-decalactone) or Poly(δ-dodecalactone) as the Midblock and 35% or 45% Poly(L-Lactide) Weight as the End block Midblocka

PDDL

PDDL

PDDDL

PDDDL

Target PLLAb (wt%)

35

45

35

45

Actual PLLAc (wt%)

31

44

32

43

Theoretical Mnd (kg mol-1)

31.8

37.5

37.5

44.2

NMR Mnc (kg mol-1)

31.0

30.8

35.6

37.4

% Yielde

50

61

53

56

Đe

1.39

1.3

1.37

1.4

PDDL = poly(δ-decalactone), PDDDL = poly(δ-dodecalactone) b Target Weight % PLLA based on procedure for specific polymer product c NMR determination of weight % PLLA and Mn using initiator protons as a 4 hydrogen internal standard and relative ratio of PLLA and homopolymer peaks d Theoretical Mn calculated assuming monomer conversion of 100% e %Yield based on ratio of mass of final polymer product to the mass of all inputs f Dispersity determined using SEC with RI detector, referenced to polystyrene standards a

Conclusion In summary, a series of experiments based on the synthesis and study of ABA triblock polymers prepared from all renewable resources using green chemistry principles has been developed and performed by over two thousand students. The experimental procedure is amenable to non-stringent conditions of teaching laboratories and the resulting thermoplastic elastomer materials are suitable for analysis by 1H NMR, as well as other instrumental analysis, and mechanical testing. Considering the overwhelming presence of plastics in society and the current challenges faced in developing sustainable alternatives, the experiment presents a topic at the forefront of new innovations in the field of chemistry and germane to students of all disciplines.

Acknowledgments This work was funded by the Center for Sustainable Polymers at the University of Minnesota, a National Science Foundation (NSF)-supported Center for Chemical Innovation (CHE-1413862) and the Margaret A. Cargill Scholarship Fund (Christa Blaquiere). The authors wish to thank the many students and TAs who participated in refining the experiment, the Organic chemistry stockroom for 144 Fahey and Maelia; Green Chemistry Experiments in Undergraduate Laboratories ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

their suggestions and support, and members of the Marc Hillmyer group for use and assistance with obtaining SEC data.

References 1.


2. 3.

4.

5. 6.

7.

8.

9.

10.

11.

12. 13.

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