the plastic problem

The topic of polymers and plastics and resulting pollution is enormous. This lesson draws systems boundaries by focusing on student exploration of feedstocks and end-of-life of polymeric materials and products. These stages of plastics production are easily connected to common organic chemistry and biochemistry functional groups and molecular interactions. Additional subtopics to explore are microplastics, green processes for syntheses, or the economics of consumer products designed for circularity or not. The thermodynamics of polymer synthesis and properties are potential topics for physical chemistry and materials curriculum. All these angles of presenting the plastics problem bring into focus the magnitude of the socio-environmental and -economic stress plastics are having on our planet and society and are important for integration into chemistry curriculum.

Contextualize

The topic of "plastics" creates many opportunities for applying systems thinking in chemistry to design more sustainable materials. Most plastics are currently derived from petroleum, making them inexpensive to the consumer and efficient to synthesize from hydrocarbon starting materials. However, fossil fuels are a depleting resource, not evenly distributed around the world thus invoking political ramifications. There are major research efforts in deriving polymeric materials from renewable plant materials with increasing economic viability and end-of-life characteristics, but their high oxygen content can require more processing. Analysis of the molecular structure of monomers used and processes followed in the production of polymers will help students learn important structure-property relationships in chemistry and enable them to understand the current status of the production of plastics and sustainable options.

Focus

This infographic summarizes the main systems and components in interaction in the life cycle of petroleum-derived plastics analyzed as part of the lesson:

Define

Central Ideas

In a little over 100 years, scientists have mastered the design and synthesis of polymeric materials (plastics) to perform almost any function.
Polymeric materials can be soft and flexible, hard and durable, absorb water or repel water, recyclable or resistant to reprocessing and environmental elements. These functions are dependent on the molecular mass of the polymers, intermolecular forces between polymeric chains, and stability of the functional groups.
The class of polymers (linear, branched, crosslinked, or network) contributes to other chemical and mechanical properties such as melting temperature, recyclability, swelling properties and more.
Design for circularity (in contrast to single-use plastics) in the life cycle of plastics is key to addressing sustainability challenges

Core Practices

Generate explanations and arguments about the physical and chemical properties of polymeric materials based on the analysis of their molecular structure.
Engage in collaborative discussions and problem-solving to relate structure to function in polymeric materials and learn systems thinking strategies currently used to design new sustainable materials.

Systems Thinking Skills

System Composition: Given a polymer representation, identify the repeating monomer unit or units that make up the polymeric structure, the functional group(s) present, and overall molecular mass. Identify potential petro- or bio-based feedstocks relevant to synthesis of the polymer.
System Structure: Examine the structural features (linearity, branching, cross-linking, network) of the polymer/copolymer, and potential unreactive or reactive bonds present that contribute to properties of the polymer that determine its ability/resistance to being recycled, reprocessed, or degraded.
System Behavior: Associate a polymer’s chemical and physical properties (thermal classification, melting and glass transition temperatures, elasticity, etc..) with its use in consumer products. Explore polymers categorized with recycle codes, as compostable/degradable, and biodegradeable.
System Effects: Explore the societal, economic, and environmental impacts of plastics based on feedstocks used and intended and untended end of life fate. Identify four pillars of a polymer lifecycle, connected subsystems, and strategies for designing a sustainable polymer.

Socio-Environmental Competencies

Recognize that plastics are ubiquitous in society and responsible for raising the standard of living and longevity of human life. However, development of plastics for function without a systems thinking approach leads to vast unintended consequences resulting in negative impacts on the health of the planet.
Evaluate both the benefits of plastic products and their potential negative environmental impact based on responsible and negligent human actions.
Evaluate the impact of daily personal decisions on what products to purchase and use on the socio-environmental problems caused by plastic materials.
Understand the concept of a circular economy for plastics and importance for reducing waste and preventing environmental harm.

Design

The following presentation introduces the topic of plastics as macromolecular chemistry that is pervasive in every aspect of human life. Class discussions about the socio-scientific benefits and environmental problems set the stage for exploring polymeric structures and properties that correlate to functional properties and consumer products. This 3-class-period (50-minute) lesson is designed for a second semester organic chemistry lecture or laboratory course or above where functional group recognition and reactivity is familiar. Adaption to a general chemistry level is possible by using general polymer representations. A fourth-class session is proposed for rotating class poster presentations (small classes) or virtual poster presentations (larger).

Plastics Problem.pptx

The lesson reviews the basic concepts of how to represent polymer structure, repeating units, and common terms used to characterize polymers including crystallinity, melting temperature and glass transition temperature.
The relationship of thermoplastics/thermoplastic elastomers to recyclability and thermoset cross-linking to non-recyclability is taught.
The lifecycle of a polymer is presented through a graphic with four pillars of “feedstock”, “process”, “intended use” and “end-of-use”. The graphic is used to understand a systems approach to design for sustainability and the importance of circularity.
Recognizing the limit of time in most courses for polymers, a focus is placed on thinking in systems for feedstocks and end-of-use.

The example lesson includes a set of interspersed activities (labeled "Let's Think") that students are expected to complete in small collaborative groups and then share their ideas in whole class discussions. These activities ask students to share and explore/consider: 1) their understanding of both the benefits and negative impact of plastics in our lives; 2) the planetary boundaries and plastics as a “novel entities” introduced on the planet through humans; 3) why plastics are now a problem (developed for functionality and performance with little consideration of interconnecting subsystems); 4) connecting the UN SDGs to plastics ; 5) what are subsystems connected to the life cycle of a polymer; and 5) their understanding of polymer functional groups to degradation by hydrolysis as an example.
The SOCKit tool is introduced to be used with their project assignment.
The project assignment details are presented and reviewed in class.

Map Out

During the "map out" phases of a lesson, students are introduced to the socioenvironmental problem under analysis to identify the systems in interaction. This phase should allow them to develop an overall view of the nature and complexity of the problem or phenomenon to be analyzed. As illustrated in this example lesson on plastics, students are asked to recognize the prevalence of plastics in their lives and contemplate both their benefits and harm to humans and the environment. Data is shared to provide context for the magnitude of the plastic’s problem and an introduction to plastics as “novel entities” in the Planetary Boundaries. The concept of polymer design for function only, without regard for unintended consequences on connected systems, provides an introduction to systems thinking.

Zoom In

In the example lesson on plastics, the chemistry of polymers is reviewed with a focus on their large molecular masses, representations, types of structures, and classifications based on thermal stability. These concepts will be used to later connect to recyclability, degradability, or resistance to degradation. Instructors can tailor the depth of discussion according to their class objectives. Student understanding is accessed with a “let’s think” activity that models the final connection activity.

A second “zoom in” phase involves relating structure to end-of-life degradability considerations for polymers. Simple functional group hydrolysis is used as an example, however the many methods and complexity of polymer degradation, including recycling, are presented as well as distinguishing between biodegradability and compostability.

Zoom Out

Once students have an appreciation for the chemistry and synthesis of polymers, they can consider the life cycle of a polymer from raw materials to end-of-life. A representative graphic based on petroleum-derived polymers (estimated as the source for 99% of today’s plastics) is used to analyze each step. A focus is then placed on the important consideration of sustainability of feedstocks used for monomers synthesis. A comparison of petroleum-based feedstocks and renewable feedstocks are explored and connections to subsystems considered.

A second “zoom out” activity considers the broader picture of what defines a “sustainable polymer” based on every stage from raw material extraction to end-of-life. This includes feedstocks, processing, intended use, and end-of-use fate. Students will quickly recognize the importance of human actions at several of these stages and are asked to consider subsystem interactions. A broader view incorporates the relevance to achieving the UN Sustainable Development Goals and the importance of design for circularity.

Connect

In the "Connect" phases of the lesson, students engage in activities that allow them to explore the effects of interactions between relevant subsystems, as well as apply their knowledge in making decisions and suggesting individual or collective actions directed at addressing the societal and/or environmental problem under consideration:

Student Group Assignment: Groups of 2-4 students select a single-use plastic product or packaging to explore. The order of selection - product first or plastic type - can be decided by the students. Use of recycle codes 1-7 as well as new materials on the market made from biomass (mycelium, starches, algae etc..) provide ideas for students. Examples are fossil-fuel based plastic water bottle (Recycle code 1) or partially bio-based or fully bio-based bottles, dissolvable packing peanuts, valorized waste products such as whey, or recycled consumer products such as carpet. Ideally, each group explores a different material so that other students see and learn a breadth of polymer and plastics structures through the poster presentations.

Student Group Poster Presentation: To communicate the results of their investigation, students are asked to design a poster using the template provided to summarize the important aspects of the plastic polymer their group investigated. They are expected to:

Present the elements that highlight the polymer’s important characteristic and social and economic benefits.
Include the SOCKit mapping diagram for end-of-life/use.
Be prepared to give a 3-minute presentation on their polymer during a poster session involving all groups.

Evaluate

The "Let's Apply" activity described above (Connect section) can be use as a summative assessment for the lesson. The different “Let’s Think” activities completed in class are also useful for evaluating student engagement as well as student skill level in understanding polymer structure and applying systems thinking skills. Two sample class engagement activities illustrated below include a) an initial use of the SOCKit tool to identify connected subsystems to renewable feedstocks and b) observing the affect of stereochemistry and packing of polymer chains to the melting temperature of poly(lactic acid) isomers.

Reflect

The quality and accuracy of the student group’s reports and poster presentations can be used to narrow or expand the description of polymer products and materials useful for student learning and engagement.