The Chemistry of Microplastics: How Polymers Fragment, Weather, and Persist

What a microplastic actually is, chemically

A microplastic is any solid, water-insoluble synthetic polymer particle between roughly 1 µm and 5 mm; below 1 µm we call them nanoplastics. The 5 mm upper bound is an operational convention (a net mesh size), not a chemical one — the underlying material is identical to the macroplastic it broke off from.

Two origins are distinguished:

• Primary microplastics — manufactured small: nurdle pellets, cosmetic microbeads, and the polyester/acrylic microfibers shed by textiles (a single wash can release hundreds of thousands of fibers).
• Secondary microplastics — the dominant source by mass, produced when a bottle, bag, or fishing rope is weathered and fragmented in the environment.

Chemically, the commodity plastics fall into two camps. All-carbon-backbone plastics (the polyolefins PE and PP, plus styrenic PS) have a backbone joined only by C–C and C–H bonds — there is no polar, hydrolyzable linkage for water or enzymes to grab, which is exactly why they persist. Heteroatom polymers (PET, nylon, PVC) contain ester, amide, or C–Cl bonds that offer a chemical foothold: PET can slowly hydrolyze, PVC can eliminate HCl. This single structural difference — pure C-backbone vs. a cleavable linkage — governs almost everything about how a given plastic weathers.

The core reaction: UV-driven radical photo-oxidation

Sunlight reaching Earth's surface carries UV-A/UV-B photons of ~300–400 nm, worth about 300–400 kJ/mol — comparable to a C–C bond and more than enough to break the weaker bonds at defect sites (a residual C=O chromophore, a catalyst remnant, a hydroperoxide). Pure polyethylene shouldn't absorb visible or near-UV light at all; it degrades because manufacturing leaves trace chromophores that seed the chain. Once initiated, the classic auto-oxidation radical chain runs, with R = the polymer chain:

• Initiation: RH + hν → R• + H• (abstraction of a backbone hydrogen creates a carbon radical)
• Propagation (O₂ addition): R• + O₂ → ROO• (a peroxyl radical, diffusion-limited)
• Propagation (H-abstraction): ROO• + RH → ROOH + R• (regenerates R•, so one photon damages many bonds)
• Chain branching: ROOH + hν → RO• + •OH (the hydroperoxide splits, multiplying radicals)
• Termination: R• + R• → R–R, or ROO• + ROO• → non-radical products + O₂

The alkoxyl radical RO• breaks the chain directly by β-scission, and the carbonyl (ketone) groups it leaves behind then absorb UV and cleave the chain further via the Norrish reactions: Norrish type I cleaves the C–C bond α to a carbonyl homolytically; Norrish type II abstracts a γ-hydrogen through a six-membered transition state and then snaps the backbone, leaving a ketone plus a terminal vinyl (C=C) group. Each scission converts one long chain into two shorter ones — molecular weight falls, crystallinity rises, and the plastic goes brittle.

Fragmentation: from bottle to microbead to nanoplastic

Photo-oxidation is a surface process — O₂ and photons only penetrate the outermost tens of micrometers — so weathering builds a stiff, oxidized, cracked skin over a still-tough core. Mechanical stress from waves, sand abrasion, and freeze–thaw then does the cutting. The embrittled surface fails along the cracks and spalls off millions of micrometer-scale flakes, exposing fresh polymer that oxidizes in turn. It is a self-perpetuating comminution engine.

You can watch it happen with infrared spectroscopy. The single most useful metric is the carbonyl index (CI): the ratio of the FTIR absorbance of the new C=O stretch (~1715 cm⁻¹) to a stable reference C–H band (~1465 cm⁻¹). Fresh PE has CI ≈ 0; heavily weathered beach PE reaches CI of 0.3–1 or more. Because each scission event doubles the particle count while roughly halving the mass per particle, the number distribution explodes as size shrinks — there are vastly more nanoplastics than microplastics, even though they are nearly invisible and mostly uncounted by today's 300-µm nets. This is why total microplastic inventories are almost certainly underestimated: our instruments miss the smallest, most numerous, and most bioavailable fraction.

Additives, sorbed pollutants, and the plastisphere

A microplastic is rarely pure polymer. Plastics ship with additives that are not chemically bonded to the chain and therefore leach:

• Plasticizers — phthalates like DEHP (up to tens of % by mass in flexible PVC) that soften the material and diffuse out over time.
• Stabilizers — UV absorbers (benzophenones, hindered amine light stabilizers, HALS) that intercept radicals to slow the very oxidation described above; ironically their depletion is what lets weathering accelerate.
• Flame retardants and pigments — brominated compounds, and metal-bearing colorants (Pb, Cd, Cr) in legacy plastic.

Beyond what leaches out, microplastics act as hydrophobic sponges. The apolar polyolefin surface partitions persistent organic pollutants (POPs) — PCBs, DDT, PAHs — out of seawater, concentrating them by factors of 10⁵–10⁶ relative to the surrounding water (a partition coefficient issue governed by high log Kₒw). A single weathered pellet can carry ppm-level PCBs into a fish gut, where warm, surfactant-rich, lower-pH conditions promote desorption. Finally, the surface hosts the plastisphere: a distinct microbial biofilm — including hydrocarbon degraders and, worryingly, some pathogens and antibiotic-resistance carriers — that colonizes plastic differently from natural substrates.

Why heteroatom polymers behave differently

PVC and PET break the polyolefin rulebook because their backbones contain reactive linkages. PVC (–CH₂–CHCl–) undergoes dehydrochlorination: heat or UV triggers loss of HCl in a zipper-like elimination that leaves a conjugated polyene and turns the plastic yellow-to-brown:

• –CH₂–CHCl–CH₂–CHCl– → –CH=CH–CH=CH– + 2 HCl

The released HCl is autocatalytic and locally acidic, and the growing polyene chromophore then absorbs light and feeds straight into radical oxidation. PET — poly(ethylene terephthalate) — carries a hydrolyzable ester linkage. Its slow abiotic breakdown is ester hydrolysis:

• –CO–O–CH₂– + H₂O → –COOH + HO–CH₂– (yielding terephthalic acid and ethylene glycol fragments)

This is genuinely reversible chemistry (esterification ⇌ hydrolysis), which is why PET is the one commodity plastic with a real enzymatic answer: engineered PETases and the thermophilic LCC cutinase hydrolyze that same ester bond, depolymerizing PET back to its monomers for true chemical recycling. No comparable enzyme exists for the inert C–C backbone of PE or PP — the great chemical asymmetry of the plastic problem.

Scale, health, and what chemistry can do about it

The numbers are large and getting larger. Global plastic production is roughly 400 million tonnes per year, of which on the order of 10+ million tonnes reach the ocean annually. Microplastics have been measured in Arctic sea ice, in the deepest ocean trenches (>10 km), in rain and remote mountain snow, in tap water and bottled water, and in human blood, placenta, and lung tissue. A 2024 study reported roughly a quarter-teaspoon of plastic (grams) per human brain on average, rising in dementia cases — an area of active, contested research.

The chemical hazards are threefold: physical (particle irritation and inflammation, worst for the ~sub-micron fraction that can cross membranes), additive leaching (endocrine-disrupting phthalates and bisphenols), and vectoring (delivering sorbed POPs and heavy metals). The interventions are also chemical:

• Design for degradation — real biodegradables like PLA (polylactic acid) and PHA use ester backbones that hydrolyze; "oxo-degradable" additives that merely accelerate fragmentation into microplastics are being banned as counterproductive.
• Enzymatic and chemical depolymerization — PETase/LCC and glycolysis/methanolysis routes recover monomers instead of downcycling.
• Capture — filtration of wastewater and washing-machine effluent to stop microfibers at the source.

The uncomfortable thermodynamic truth is that a C–C backbone is stable, and dispersal is entropically favored — once fragmented and spread across the biosphere, microplastics are effectively impossible to recover. Prevention, not cleanup, is the only chemistry that scales.

How the five commodity polymers differ in structure, weathering chemistry, and behavior in the ocean

Polymer	Repeat unit / linkage	Density (g/cm³)	Dominant degradation route	Ocean fate
Polyethylene (PE)	–CH₂–CH₂–, pure C–C	0.91–0.97	UV photo-oxidation (Norrish I/II)	Floats; most abundant
Polypropylene (PP)	–CH₂–CH(CH₃)–, tertiary C–H	0.90	Photo-oxidation (fast, tertiary H)	Floats; embrittles fast
PET	aromatic ester –CO–O–	1.38	Photolysis + ester hydrolysis	Sinks; slow hydrolysis
PVC	–CH₂–CHCl–	1.38–1.45	Dehydrochlorination → HCl + polyenes	Sinks; releases Cl⁻
Polystyrene (PS)	–CH₂–CH(C₆H₅)–	1.05	Photo-oxidation, yellowing	Sinks slowly; leaches styrene