{ "id": "epx3vjf0gm", "slug": "polyaddition", "title": "Polyaddition", "updated": "2026-10-21", "topic_path": [ { "id": "physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/topics/physical" }, { "id": "physical.chemistry", "label": "Chemistry", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.chemistry" }, { "id": "physical.chemistry.methods", "label": "Chemical principles and methods", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.chemistry.methods" }, { "id": "physical.chemistry.methods.chemical-synthesis", "label": "Chemical synthesis", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.chemistry.methods.chemical-synthesis" }, { "id": "physical.chemistry.methods.chemical-synthesis.polymer-synthesis", "label": "Polymer synthesis", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.chemistry.methods.chemical-synthesis.polymer-synthesis" } ], "geo": [ { "id": "geo.nongeo.t1800.physical.chemistry.methods.chemical-synthesis.polymer-synthesis", "label": "Non-geographic · 1800 to 1945: Polymer synthesis", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical.chemistry.methods.chemical-synthesis.polymer-synthesis", "path": [ { "id": "geo.nongeo", "label": "Non-geographic", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo" }, { "id": "geo.nongeo.t1800", "label": "Non-geographic · 1800 to 1945", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800" }, { "id": "geo.nongeo.t1800.physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical" }, { "id": "geo.nongeo.t1800.physical.chemistry", "label": "Chemistry", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical.chemistry" }, { "id": "geo.nongeo.t1800.physical.chemistry.methods", "label": "Chemical principles and methods", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical.chemistry.methods" }, { "id": "geo.nongeo.t1800.physical.chemistry.methods.chemical-synthesis", "label": "Chemical synthesis", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical.chemistry.methods.chemical-synthesis" }, { "id": "geo.nongeo.t1800.physical.chemistry.methods.chemical-synthesis.polymer-synthesis", "label": "Polymer synthesis", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical.chemistry.methods.chemical-synthesis.polymer-synthesis" } ] }, { "id": "geo.nongeo.t1946.physical.chemistry.methods.chemical-synthesis.polymer-synthesis", "label": "Non-geographic · 1946 to 2000: Polymer synthesis", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.chemistry.methods.chemical-synthesis.polymer-synthesis", "path": [ { "id": "geo.nongeo", "label": "Non-geographic", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo" }, { "id": "geo.nongeo.t1946", "label": "Non-geographic · 1946 to 2000", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946" }, { "id": "geo.nongeo.t1946.physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical" }, { "id": "geo.nongeo.t1946.physical.chemistry", "label": "Chemistry", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.chemistry" }, { "id": "geo.nongeo.t1946.physical.chemistry.methods", "label": "Chemical principles and methods", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.chemistry.methods" }, { "id": "geo.nongeo.t1946.physical.chemistry.methods.chemical-synthesis", "label": "Chemical synthesis", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.chemistry.methods.chemical-synthesis" }, { "id": "geo.nongeo.t1946.physical.chemistry.methods.chemical-synthesis.polymer-synthesis", "label": "Polymer synthesis", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.chemistry.methods.chemical-synthesis.polymer-synthesis" } ] } ], "excerpt": "Polyaddition is a step polymerization in which monomers with two or more functional groups add together, forming polymer with no low-molar-mass by-product; polyurethane is the best-known example.", "snippet": "Polyaddition is a step polymerization in which monomers with two or more functional groups add together, forming polymer with no low-molar-mass by-product; polyurethane is the best-known example.", "node": "physical.chemistry.methods.chemical-synthesis.polymer-synthesis", "markdown": "# Polyaddition\n\nPolyaddition is a step polymerization in which monomers carrying two or more functional groups add to each other across multiple bonds or by opening rings, forming polymer with no low-molar-mass by-product; polyurethane from a diol and a diisocyanate is the best-known example.[1](https://iupac.org/wp-content/uploads/2025/08/PAC-REC-2025-0490.R1_PR20250813.pdf) • [2](https://www.degruyter.com/document/doi/10.1515/pac-2021-0115/pdf) It differs from polycondensation, which releases small molecules such as water, and from chain polymerization, in which each macromolecule grows extremely fast from a reactive center while polyaddition molar mass rises hyperbolically with conversion.[1](https://iupac.org/wp-content/uploads/2025/08/PAC-REC-2025-0490.R1_PR20250813.pdf) • [2](https://www.degruyter.com/document/doi/10.1515/pac-2021-0115/pdf) IUPAC's 2025 recommendations make polyaddition the synonym for additive step polymerization and note that the historical terms \"addition polymerization\" and \"step-growth polymerization\" should be avoided because they have been defined inconsistently.[1](https://iupac.org/wp-content/uploads/2025/08/PAC-REC-2025-0490.R1_PR20250813.pdf)\n\n| Key fact | Detail |\n|---|---|\n| Definition | Step polymerization without low-molar-mass by-products; IUPAC synonym for additive step polymerization[1](https://iupac.org/wp-content/uploads/2025/08/PAC-REC-2025-0490.R1_PR20250813.pdf) |\n| Canonical reaction | n O=C=N–R–N=C=O + n HO–R′–OH → –[C(O)NH–R–NHC(O)O–R′–O]n– (polyurethane)[2](https://www.degruyter.com/document/doi/10.1515/pac-2021-0115/pdf) |\n| Molar-mass control | Carothers equation, \\( X_{n} = (1+r)/(1+r-2rp) \\), with stoichiometric ratio \\( r \\) and conversion \\( p \\); monofunctional chain stoppers also work[2](https://www.degruyter.com/document/doi/10.1515/pac-2021-0115/pdf) • [3](https://ptacts.uspto.gov/ptacts/public-informations/petitions/1501655/download-documents?artifactId=L5BOOA3ntvZnk9mM2-SNv1V0_7Lh-ZE8jeB26a9EKIHCo5Vo4l3oxe4) |\n| Typical catalysts | Organotin compounds and tertiary amines[4](https://pubs.acs.org/doi/10.1021/bk-2021-1380.ch001) |\n| Kinetics | Urethane formation activation energies of 17–54 kJ/mol; initially second-order rate law[5](https://real.mtak.hu/100997/1/polymers-11-01543-v2.pdf) • [6](https://www.degruyterbrill.com/document/doi/10.1515/revce-2025-0014/html) |\n| Introduced | Otto Bayer, I.G. Farbenindustrie, 1937[7](https://onlinelibrary.wiley.com/doi/10.1002/ange.19470590901) • [8](https://pubs.rsc.org/en/content/articlelanding/2022/py/d2py00086e) |\n| Main uses | Elastomers, rigid and elastic foams, adhesives, coatings[9](https://www.sciencedirect.com/science/article/pii/S0040603119306513) |\n\n## How it works\n\nGrowth occurs by reactions between monomer, oligomer, or polymer molecules of any length, each carrying functional groups of functionality at least 2.[1](https://iupac.org/wp-content/uploads/2025/08/PAC-REC-2025-0490.R1_PR20250813.pdf) In the urethane case, an alcohol adds nucleophilically across the N=C=O bond of an isocyanate; the atoms of both reactants end up in the polymer, so nothing is expelled.[10](https://openstax.org/books/organic-chemistry/pages/31-4-step-growth-polymers) The reaction is formally bimolecular, but theoretical calculations show the direct-addition barrier exceeds 100 kJ/mol and drops substantially when one or two additional alcohol molecules assist the transition state.[5](https://real.mtak.hu/100997/1/polymers-11-01543-v2.pdf)\n\n[Molar mass](https://www.edgepedia.org/molar-mass) follows the Carothers equation, \\( X_{n} = (1+r)/(1+r-2r \\cdot p) \\), where \\( r = N_{AA}/N_{BB} \\) is the stoichiometric ratio and \\( p \\) the extent of reaction; near-perfect stoichiometry and high conversion are both required for high \\( X_{n} \\).[2](https://www.degruyter.com/document/doi/10.1515/pac-2021-0115/pdf) A small amount of monofunctional monomer acts as a chain stopper when a lower molar mass is wanted.[3](https://ptacts.uspto.gov/ptacts/public-informations/petitions/1501655/download-documents?artifactId=L5BOOA3ntvZnk9mM2-SNv1V0_7Lh-ZE8jeB26a9EKIHCo5Vo4l3oxe4)\n\nOne classification subtlety deserves note. A 2015 simulation study concluded that \"catalysed reactions follow a chain growth mechanism while the uncatalysed reactions undergo a step growth mechanism\", via an active catalytic complex, so both mechanisms may operate simultaneously in polyurethane formation.[11](https://pubs.rsc.org/en/content/articlehtml/2015/ra/c5ra10725c) This is unresolved against the IUPAC step-polymerization classification; at equal conversion a chain-growth component produces fewer, higher-molecular-weight polymer molecules and higher viscosity.[11](https://pubs.rsc.org/en/content/articlehtml/2015/ra/c5ra10725c)\n\n## How it is done\n\nA practitioner making a polyurethane selects a polyol and a diisocyanate or polyisocyanate, each with functionality of at least 2; the reaction can be run at room temperature under mild conditions.[4](https://pubs.acs.org/doi/10.1021/bk-2021-1380.ch001) The diol is usually a low-molecular-weight polymer of about 1000 amu with hydroxyl end groups, and the diisocyanate is often toluene-2,4-diisocyanate.[10](https://openstax.org/books/organic-chemistry/pages/31-4-step-growth-polymers) Elastic polyurethanes use linear, high-molecular-weight, low-functionality polyols; rigid foams use low-molecular-weight polyols with aromatic groups and higher functionality for cross-linking.[4](https://pubs.acs.org/doi/10.1021/bk-2021-1380.ch001)\n\nStoichiometry is set from the NCO:OH ratio, and cure is tracked by gelation time and viscosity build-up; for a PMDI–castor oil system (a trifunctional polyol), these are quantified by isoconversional analysis of DSC data plus rheology.[12](https://onlinelibrary.wiley.com/doi/10.1002/macp.200800345) Organotin and tertiary amine compounds are the common catalysts, and catalyst choice affects both rate and [degree of polymerization](https://www.edgepedia.org/degree-of-polymerization), which matters in foam engineering.[4](https://pubs.acs.org/doi/10.1021/bk-2021-1380.ch001) • [11](https://pubs.rsc.org/en/content/articlehtml/2015/ra/c5ra10725c) The diisocyanate–diol reaction is initially second-order, with rate \\( R = k \\cdot [\\mathrm{A\\sim A}] \\cdot [\\mathrm{B\\sim B}] \\) for an uncatalyzed A~A/B~B system.[6](https://www.degruyterbrill.com/document/doi/10.1515/revce-2025-0014/html) • [13](https://ptgmedia.pearsoncmg.com/images/0130181684/samplechapter/0130181684_ch02.pdf) Experimental activation energies for aryl isocyanates with alcohols span 17–54 kJ/mol depending on solvent and reactant ratio.[5](https://real.mtak.hu/100997/1/polymers-11-01543-v2.pdf)\n\n## Origin\n\nThe step-growth lineage begins with L. H. Baekeland, whose Bakelite, the first polymer network from phenol–formaldehyde condensation, was reported in [Scientific American](https://www.edgepedia.org/scientific-american) in 1909.[14](https://doi.org/10.1038/scientificamerican11201909-322supp) The terms \"addition or A polymers\" and \"condensation or C polymers\" could not accommodate polyurethanes, which expel no by-product yet grow stepwise.[8](https://pubs.rsc.org/en/content/articlelanding/2022/py/d2py00086e) • [7](https://onlinelibrary.wiley.com/doi/10.1002/ange.19470590901) Because the process proceeds like a condensation polymerization but releases no low-molar-mass by-product, it is called a \"polyaddition\".[8](https://pubs.rsc.org/en/content/articlelanding/2022/py/d2py00086e) Detailed kinetic investigations of uncatalyzed and catalyzed urethane formation concluded the apparently bimolecular addition is catalyzed by both the alcohol reactant and the urethane product.[5](https://real.mtak.hu/100997/1/polymers-11-01543-v2.pdf)\n\n## Variants\n\n**Polyurethane** is the defining case, from a diisocyanate and a diol as above.[2](https://www.degruyter.com/document/doi/10.1515/pac-2021-0115/pdf) **Epoxy–amine addition** curing: amines are more nucleophilic than hydroxyls and attack the epoxy group to open the ring, yielding β-hydroxy amine linkages at room temperature, while urethane formation often needs higher temperatures and catalysts, especially with aliphatic diisocyanates.[20](https://doi.org/10.5282/edoc.24113) • [6](https://www.degruyterbrill.com/document/doi/10.1515/revce-2025-0014/html)\n\n**Thiol–ene and Michael-type polyaddition** covers the 1,4-addition of thiols, alcohols, or amines onto double bonds activated by electron-withdrawing groups; the [Michael addition](https://www.edgepedia.org/michael-addition) is the addition of sodiomalonate esters onto α,β-unsaturated esters.[15](https://pmc.ncbi.nlm.nih.gov/articles/PMC9609322/) Thia-Michael addition is more efficient, rapid, and selective than radical-mediated thiol–ene reaction and runs under neat conditions at low temperature; thiol–ene polymerization of a dithiol and an α,ω-diene is itself a classification curiosity, forming polymer in a step-growth pattern via a radical chain process.[15](https://pmc.ncbi.nlm.nih.gov/articles/PMC9609322/) • [8](https://pubs.rsc.org/en/content/articlelanding/2022/py/d2py00086e)\n\n**Polyhydroxyurethanes (NIPU route)** come from the aminolysis of cyclic carbonates with amines; each urethane bond formed also creates a hydroxyl group.[16](https://pubs.acs.org/apaccd/article/6/4/987/5232894/Glycerol-as-a-Sustainable-Source-for-Nonisocyanate)\n\n## Applications\n\nThe basic method of polyurethane synthesis, polyaddition between polyol and di- or polyisocyanate, yields elastomers, rigid or elastic foams, adhesives, and coatings.[9](https://www.sciencedirect.com/science/article/pii/S0040603119306513) Non-isocyanate polyurethane adhesives rely on the cyclic carbonate–amine polyaddition and β-hydroxy urethane chemistry, with epoxy hybridization, siloxane modification, and vitrimer-like networks used to improve curing efficiency and durability.[17](https://link.springer.com/article/10.1007/s42114-026-02044-7) Fully bio-based NIPUs with targeted crosslinking density have been made from carbonated soybean oil (carbonated in supercritical CO₂) and diamines, cured at 90 °C for 6 h at an amine/cyclic carbonate ratio of 1.1.[18](https://www.mdpi.com/2073-4360/15/6/1589)\n\n## Limitations and alternatives\n\n**Moisture and side reactions.** Isocyanates react with water to form amines and carbon dioxide; this drives foam synthesis but ruins elastomers, since the amines convert to urea moieties, giving hardened, unusable products.[19](https://pmc.ncbi.nlm.nih.gov/articles/PMC12114677/) Allophanate (urethane + isocyanate) and biuret (urea + isocyanate) formation cause branching and cross-linking; allophanate production is especially favored above 60 °C for both aromatic and aliphatic diisocyanates when catalysts are present, consuming isocyanate beyond the initial NCO:OH ratio.[6](https://www.degruyterbrill.com/document/doi/10.1515/revce-2025-0014/html) Stoichiometric imbalance limits molar mass, per the Carothers equation.[2](https://www.degruyter.com/document/doi/10.1515/pac-2021-0115/pdf)\n\n**Alternatives.** Four main routes to non-isocyanate polyurethanes exist: polycondensation, rearrangement, ring-opening polymerization, and polyaddition of cyclic carbonates with amines; the first three rely on phosgene derivatives, acylazides, or carboxamides and generate by-products such as HCl or alcohols.[18](https://www.mdpi.com/2073-4360/15/6/1589) The polyaddition route's two disadvantages are the low reactivity between cyclic carbonate carbonyls and amines, and limited room-temperature conversion, giving low-molecular-weight PHUs; five-membered cyclic carbonates need elevated temperatures and catalysts relative to the highly reactive isocyanate–polyol pair.[19](https://pmc.ncbi.nlm.nih.gov/articles/PMC12114677/) • [6](https://www.degruyterbrill.com/document/doi/10.1515/revce-2025-0014/html) PHUs do gain hydrolytic stability from intramolecular hydrogen bonds that block the carbonyl carbon, with chemical resistance 1.5–2 times greater than analogous materials without such bonds.[19](https://pmc.ncbi.nlm.nih.gov/articles/PMC12114677/)\n\n**Recent progress.** Ring-opening of the eight-membered cyclic carbonate CC8 by amines is kinetically more favorable owing to higher ring strain, enabling NIPUs with \\( M_{n} \\) of 47,000 g·mol⁻¹, and thioether-substituted tetrasubstituted cyclic carbonates undergo fast, complete, catalyst-free aminolysis at room temperature.[16](https://pubs.acs.org/apaccd/article/6/4/987/5232894/Glycerol-as-a-Sustainable-Source-for-Nonisocyanate) Slow curing kinetics and wet durability remain the key barriers for NIPU adhesives.[17](https://link.springer.com/article/10.1007/s42114-026-02044-7) Published comparisons do not give numeric cure times or exotherm temperatures for conventional polyaddition curing, nor quantitative rate constants for industrially relevant diisocyanate/polyol pairs beyond the [phenyl isocyanate](https://www.edgepedia.org/phenyl-isocyanate) model system.\n\n## References\n\n1. [Basic Classification and Definitions of Polymerization Reactions (IUPAC Recommendations 2025)](https://iupac.org/wp-content/uploads/2025/08/PAC-REC-2025-0490.R1_PR20250813.pdf)\n2. [A brief guide to polymerization terminology (IUPAC Technical Report)](https://www.degruyter.com/document/doi/10.1515/pac-2021-0115/pdf)\n3. [USPTO petition document citing Carothers' 1929 classification (with Odian, Principles of Polymerization, ch. 1 excerpt)](https://ptacts.uspto.gov/ptacts/public-informations/petitions/1501655/download-documents?artifactId=L5BOOA3ntvZnk9mM2-SNv1V0_7Lh-ZE8jeB26a9EKIHCo5Vo4l3oxe4)\n4. [Introduction to Polyurethane Chemistry (ACS Symposium Series, 2021)](https://pubs.acs.org/doi/10.1021/bk-2021-1380.ch001)\n5. [Urethane Formation with an Excess of Isocyanate or Alcohol: Experimental and Ab Initio Study (Polymers 2019, 11, 1543; repository copy)](https://real.mtak.hu/100997/1/polymers-11-01543-v2.pdf)\n6. [State-of-the-art and recent progress in the synthesis of polyurethanes and non-isocyanate polyurethanes (Rev. Chem. Eng., 2025)](https://www.degruyterbrill.com/document/doi/10.1515/revce-2025-0014/html)\n7. [Das Di-Isocyanat-Polyadditionsverfahren (Polyurethane)](https://onlinelibrary.wiley.com/doi/10.1002/ange.19470590901)\n8. [Reconsidering terms for mechanisms of polymer growth: the \"step-growth\" and \"chain-growth\" dilemma (Polym. Chem., 2022)](https://pubs.rsc.org/en/content/articlelanding/2022/py/d2py00086e)\n9. [Kinetics of cross-linking processes of fast-curing polyurethane system (2019)](https://www.sciencedirect.com/science/article/pii/S0040603119306513)\n10. [31.4 Step-Growth Polymers (OpenStax Organic Chemistry)](https://openstax.org/books/organic-chemistry/pages/31-4-step-growth-polymers)\n11. [Chain growth polymerization mechanism in polyurethane-forming reactions (RSC Advances, 2015)](https://pubs.rsc.org/en/content/articlehtml/2015/ra/c5ra10725c)\n12. [Reaction Kinetics of Polyurethane Formation Using a Commercial Oligomeric Diisocyanate Resin Studied by Calorimetric and Rheological Methods (Macromol. Chem. Phys., 2008)](https://onlinelibrary.wiley.com/doi/10.1002/macp.200800345)\n13. [Polymer Synthesis, Chapter 2: Step-Growth Polymerization (Pearson textbook sample)](https://ptgmedia.pearsoncmg.com/images/0130181684/samplechapter/0130181684_ch02.pdf)\n14. [L. H. Baekeland (1909). Bakelite, a New Composition of matter. Scientific American.](https://doi.org/10.1038/scientificamerican11201909-322supp)\n15. [Thia-Michael Reaction: The Route to Promising Covalent Adaptable Networks (Molecules, 2022)](https://pmc.ncbi.nlm.nih.gov/articles/PMC9609322/)\n16. [Glycerol as a Sustainable Source for Nonisocyanate Polyurethanes (ACS Polymers Au, 2025)](https://pubs.acs.org/apaccd/article/6/4/987/5232894/Glycerol-as-a-Sustainable-Source-for-Nonisocyanate)\n17. [Non-isocyanate polyurethane (NIPU) adhesives: Chemistry to interface engineering and applications (Adv. Compos. Hybrid Mater., 2026)](https://link.springer.com/article/10.1007/s42114-026-02044-7)\n18. [Tailor-Made Bio-Based Non-Isocyanate Polyurethanes (NIPUs) (Polymers, 2023)](https://www.mdpi.com/2073-4360/15/6/1589)\n19. [Toward Sustainable Polyurethane Alternatives: A Review of the Synthesis, Applications, and Lifecycle of Non-Isocyanate Polyurethanes (NIPUs)](https://pmc.ncbi.nlm.nih.gov/articles/PMC12114677/)\n20. [Rlmydk69wk1 (exa.ai)](https://doi.org/10.5282/edoc.24113)\n\n---\n*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Chemical principles and methods › Chemical synthesis › Polymer synthesis*\n\n*Initially written Sep 29, 2026 · Reviewed: Sep 30, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Sep 30, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n", "same_as": [], "url": "https://www.edgepedia.org/polyaddition", "markdown_url": "https://www.edgepedia.org/polyaddition.md", "license": { "name": "Edgepedia Community License 1.0", "url": "https://www.edgepedia.org/edgepedia/license", "summary": "Free with credit, commercial use included. AI training is open to everyone. For other uses, organizations over USD 100M in revenue or 100M monthly users license separately.", "spdx": "LicenseRef-Edgepedia-Community-1.0" }, "credit": "\"Polyaddition\", Edgepedia (EdgeChat), https://www.edgepedia.org/polyaddition. Edgepedia Community License 1.0.", "credit_md": "\"[Polyaddition](https://www.edgepedia.org/polyaddition)\", Edgepedia (EdgeChat), [https://www.edgepedia.org/polyaddition](https://www.edgepedia.org/polyaddition). [Edgepedia Community License 1.0](https://www.edgepedia.org/edgepedia/license).", "credit_html": "\"Polyaddition\", Edgepedia (EdgeChat), https://www.edgepedia.org/polyaddition. Edgepedia Community License 1.0.", "speakable": "Polyaddition is a step polymerization in which monomers with two or more functional groups add together, forming polymer with no low-molar-mass by-product; polyurethane is the best-known example." }