Polymerization Methods — Cross-Cutting Comparison
This note compares every major polymerization mechanism described across the Chemistry library on the axes that actually decide which one a chemist or process engineer picks: chain-growth vs step-growth kinetics, achievable dispersity (Đ), tacticity control, monomer scope, scale-up tractability, and the dominant initiator/catalyst family. Each row maps to a real polymer product line so the table doubles as a polymer-architecture index. Read the dimension tables first, then the decision tree for picking-by-objective.
See also
- polymer-chemistry
- organic-chemistry-foundations
- materials-chemistry
- green-chemistry-and-process-intensification
- medicinal-and-photo-chemistry
- catalyst-instrumentation-and-monomers
- reagent-and-reaction-catalog
1. The eight families
All industrial polymerization fits in eight mechanistic boxes. Everything more exotic (RAFT, NMP, ATRP, Grubbs ROMP, thiol-ene click, photoiniferter, ionic liquids, supercritical-CO2 dispersion) is a refinement of one of these eight — usually a cleverer way to control initiation, propagation, or termination.
chain-growth step-growth
| |
radical ionic coordination polycondensation
| | /insertion |
FRP cationic Ziegler-Natta polyester
ATRP anionic metallocene nylon (PA)
RAFT living-A post-metallocene polyurethane
NMP ROMP (Grubbs) epoxy / phenolic
ADMET polycarbonate
silicone
↓
ring-opening enzymatic photopolymerization
| | |
lactones (PLA, PCL) lipase-CALB acrylate UV-cure
lactams (nylon-6) cutinase thiol-ene click
epoxides (PEO) hydrolase CuAAC (azide-alkyne)
cyclic siloxanes (PDMS) SuFEx
The Sharpless-Meldal-Bertozzi 2022 Nobel Prize in Chemistry was awarded for click chemistry (CuAAC) and bioorthogonal chemistry; the same year, the Yves Chauvin / Robert Grubbs / Richard Schrock 2005 Nobel for olefin metathesis unlocked ROMP for industrial use. Karl Ziegler and Giulio Natta won in 1963 for the catalysts that enabled isotactic polypropylene.
2. Mechanism, monomer scope, dispersity — the deciding axes
| Family | Mechanism | Monomer types | Đ (PDI) typical | Tacticity control | Key initiator / catalyst | Industrial products |
|---|---|---|---|---|---|---|
| Free radical (FRP) | chain, radical, irreversible termination | vinyl, acrylate, styrene, vinyl chloride | 2.0–10 | none (atactic) | BPO, AIBN, persulfate, redox pair | LDPE, PS, PMMA, PVAc, PVC, NBR |
| ATRP (Matyjaszewski 1995) | reversible-deactivation radical via Cu–halide | acrylates, styrene, MA | 1.05–1.30 | none (atactic) | CuBr/PMDETA, Cu(0) (ARGET, SARA) | precision drug-delivery PEG-acrylate blocks |
| RAFT (Rizzardo/Moad/Thang 1998) | reversible-deactivation radical via chain-transfer agent | almost all vinyl monomers | 1.05–1.30 | none (atactic) | dithioesters, trithiocarbonates, xanthates | block copolymers, dispersants, gradient copos |
| NMP (Hawker, Georges) | reversible-deactivation via nitroxide capping | styrenes, some acrylates | 1.10–1.40 | none (atactic) | TEMPO, SG1, BlocBuilder | bulk-scale block copo coatings (Arkema) |
| Cationic | chain, carbocation, fast irreversible | isobutylene, vinyl ethers, styrene-α-methyl | 2–5 (uncontrolled), 1.05–1.20 (living) | partial | BF3, AlCl3, TiCl4/H2O (Kennedy living) | butyl rubber (IIR), polyisobutylene (PIB) |
| Anionic / living anionic (Szwarc 1956) | chain, carbanion, no termination | styrene, butadiene, isoprene, EO, lactams | 1.02–1.10 | high (stereo via Li counterion) | sec-BuLi, n-BuLi, Na-naphthalide | SBR, SBS/SIS Kraton, monodisperse PS standards |
| Polycondensation | step, condensation (–H2O, –HCl, –ROH) | difunctional acid/alcohol, diamine, etc | 1.5–2.5 (Carothers/Flory most-probable: Đ → 2) | n/a | acid/base, transesterification cats (Sb, Ti, Ge, Sn) | PET, PBT, nylon-6,6, polyurethane, polyester resins, polycarbonate (interfacial / melt) |
| Polyaddition step | step, no leaving group (urethane, epoxy) | diol+diisocyanate, diamine+epoxide | 2–4 | n/a | tin/Sn catalysts (DBTDL), tertiary amines | PU foams (Bayer 1937), epoxy resins, RTV silicone |
| Ring-opening (ROP) | chain or step from cyclic monomer | lactones, lactams, epoxides, cyclic siloxanes, NCAs | 1.05–1.50 (living) up to 2 (cationic) | high (Sn(Oct)2, salen-Al) | Sn(Oct)2, salen-Al/Zn, organocatalysts (DBU, TBD, NHC) | PLA, PCL, PGA, nylon-6, PEG, PDMS, peptides |
| Ziegler-Natta (heterogeneous) | coordination insertion, multi-site | α-olefins | 4–20 (multi-site) | high (TiCl3/MgCl2 isotactic PP) | TiCl4/MgCl2 + AlEt3 | LLDPE, HDPE, PP (commodity) |
| Metallocene (single-site) | coordination insertion, single-site | α-olefins, polar-fct via post-met | 2.0–2.5 | very high (C2-symmetric isotactic, Cs syndio) | Cp2ZrCl2/MAO, ansa-bridged systems | mLLDPE (Exxon Exact), mPE (Dow Affinity), iPP (LyondellBasell) |
| Post-metallocene (Brookhart, Bercaw, Mitsui) | coordination, polar-monomer-tolerant | ethylene, polar comonomers | 2.0–3.0 | high | bis(imino) Pd/Ni (Brookhart), FI-cat (Mitsui), Pt-diimine | branched PE, EVA via Pd, OBC (Dow INFUSE) |
| ROMP (Grubbs, Nobel 2005) | chain, metathesis, ring-opening of cyclic olefin | norbornenes, cyclooctene, DCPD | 1.05–1.20 (living Grubbs G2/G3) | n/a | Grubbs G1/G2/G3 Ru, Schrock Mo/W | poly-DCPD (Telene/Pentam), Vestenamer (Evonik), pNB optical films |
| ADMET (acyclic diene metathesis, Wagener) | step, metathesis of α,ω-diene | α,ω-dienes | 1.8–2.2 | n/a | Grubbs Ru, Mo Schrock | linear precision PE w/ defects at chosen spacings |
| Enzymatic | chain or step via enzyme | hydroxy acids, lactones, polyols/diacids | 1.2–2.0 | partial | lipase CALB (Novozyme 435), cutinase | PCL, PHA-mimics, polyesters in biomedical (limited scale) |
| Photopolymerization (acrylate UV) | chain radical, photoinitiated | acrylate / methacrylate / vinyl ether / thiol-ene | 2–5 (network, no Đ; uncrosslinked: 2) | none | benzophenone, TPO, Irgacure 184/819, BAPO | UV-cure coatings, dental composites, SLA/DLP resins (Formlabs), nail-gel |
| Thiol-ene click (Sharpless–Meldal–Bertozzi 2022) | step, radical chain transfer via thiyl | thiol + ene (alkene, norbornene, vinyl ether) | 1.1–2.0 | n/a | photoinitiator (TPO) or thermal AIBN | dental sealants (3M), nail gels, microfluidic seals (Carbon DLS resins) |
The single highest-impact axis is dispersity Đ. FRP at Đ = 2–10 gives every commodity plastic in the world. Living/controlled radical (ATRP, RAFT, NMP) drops Đ to ~1.1 and makes block copolymers possible — that is the whole reason RDRP exists. Anionic at Đ ≈ 1.05 is still the gold standard for monodisperse standards in GPC/SEC calibration. Step-growth converges to Đ = 2 (Flory’s most-probable distribution) at full conversion and cannot do better without subtractive fractionation. Ziegler-Natta heterogeneous catalysts give Đ = 4–20 because there are multiple active site types on the catalyst surface, each with its own propagation rate; single-site metallocenes collapse Đ to ~2.0 by definition (one site → Schulz-Flory).
3. Tacticity, branching, architecture
Most chain-growth families have tacticity options; step-growth does not (no stereocenter in the backbone unless the monomer has one).
| Family | Isotactic | Syndiotactic | Atactic | Branched | Star/comb | Block | Gradient |
|---|---|---|---|---|---|---|---|
| FRP | ✗ | ✗ | ✓ (default) | ✓ (long-chain via H-abstraction) | ✗ | ✗ | ✗ |
| ATRP/RAFT/NMP | ✗ | ✗ | ✓ | ✓ | ✓ (multifunctional initiator) | ✓ (sequential monomer addition) | ✓ (continuous monomer feed) |
| Anionic | ✓ (Li in nonpolar) | partial | ✓ (in polar solvent) | ✓ | ✓ | ✓ (canonical Kraton SBS) | ✓ |
| Ziegler-Natta | ✓ (iPP, Natta 1955) | ✓ (sPP w/ Cs catalyst) | ✓ | ✓ (LDPE-like via copolymer) | ✗ | partial (via reactor train) | ✗ |
| Metallocene | ✓ (C2-symmetric) | ✓ (Cs-symmetric) | ✓ (C1 / oscillating) | ✓ (constrained-geometry Dow INSITE) | ✗ | ✓ (chain-shuttling Dow INFUSE) | ✗ |
| Post-metallocene | ✓ | ✓ | ✓ | ✓ | ✗ | ✓ (Dow INFUSE) | ✗ |
| ROMP (Grubbs) | n/a | n/a | n/a | ✓ (Grubbs G3) | ✓ (multifunctional CTA) | ✓ (sequential monomer) | ✓ |
| Polycondensation | (cf monomer) | — | — | ✓ (hyperbranched via A2+B3) | ✓ | partial (via interchange) | ✗ |
| ROP | ✓ (PLA, salen-Al) | ✓ (rac-LA w/ Aida-Al) | ✓ | ✓ (multifunctional initiator) | ✓ | ✓ | ✓ |
The Dow INFUSE olefin block copolymers (OBC) deserve their own mention — chain-shuttling catalysis uses two catalysts and a chain-shuttling agent (diethyl zinc) to make multiblock PE/octene with sharp blocks, commercialized 2007. Until then, blocks in polyolefins required an anionic-style living polymerization that olefins simply don’t permit.
4. Scale, cost, real production tonnage
| Family | Annual global tonnage (2024) | Capex intensity | Reactor type | Solvent | Polymer cost |
|---|---|---|---|---|---|
| Ziegler-Natta / metallocene | ~160 Mt (PE + PP) | very high ($1-2B for world-scale unit) | slurry, gas-phase fluidized-bed (Unipol, Spheripol), solution (Dow, Nova) | hexane / iC4 / none | $1-2/kg |
| Polycondensation (PET) | ~80 Mt | high ($300-500M) | melt polycondensation | none (melt) | $1.5/kg |
| Polycondensation (nylon) | ~9 Mt | high | melt, autoclave | none (melt) | $3-5/kg |
| Polycondensation (PU) | ~22 Mt | medium | RIM, slabstock, spray | none (reactive) | $2-4/kg |
| FRP (PS, PVAc, acrylics) | ~30 Mt PS, 25 Mt PVC | medium | bulk, suspension, emulsion | water (emulsion), monomer (bulk), org (sol) | $1.5-2/kg |
| Anionic | ~5 Mt SBR/SBS | medium | solution, cyclohexane | cyclohexane | $3-5/kg |
| ATRP/RAFT/NMP | < 0.1 Mt | low (specialty) | batch | various | $50-1000/kg (specialty) |
| ROP (PLA, PCL, PEO) | ~0.5 Mt PLA, 0.1 Mt PCL | medium | melt, solution | none (melt) | $2-5/kg PLA, $10-30/kg PCL |
| ROMP (DCPD, NB) | ~0.1 Mt (Telene + Pentam) | low | RIM | none | $5-20/kg |
| Enzymatic | < 0.001 Mt | very low (specialty) | batch | aq or org | $100-1000/kg |
| Photopolymerization | ~3 Mt (UV-cure coatings + nails + AM resins) | very low (formulator) | static-mix + UV cure | reactive monomer | $10-200/kg formulated |
| Thiol-ene | < 0.05 Mt | very low | static-mix + UV cure | reactive | $30-500/kg |
For perspective: a single LyondellBasell Spheripol PP line in the Gulf Coast produces ~600 kt/year. The world’s entire RAFT polymer output, by contrast, is well under 5 kt/year. Industrial polymerization is dominated by Ziegler-Natta and polycondensation; controlled-radical is a precision specialty.
5. Reactor mode — bulk, solution, emulsion, suspension, precipitation, dispersion
Orthogonal to the mechanism — every chain-growth method can run in multiple reactor modes, each with trade-offs.
| Mode | Continuous phase | Particle size | Conversion-limit driver | Examples |
|---|---|---|---|---|
| Bulk | monomer | n/a (continuous) | viscosity, hotspot, gel effect | bulk PS, bulk PMMA, bulk LDPE (tubular) |
| Solution | inert solvent | n/a | solvent recovery cost | solution SBR (cyclohexane), Dow solution PE |
| Emulsion | water + surfactant | 0.05–0.5 µm | nucleation locus + chain transfer | latex paint, NBR, SBR (E-SBR), PVC paste |
| Suspension | water + suspending agent | 50–500 µm | droplet shear vs coalescence | PVC bottle resin, PS expandable beads (EPS), PMMA cast |
| Precipitation | poor solvent | porous solids | precipitation onset | gas-phase PE (Unipol; ethylene as solvent), PVDF in scCO2 |
| Dispersion (incl. scCO2) | non-solvent + stabilizer | 1–10 µm | stabilizer adsorption | scCO2 fluoropolymers (DuPont Teflon-AF), pharma microspheres |
| Interfacial | two immiscible liquids | film | mass transfer across interface | polycarbonate (BPA + phosgene/CH2Cl2 + NaOH aq) |
| Reactive extrusion (REX) | melt | bulk | residence time, screw config | nylon, PETG modification, peroxide-initiated PP rheology mod |
Emulsion is the safety / heat-management champion (water as continuous phase absorbs ΔH). Bulk is the highest space-time-yield but the worst for heat (PMMA bulk cast sheets need slow cure to avoid runaway). Supercritical CO2 dispersion eliminates VOCs and is the only practical solvent for fluoropolymer chains that swell every conventional solvent.
6. Scale-up tractability
| Family | Lab → pilot → world-scale | Bottleneck | Notable failures |
|---|---|---|---|
| Ziegler-Natta / metallocene | well-trodden; thousands of plants | catalyst poisoning by H2O/O2/CO ppm-level | INEOS Köln 2008 (loss of cooling); Pemex Pajaritos 2016 (VCM not Ziegler but exemplifies the safety frame) |
| Polycondensation | mature | dust + finishing (chip/pellet handling), Sb/Sn residue | DowDuPont nylon-6,6 ADN shortage 2018 |
| FRP (bulk + emulsion + suspension) | mature; well-understood | gel effect runaway (Trommsdorff–Norrish) | T2 Lab 2007 (Jacksonville FL) — sodium-aided MCMT runaway, killed 4 |
| Anionic | mature for SBR/SBS; specialty otherwise | absolute exclusion of H2O/O2/CO2 | rare incidents — failures are usually capacity scale-up, not safety |
| ATRP/RAFT/NMP | tough; only specialty scale | copper removal (ATRP), CTA odor + color (RAFT), nitroxide cost (NMP) | none material |
| ROP (PLA) | mature (NatureWorks Blair NE 150 kt) | water sensitivity (Sn(Oct)2 deactivated), monomer purity | recurring water-ingress issues, lost batches |
| ROMP (DCPD) | mature for poly-DCPD via RIM | catalyst cost (Grubbs G2 was $10000/kg in 2010s) | none material |
| Enzymatic | not scaled beyond demo | enzyme cost + thermal stability + slow rate (TON limit) | many academic demos, no commodity scale |
| Photopolymerization | scaled but only in thin layers / 3D voxels | UV penetration depth ~100 µm — limits thickness | not a “plant-scale” mechanism by nature |
| Thiol-ene | scaled in coatings / AM | thiol odor (alkyl thiols smell vile) | none |
The two pathological behaviors that have killed people in industry are the Trommsdorff–Norrish gel effect (auto-acceleration in bulk FRP when viscosity rises and termination becomes diffusion-limited — the polymerization rate runs away, generating heat faster than the jacket can remove) and the monomer flash on loss of cooling (the 1989 Phillips Pasadena disaster — ethylene fed into a polymerization reactor with stuck-open valve; the cloud ignited, killed 23). Both are taught in PSM (process safety management) courses and informed the OSHA 1992 Process Safety Management standard, the EPA RMP (40 CFR 68), and Seveso II/III in the EU.
7. Stereochemistry — the catalyst spectrum
Stereoregular polymerization is what makes polypropylene useful. Atactic PP is a tacky, soft, low-MW mess; isotactic PP (Tm 165°C) is a crystalline engineering plastic. The catalyst maps:
- TiCl3 (heterogeneous), Ziegler 1953, Natta 1955 — isotactic PP. Multi-site, broad Đ.
- TiCl4/MgCl2/donor (4th gen Z-N) — ~95% isotactic, single product line for all commodity iPP.
- C2-symmetric ansa-metallocene (Brintzinger, Ewen, Spaleck) — isotactic. Single-site, Đ ~2.0, very high stereo control.
- Cs-symmetric ansa-metallocene (Ewen 1988) — syndiotactic PP, Tm 137°C, crystalline.
- C1-symmetric (oscillating) — atactic-isotactic stereoblock (Coates–Waymouth 1995, nice academic curiosity).
- Salen-Al / salen-Cr / salen-Zn (ROP) — rac-lactide → isotactic PLA via enantiomorphic site control. Coates, Feijen, Spassky.
8. Modern catalyst innovations (2020–2026)
- Photoiniferter (visible-light) ATRP/RAFT — Hawker, Boyer 2018 onward. Avoids transition metals; ppm-level catalyst with blue/green LED. Now used commercially for high-purity biomedical polymers (Polymer Factory).
- Aqueous ATRP (Matyjaszewski 2018+) — ARGET/SARA with parts-per-million copper, viable in industrial water-based formulations.
- Living cationic polymerization at room temperature — Kennedy → Aoshima/Sawamoto/Kamigaito. Polyvinyl ethers at controlled MW.
- Group transfer polymerization (GTP) — Webster/DuPont 1983, mostly historical, but resurfaced 2020+ for high-throughput MA-derivative libraries.
- Stereoselective ROMP — Schrock W/Mo Z-selective catalysts (2011+), Grubbs Z-selective (2013+). Make all-cis poly(norbornene).
- Chain-shuttling olefin block copolymers (OBC) — Dow INFUSE/INTUNE, 2007 introduction, now ~100 kt/year specialty. The single largest mechanism innovation in olefin polymerization since metallocenes.
- Light-driven step-growth (SuFEx) — Sharpless 2014, click 2.0. Sulfur(VI)-fluoride exchange chemistry. Quietly going through scale-up at multiple specialty firms.
- Catalytic chain-transfer polymerization (CCTP) — Co(II) catalysts make low-MW macromonomers in FRP, useful for graft architectures.
9. End-of-life and recycling implications
Polymerization mechanism informs depolymerizability:
| Family | Depolymerizable? | Mechanism | Note |
|---|---|---|---|
| Step (PET, nylon) | yes | hydrolysis, methanolysis, glycolysis | Loop Industries, IBM VolCat, Eastman ICI for PET |
| ROP (PLA, PCL) | yes | hydrolysis (industrial compost) | NatureWorks |
| ROMP (poly-DCPD) | partial (back-biting under heat with Ru cat) | retro-metathesis | academic |
| FRP commodities (PE, PP, PS) | no | radical depolymerization off-route except PS → styrene at 350°C+ | mechanical / chemical (pyrolysis) recycling |
| Ziegler-Natta PE | no | pyrolysis only | Plastic Energy, Loop |
| Anionic SBR/SBS | no | pyrolysis | tire recycling |
| Photopolymer networks | no | crosslinked thermoset | landfill / pyrolysis |
| Polyurethane | partial | glycolysis (BASF Loopamid), hydrolysis | BASF Schwarzheide demo plant 2024 |
| Epoxy | hard | vitrimers (Leibler 2011) reversible at high T | new and growing |
Vitrimers (Ludwik Leibler, ESPCI 2011) sit between thermoplastic and thermoset — covalent network with dynamic exchange bonds (transesterification, imine, disulfide). Mallinda’s epoxy vitrimer is shipping for aerospace + wind-blade applications as of 2025.
10. Side-by-side — when to pick each
| If your priority is… | Pick this mechanism |
|---|---|
| Cheapest polymer per kg, any properties | Ziegler-Natta PE or polycondensation PET |
| Isotactic crystalline PP | Ziegler-Natta 4th gen or C2-metallocene |
| Block copolymer (well-defined) | living anionic > RAFT > ATRP |
| Block copolymer (commodity scale) | chain-shuttling olefin (Dow INFUSE) |
| Star or comb polymer | ATRP/RAFT/NMP with multifunctional initiator |
| Functional-group tolerance + radical chemistry | RAFT > ATRP > NMP |
| Bioresorbable (medical) | ROP of lactide / lactone / NCAs |
| UV-cure coating | acrylate photopolymerization |
| Dental composite or adhesive | thiol-ene click + methacrylate |
| All-cis polymer | Schrock Z-selective metathesis |
| Aqueous emulsion latex | FRP emulsion (BPO/persulfate redox) |
| RIM thermoset part (auto bumper) | poly-DCPD ROMP RIM |
| Recyclable polyester | melt polycondensation PET (existing infra) |
| Recyclable polycarbonate | Bayer CO2-based PC (Covestro 2016+) |
| Bio-based polyamide | ROP of lactam (nylon-11 from castor, Arkema) |
| Network with dynamic bonds | vitrimer (epoxy + transesterification) |
| Living cationic polyvinyl ether | Aoshima HI/I2-based system |
11. Decision tree
What polymer architecture do you need?
├─ Random copolymer, broad Đ, cheap, commodity scale
│ → Ziegler-Natta (LLDPE), polycondensation (PET), FRP emulsion (latex)
├─ Random copolymer, narrow Đ, specialty
│ → RAFT > ATRP > NMP (radical-tolerant)
│ → living anionic (high purity, dry box)
├─ Block copolymer
│ ├─ commodity scale + olefins → chain-shuttling (Dow OBC)
│ ├─ specialty scale + styrenics → anionic (Kraton SBS)
│ ├─ acrylates → RAFT or ATRP
│ └─ polyester/polylactam → sequential ROP (PLA-b-PCL)
├─ Stereoregular (iso/syndio)
│ ├─ α-olefin → Ziegler-Natta or metallocene
│ ├─ polylactide → salen-Al ROP
│ └─ polyvinyl ether → living cationic with stereodirecting cat
├─ Crosslinked network
│ ├─ thermoset rigid → epoxy/phenolic step
│ ├─ elastomer → PU step
│ ├─ optical/dental → thiol-ene or acrylate UV
│ └─ recyclable network → vitrimer (epoxy + transesterification)
├─ Star / comb / hyperbranched
│ ├─ star → multifunctional initiator ATRP/RAFT or anionic
│ ├─ comb → grafting-from ATRP
│ └─ hyperbranched → A2+B3 step
├─ Bioresorbable medical implant
│ └─ ROP (PLA, PCL, PGA, PLGA), Sn(Oct)2 or organocat
└─ Recyclable / depolymerizable
├─ step (PET, nylon) → existing
└─ vitrimer / dynamic covalent → new (Leibler, Du Prez)
Adjacent
- Tier-3 indices — catalyst-instrumentation-and-monomers, reagent-and-reaction-catalog cover the specific catalyst systems and monomer feedstocks.
- Engineering-side polymer notes — materials-polymers, polymers-taxonomy cover finished-resin selection, additives, and processing.
- Process-engineering view — chemical-process-fundamentals for reactor design; pharma-process-engineering for GMP cross-link with ROP for drug-eluting polymers.
- Green chemistry — green-chemistry-and-process-intensification for scCO2, ionic liquid, enzymatic, and circular routes.
- NMR — nmr-spectroscopy-deep for tacticity quantification (¹³C triad/pentad analysis) and end-group analysis.
When to pick what
The fastest narrowing is: commodity (>1 Mt/yr) → mature step or insertion; specialty (<10 kt/yr) → RDRP or living ionic; biomedical → ROP; photo/AM → acrylate UV or thiol-ene; recyclable network → vitrimer. Within those, the catalyst/initiator system is decided by the monomer’s functional-group compatibility — radical chemistry tolerates almost everything except thiols at scale (chain transfer); anionic chemistry tolerates almost nothing protic; coordination chemistry tolerates almost no polar groups except in post-metallocene Brookhart Pd systems.