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Química de Polímeros · Polymerization mechanisms

Polimerização por abertura de anel e controlada

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O que você vai aprender

  • Decide from ring strain, entropy and temperature whether a cyclic monomer can polymerize, and calculate a ceiling temperature or an equilibrium monomer concentration.
  • Draw anionic and cationic ring-opening polymerization of epoxides and tetrahydrofuran, and predict the regiochemistry and chain-transfer reactions of propylene oxide.
  • Draw the ring opening of lactones, lactide, cyclic carbonates and N-carboxyanhydrides by nucleophiles and acids, and identify which bond of the ring breaks.
  • Explain the coordination–insertion mechanism of tin(II) octanoate and the side reactions (transesterification, epimerization) that broaden or scramble polyester chains.
  • Use [M]₀/[I]₀, conversion and end-group analysis to calculate the molar mass of a living polymer, and plan block copolymers.
  • Explain how ATRP, RAFT, ROMP and Ziegler–Natta or metallocene catalysts control chain growth, chain-end structure and tacticity.

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Why a ring opens: strain against entropy

In ring-opening polymerization (ROP) an active chain end opens a cyclic monomer and adds it to the chain. The polymer usually has the same composition as the monomer, so no small molecule is released (N-carboxyanhydrides, which lose CO₂, are the exception). Turning many free monomers into one chain costs translational entropy (ΔSp < 0), so polymerization happens only when ring opening releases enough enthalpy (ΔHp < 0). That enthalpy comes from ring strain. Three- and four-membered rings (ethylene oxide, oxetane, β-lactones) are strongly strained and polymerize readily. Cyclohexane, a strain-free six-membered carbocycle, cannot be polymerized, and the nearly strain-free five-membered γ-butyrolactone does not give high polymer under ordinary conditions. A planar ester or carbonate group built into a ring changes this: ε-caprolactone (drawn) and the six-membered δ-valerolactone, lactide and trimethylene carbonate have enough strain, from bond-angle distortion, eclipsing and transannular interactions, to polymerize. Because ΔSp is negative, ΔGp = ΔHp − TΔSp becomes zero at the ceiling temperature Tc = ΔHp/(ΔSp° + R ln[M]). Above Tc the chain unzips back to monomer, and at any temperature a living ROP stops at an equilibrium monomer concentration given by ln[M]eq = ΔHp/RT − ΔSp°/R. Weakly strained monomers such as tetrahydrofuran (bulk Tc about 80 °C) and lactide therefore never reach full conversion.

Cyclic ethers: anionic and cationic ring opening

Alkoxides open epoxides by SN2 attack at a ring carbon. Each opening creates a new alkoxide at the chain end, which opens the next epoxide, so ethylene oxide gives poly(ethylene glycol) with a living alkoxide end. The counterion matters: potassium and caesium alkoxides propagate, while lithium alkoxides are so tightly aggregated that they add only one ethylene oxide unit. Propylene oxide (drawn) is attacked at the less hindered CH₂, giving a secondary alkoxide and a head-to-tail chain. The alkoxide can also remove a proton from the methyl group of propylene oxide; this E2-like ring opening gives an allyl alkoxide that starts a new chain. This chain transfer to monomer limits the molar mass of base-made poly(propylene oxide) to a few thousand g/mol and leaves allyl ether end groups. Tetrahydrofuran is too weakly strained for anionic attack, but it polymerizes cationically. A strong alkylating agent such as methyl triflate alkylates the ring oxygen, and the next THF molecule attacks a CH₂ next to the oxonium oxygen, opening that ring and forming a new cyclic oxonium ion at the chain end. The counterion must be very weakly nucleophilic (triflate, SbF₆⁻); a chloride would close the chain end as a covalent chloride.

Lactones, lactide and cyclic carbonates

Alkoxides add to the carbonyl carbon of a lactone, and the tetrahedral intermediate expels the ring oxygen: acyl–oxygen cleavage. The chain grows from an alkoxide end and carries the initiator as an ester end group. L-Lactide (drawn), the cyclic dimer of lactic acid, has two stereocentres, (S,S); D-lactide is (R,R) and meso-lactide is (R,S). Ring opening does not touch the stereocentres, so L-lactide gives isotactic poly(L-lactide). Two side reactions spoil control. Transesterification, attack of a chain-end alkoxide on an ester inside another chain (or its own chain), shuffles chain lengths and pushes the dispersity toward 2. Strong bases also remove the acidic C–H next to the carbonyl, and reprotonation of the enolate on the other face epimerizes the monomer and the chain. β-Lactones behave differently with weak nucleophiles: a carboxylate attacks the ring CH carbon (SN2, inversion) and breaks the alkyl–oxygen bond. Six-membered trimethylene carbonate polymerizes without losing CO₂, while the nearly strain-free five-membered ethylene carbonate does not polymerize under mild conditions. Industrially, poly(lactide) is made with tin(II) 2-ethylhexanoate, Sn(Oct)₂, and an alcohol. The alcohol converts Sn(Oct)₂ into a tin alkoxide; lactide coordinates to tin through a carbonyl oxygen, the tin alkoxide adds to that carbonyl carbon (insertion), and acyl–oxygen cleavage leaves a new tin alkoxide at the chain end. Strong acids such as methanesulfonic acid work the other way round: they protonate the monomer, and a neutral alcohol chain end attacks it (activated-monomer mechanism).

N-Carboxyanhydrides: polypeptides by ring opening

α-Amino acid N-carboxyanhydrides (NCAs, drawn for L-alanine) polymerize to polypeptides. A primary amine attacks the C5 carbonyl next to the α-carbon, the ring opens at that acyl–oxygen bond, and the carbamic acid that forms loses CO₂. The new chain end is again a primary amine, so the cycle repeats: the normal amine mechanism, which gives Mn ≈ [NCA]₀/[amine]₀ × conversion × residue mass. Bases that cannot add and form an amide end, such as tertiary amines (no N–H) or alkoxides, instead remove the ring N–H proton. The NCA anion then attacks another monomer (the activated-monomer mechanism), which is fast but gives poor control of molar mass. Loss of CO₂ makes each step irreversible, and water must be excluded because it initiates chains too.

Living control: molar mass, end groups and blocks

When initiation is fast and there is no termination or transfer, every initiator starts one chain and all chains grow together. Then Xn = ([M]₀/[I]₀) × conversion, Mn = Xn × M(monomer) + M(end groups), Mn rises linearly with conversion, and the dispersity stays low (about 1.05–1.2). End groups give Mn directly: in poly(ε-caprolactone) started from benzyl alcohol, the benzyl CH₂ (δ 5.11 ppm, 2H per chain) is compared with the CH₂O signals of the caprolactone units: δ 4.06 ppm for every in-chain unit and δ 3.65 ppm for the CH₂OH of the last unit, 2H per unit in both. Living ends can start a second monomer, so block copolymers are made in sequence; a hydroxyl-terminated poly(ethylene glycol) is a macroinitiator for lactide. Radical polymerization can be made controlled by keeping the radical concentration tiny. In ATRP a Cu(I) complex reversibly removes a halogen atom from a dormant alkyl halide chain end, giving a radical and Cu(II)–X, and the equilibrium lies far on the dormant side. In RAFT a thiocarbonylthio compound exchanges the growing radical between chains (degenerative transfer); a conventional initiator still supplies radicals, and the number of chains is set by the RAFT agent.

Metal-catalysed chain growth: ROMP and Ziegler–Natta

In ring-opening metathesis polymerization (ROMP) a metal alkylidene, such as a ruthenium Grubbs catalyst or a molybdenum Schrock catalyst, adds across the C=C bond of a strained cycloalkene to give a metallacyclobutane, which breaks the other way to open the ring. The new alkylidene sits at the chain end, so ROMP is living when initiation is fast. Each repeat unit keeps one C=C bond in the backbone. Release of ring strain drives the reaction: norbornene (drawn, strain about 114 kJ/mol) polymerizes readily, cis-cyclooctene (about 31 kJ/mol) polymerizes, and cyclohexene (about 10 kJ/mol) does not give polymer. Ruthenium catalysts tolerate water, alcohols and acids; molybdenum catalysts are more active but air- and moisture-sensitive. Ziegler–Natta catalysts (TiCl₄ or TiCl₃ with an aluminium alkyl) and metallocenes activated by methylaluminoxane polymerize propene by repeated coordination and migratory insertion into the metal–carbon bond. The symmetry of the catalyst site sets the tacticity: C₂-symmetric ansa-metallocenes and heterogeneous Ziegler–Natta catalysts give isotactic polypropylene (crystalline, Tm about 165 °C), Cs-symmetric catalysts give syndiotactic polymer, and achiral Cp₂ZrCl₂ gives atactic, amorphous polymer.

What to carry forward

A ring opens when strain enthalpy beats the entropy lost on polymerization, and weakly strained monomers stop at an equilibrium monomer concentration. Find the nucleophilic or electrophilic chain end, decide which ring bond breaks (C–O of an epoxide, acyl–O of a lactone, the alkyl C–O at the β-carbon of a β-lactone with carboxylates), and remember that living chains give Mn = ([M]₀/[I]₀) × conversion × M + end groups, until transesterification, epimerization or transfer intervene.

Problemas de prática (40)

Praticar é grátis com uma conta.

Desenhe o mecanismo 12

Lactones, lactide and catalysts

  • Acetate opens (S)-β-butyrolactone by alkyl–oxygen cleavageDraw the SN2 opening of β-butyrolactone by a carboxylate at the ring carbon bonded to oxygen, show inversion at that stereocentre, and show the carboxylate chain end opening the next monomer.
  • tert-Butoxide opens ε-caprolactone by acyl–oxygen cleavageDraw the addition of an alkoxide to the carbonyl of ε-caprolactone and the collapse of the tetrahedral intermediate that opens the seven-membered ring at the acyl–oxygen bond.
  • Methoxide opens L-lactide with retention at both stereocentresDraw the ring opening of L-lactide by an alkoxide through a tetrahedral intermediate and acyl–oxygen cleavage, and show that both stereocentres keep their S configuration.
  • Alkoxide-induced epimerization of L-lactide to meso-lactideDraw deprotonation of L-lactide at a stereocentre to give an ester enolate, and reprotonation on the opposite face that gives meso-lactide.
  • Chain scrambling by intermolecular transesterification in poly(ε-caprolactone)Draw the attack of a polyester chain-end alkoxide on an ester linkage inside another chain, and explain why this exchange broadens the molar-mass distribution without changing Mn.
  • Benzyl alkoxide opens trimethylene carbonateDraw the addition of an alkoxide to the carbonyl of the six-membered cyclic carbonate and the acyl–oxygen cleavage that gives a carbonate-linked chain with a primary alkoxide end.
  • Methanesulfonic acid-catalysed ring opening of δ-valerolactoneDraw the activated-monomer mechanism of acid-catalysed lactone ROP: protonation of the lactone, attack by a neutral alcohol, proton transfers, and acyl–oxygen cleavage that leaves a neutral hydroxyl chain end and regenerates the acid.

Anionic and cationic ring opening

  • Methoxide starts a living poly(ethylene glycol) chainDraw initiation and propagation of the anionic ring-opening polymerization of ethylene oxide by methoxide, and explain why the alkoxide chain end keeps opening epoxides.
  • n-Butylamine opens L-alanine N-carboxyanhydrideDraw the normal amine mechanism of NCA polymerization: amine attack at the C5 carbonyl, proton relay, acyl–oxygen ring opening to a carbamate, and loss of CO₂ that regenerates an amine chain end.
  • Head-to-tail anionic polymerization of propylene oxideDraw the anionic ring-opening polymerization of propylene oxide, showing attack at the less hindered ring carbon and the secondary alkoxide chain end that gives a head-to-tail chain.
  • Chain transfer to propylene oxide gives an allyl alkoxideDraw the proton transfer from the methyl group of propylene oxide to an alkoxide chain end that opens the ring to an allyl alkoxide, and the start of a new chain by that allyl alkoxide.
  • Methyl triflate starts the cationic polymerization of THFDraw initiation of the cationic ring-opening polymerization of tetrahydrofuran by methyl triflate and the propagation step in which THF opens the cyclic oxonium chain end.

Preveja o produto 4

ROMP and coordination polymerization

  • Metallocene symmetry and polypropylene tacticityPredict polypropylene tacticity from the symmetry of a metallocene catalyst.
  • Ring-opening cross metathesis of norbornene with ethylenePredict the product of ring-opening cross metathesis of norbornene with excess ethylene by locating the cleaved C=C bond in the bicyclic skeleton.

Anionic and cationic ring opening

  • Two-unit poly(propylene oxide)Predict the connectivity of a short anionic poly(propylene oxide) chain from the regiochemistry of each ring opening.

Lactones, lactide and catalysts

  • Order the coordination–insertion stepsPut the events of the tin(II) octanoate coordination–insertion mechanism for lactide polymerization in order.

Classificar 4

Living control and end groups

  • Recognize a living polymerizationClassify a polymerization as living from how Mn and dispersity change with conversion and on adding more monomer.
  • What RAFT does and does not doClassify statements about RAFT polymerization by the role of the thiocarbonylthio agent and the radical initiator.

Lactones, lactide and catalysts

  • Identify meso-lactideClassify the stereoisomers of lactide and identify the meso form from its configurations.

ROMP and coordination polymerization

  • Crystalline versus amorphous polypropyleneClassify polypropylene samples as isotactic or atactic from their crystallinity and the face selectivity of the catalyst.

Comparar 2

ROMP and coordination polymerization

  • Rank cycloalkenes by ROMP driving forceRank cycloalkenes by the ring-strain driving force for ring-opening metathesis polymerization.

Ring strain and polymerizability

  • Same polyester, two routesContrast the ring-opening and polycondensation routes to poly(ε-caprolactone) in by-products, mechanism type and molar-mass control.

Calcular 4

Ring strain and polymerizability

  • Ceiling temperature of lactideCalculate the ceiling temperature of a weakly strained cyclic monomer from ΔHp and ΔSp° at the standard-state concentration.
  • Equilibrium monomer left in a lactide polymerizationCalculate the equilibrium monomer concentration of a living ring-opening polymerization from ΔHp, ΔSp° and temperature.

Living control and end groups

  • Lactic-acid units per chainCount lactic-acid repeat units per chain in a living lactide polymerization, recognizing that each lactide supplies two units.
  • Mn of a living poly(ε-caprolactone)Calculate the number-average molar mass of a living ring-opening polymer from the monomer-to-initiator ratio, conversion and end group.

Encontre o erro 2

Anionic and cationic ring opening

  • Primary versus tertiary amine in NCA polymerizationDiagnose loss of molar-mass control in an NCA polymerization from the type of amine used to start it.

Lactones, lactide and catalysts

  • A living PLA that broadens with timeDiagnose transesterification from unchanged Mn, rising dispersity and odd-numbered chains in a lactide polymerization held at high temperature.

Interprete as evidências 1

  • Degree of polymerization from ¹H NMR end groupsInterpret ¹H NMR integrals of benzyl-initiated poly(ε-caprolactone) to obtain the number of repeat units per chain.

Trabalhe de trás para frente 2

Ring strain and polymerizability

  • Monomer for poly(ethylene glycol)Match the repeat unit of poly(ethylene glycol) to its cyclic ether monomer.
  • Monomer for poly(trimethylene carbonate)Identify the cyclic monomer that gives a polycarbonate by ring opening without loss of CO₂.

Planeje uma síntese 4

Living control and end groups

  • Choose an ATRP initiator for MMAChoose an alkyl halide that initiates the ATRP of methyl methacrylate efficiently.
  • Plan a PEG–PLA block copolymerPlan the synthesis of a poly(ethylene glycol)-block-poly(L-lactide) copolymer using a macroinitiator.

ROMP and coordination polymerization

  • A catalyst for a functionalized norborneneChoose a metathesis catalyst whose functional-group tolerance suits a hydroxyl-bearing monomer under non-rigorous conditions.

Anionic and cationic ring opening

  • Choose an initiator for THFChoose an initiator that can polymerize tetrahydrofuran from the type of active chain end it needs.

Explique por quê 5

Living control and end groups

  • How ATRP controls a radical polymerizationExplain how the copper activation–deactivation equilibrium in ATRP gives controlled molar mass and low dispersity.

Ring strain and polymerizability

  • Why γ-butyrolactone does not polymerizeExplain the failure of γ-butyrolactone to polymerize from the balance between ring-strain enthalpy and the entropy lost on polymerization.

Anionic and cationic ring opening

  • A block copolymer that stops after one unitExplain why a lithium alkoxide chain end does not polymerize ethylene oxide and choose how to make the block copolymer.

ROMP and coordination polymerization

  • How a Grubbs catalyst opens norborneneExplain the metallacyclobutane mechanism of ROMP and the structure of the resulting polymer.

Lactones, lactide and catalysts

  • What the alcohol does in tin-catalysed lactide polymerizationExplain how the alcohol co-initiator sets the molar mass and end group in Sn(Oct)₂-catalysed lactide polymerization.

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