Transcript
Page 1: Polymerization Synthetic polymers used in dentistry

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PolymerizationSynthetic polymers used in

dentistry

Pavel Bradna

Institute of Dental Research

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Dental materials• Inorganic - gypsum products,

investment materials, cements,ceramics

• Metals – denture frameworks,supraconstructions, implants, amalgams,orthodontic brackets, wires

Organic (polymer-based) materials

(C) Institute of Medical Biochemistry, 1st Faculty of Medicine, Charles University, and Institute of Dental Research in Prague

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Typical applications of polymers indentistry

• Impression materials• Obturation materials (endodoncy)• Equipment (spatulas, measures, etc)

• Dentures (bases, artificial teeth,relining materials)

• Filling materials (composites, cements,adhesives)

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What is the lecture about?

• Terms and definitions• Structure of polymer chains• Polymers in dentistry• Polymerization• Methyl methacrylate polymers• Typical dimethacrylate resins

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Definition

Polymer – a chemical compound consistingof giant molecules „MACROMOLECULES“formed by union of many „POLY“ smallrepeating units „mers“

MONO“MER“ molecules POLY“MER“ chain, macromolecule

polymerization covalent bond

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1. Chemical composition of polymers

type of monomer, its structure

What controls polymer properties?

CH2=CH

RVinyl polymers

R = H : polyethylene hydrophobic,semicrystalline polymers

R = OH : poly(vinyl alcohol), hydrophilic watersoluble polymer with gelling properties

Examples:

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• Linear polymers A-A-A-A-A-A-A-

• Nonlinear (branched) polymers

Branches

Polymer (linear)backbone

2. Topology of polymer chains

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A-B-B-A- A-B-A-B-B-A-A-B- A-B-A-B-B-A-A-B-

A-B-A-B-B-A-

A-B-B-A- A-B-A-B-B-A-

• Cross-linked polymer (polymer networks)

Cross-links (permanentconnections between

chains restricting motionof chains rigidity)

Temporary cross-linksentanglements

A-A-B-A-B-

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• Homopolymers (one type of monomer - A) A-A-A-A-A-A-A- linear/branched• Copolymers (2-3 comonomers) A-B-A-B-B-B-A- statistical (random)

A-A-A-B-B-B-B-A-A- block A-B-A-B-A-B-A- alternating -A-A-A-A-A- graft/branched -B-B-B-B-B

3. Monomer distribution in copolymer chains

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Simple compounds –composed of identicalmolecules

4. Polymer molecular weight

Molecules of polymer –strongly differ (vary by manymonomer units)

Polymers are characterized bymolecular weight/polymerization

degree average

-numberaverage

-weightaverage

(C) Institute of Medical Biochemistry, 1st Faculty of Medicine, Charles University, and Institute of Dental Research in Prague

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Number average, Mn

Mn= ΣMi ni/Σni=ΣMixi

Definitions of molecular weight averages

Mw= ΣMi (ni Mi)/Σni Mi =ΣMi (mi/mtotal)=ΣMi wi

xi states for molefraction of molecules with Mi

Weight average, Mw

Polymerization degree

wi states for weight fraction of molecules with Mi

Pw= Mw/Mo

Mo states for molecular weight of repeating unit

Polydispersity Mw/ Mn

-numberaverage

-weightaverage

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5. Supramolecular structure (molecular organization)

• Amorphous polymers – coiled irregular(random) shape of polymer chains

• Semicrystalline polymers – domains withregular (crystalline) structures acting asspecial type of cross-links

Amorphous domains

Crystallites

(C) Institute of Medical Biochemistry, 1st Faculty of Medicine, Charles University, and Institute of Dental Research in Prague

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C

C

C

C

C

C

C

C

Van-der-Waalsinteractions

NH

(CH2)5

C O

NH

NH

C O

C O

C O

NH

(CH2)5

H-bridges

Typical for polyamides, polyethylene, polypropylene etc.

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Influence of cross-links on physicalproperties of polymers

Cross-linked polymers - they cannot be reshapedon heating, do not melt, but decompose onheating, insoluble in organic liquids(thermosetting polymers).

Linear or branched polymers - flow when heated, canbe easily reshaped upon heating - thermoplasticpolymers(usually soluble in organic solvents)

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Glass transition (softening temperature)Temperature Tg

Rig

idity

Temperature at which rigidity of polymer decreases

Temperature

Glass

LiquidPlastic

Range of Tg

Linear

Low degree ofcross-linking

High degree ofcross-linking

Rigid

Rubberystate

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Polymers used in dentistryNatural polymers

- polysaccharides (agar and alginates) - poly(isoprene)

natural rubberCis-poly(1,4-isoprene) - (impression materials)

guttaperchaTrans-poly(1,4-isoprene) - (obturation points)2-methyl-1,3-butadiene

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•Step-growth polymerization of bifunctional monomers, frequently with releaseof low molecular compounds (analogous reactionto low molecular weight compounds)

Polymers used in dentistrySynthetic polymers

Prepared via polymerization reactions:

•Addition (chain-growth) polymerizationof monomers with a double bond

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Addition polymerization

• Starts from an active center (only thesemolecules are capable to react)

• π-bond of monomer is converted to σ-bondin the polymer

• Monomers add sequentially to the end of agrowing chain

• Is very fast and exothermic

• Produces high molecular weight polymers

Characteristics

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Types of addition polymerizations:

• Free-radical polymerization – active centeris a radical (contains unpaired electron) andthe propagating site of reactivity is a carbonradical.

• Cationic polymerization - the active center isan acid, and the propagating site of reactivityis positively charged

• Anionic polymerization - the active center isa nucleophile, and the propagating site ofreactivity is negatively charged

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Few common polymers prepared via addition polymerization

Name(s) Formula Monomer Properties

Polyethylene low density (LDPE) –(CH2-CH2)n– ethylene

CH2=CH2 soft, waxy solid

Polyethylene high density (HDPE) –(CH2-CH2)n– ethylene

CH2=CH2 rigid, translucent solid

Polypropylene (PP) different grades –[CH2-CH(CH3)]n–

propylene CH2=CHCH3

atactic: soft, elastic solid isotactic: hard, strong solid

Poly(vinyl chloride) (PVC) –(CH2-CHCl)n– vinyl chloride

CH2=CHCl strong rigid solid

Polystyrene (PS) –[CH2-CH(C6H5)]n–

styrene CH2=CHC6H5

hard, rigid, clear solid soluble in organic solvents

Polytetrafluoroethylene (PTFE, Teflon) –(CF2-CF2)n– tetrafluoroethylene

CF2=CF2 resistant, smooth solid

Poly(methyl methacrylate) (PMMA, Lucite, Plexiglas)

–[CH2-C(CH3)CO2CH3]n–

methyl methacrylateCH2=C(CH3)CO2CH3

hard, transparent solid

cis-Polyisoprene natural rubber

–[CH2-CH=C(CH3)-CH2]n–

isoprene CH2=CH-C(CH3)=CH2

soft, sticky solid

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Step-growth polymerization

• Proceeds by conventional functional group reactions (condensation, addition)

• Needs at least 2 functional groups per reactant

• Any monomer molecule has the „same“ probabilityto react

• After an elementary reaction – ability to grow remains

• Combines two different reactants in analternating structure

Characteristics

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• Polymers are formed more slowly than by additionpolymerization

• Polymers are generally of lower molecular weight

Types of step-growth polymerization

• Polycondensation

• Polyaddition

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Formula Type Components Tm ºCpolyester

MylarPhthalic acids Instruments, matrices

polycarbonate (Bisphenol A)phosgene

Artificial teeth,Veneering shells,

Prefabricated crowns,orthodontic brackets

~[CO(CH2)4CO-NH(CH2)6NH]n~

polyamideNylon 66

HO2C-(CH2)4-CO2H

H2N-(CH2)6-NH2

Surgical suture materials

~[CO(CH2)5NH]n~polyamide

Nylon 6Perlon

Surgical suture materials

polyamideKevlar

para HO2C-C6H4-CO2H

para H2N-C6H4-NH2

Fibre reinforced splints

Toluenediisocyanate

HOCH2CH2OH Die materials

Few common polymers

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A. Stages of free radical polymerization

1. Initiation/induction – process starts- Primary radical formation

In. + In. (activation)

In. + M

– Addition of primary radical on the double bond of monomer

T, hν

In-M.

In - InInitiator Free radicals

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2. Propagation - the addition of monomerto an active center (free radical) to generatea new active center

M In-M-M.

+

In-M.

In-M-M.

yM

+

In-M-M-M.

In-M-M-M. + In-M-M-M-.. My.

M

3. Termination – the growing chain is stopped- Radical coupling/recombination (most common)

In-Mx. + .My-In In-Mx-My-In

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4. Chain transfer – An atom is transferred to the growing chain, terminating the chain growth and starting a new chain.

1. Chain termination -M-M-M. + XB -M-M-M-X + B.

2. New chain growth startsB. M+ BM.

Chain transfer agent

Polymerization is stoppedRetarder/InhibitorMol. weight regulationChain transfer agentBranchingPolymerMol. weight decreaseMonomer

ConsequencesXB

Chain transfer radical

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An inhibitor is a „retarder“ that reacts rapidly withactive radicals to form stable compounds (BX.)unable of addition to monomer completelystops or “inhibits” polymerization.

A retarder is a substance that „slowly“ reacts witha radical to form products nearly incapable ofreacting with monomer.

B. Anionic polymerization

Initiated with e.g. n-butyllithium, alkaline metalsUsed in superglues for methyl 2-cyanoacrylate

polymerization

H2C=CH(CN)+X- X-H2C-CH(CN)-phenyl, cyano or carbonyl

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C. Cationic polymerization

H2C=CHR+H+ H3C-CHR+R - alkyl, phenyl or vinyl

H+ – strong acids

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Step-growth polymerization

Polycondensation

nH2N(CH2)6NH2

+ Si(OCH3) 4

nHOOC(CH2)4COOH (-HN(CH2)6NH-CO(CH2)4CO-)n

-2n H2O

Hexamethylene diamine Adipic acid Polyamide 66

Water release from the reaction mixture:1. controls mol. weight, 2. contributes tocontraction of the system

1. Setting reaction of C-silicone impression materials

Si

O

…..O

O

OH

Si

…..O

OHO…

+

…..O

SiO

Si

HO

OHO

…..O

O

SiO

Si

O

O O

SiO

SiO

…..O

…..O

O

Si

O

O

O

…..O

…..O

… …… …

-4CH3OH

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Diglycidyl ether bisphenol A

Polyaddition

Diamine

2. Setting reaction of epoxies

O=C=N-R1-N=C=O HO-R2-OH+

1. Production of polyurethanesO=C=N-R1-N-C-O-R2-OH

H

O

Alkyl, aryl diisocyanate Diol

Root canal sealers, die materials

polyurethane

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Vinyl terminated polysiloxane prepolymer

Methylhydro-dimethylsiloxane – a cross-linking agent

Addition of -H on vinyl double bond

2. Setting reaction of A-silicone impression materials

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• Suitable manipulation/processing properties(easy to mix, shapable, simple to process andcure)

• Good mechanical properties (rigidity, strength,wear resistance)

• Biocompatible (tasteless, odourless, non-toxicor non-irritating, resistance to microbialcolonization)

Methyl methacrylate (MMA) polymersMost frequently used group of polymers in dentistry

Why? Because these materials can be easilyadopted to individual purposes (fillings, prostheses)

and can be in contact with human body

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• Aesthetic properties translucencyand transparency (colour and opticalproperties of tooth tissues)

• Chemical resistance in oralenvironment, to disinfectants etc.

• Acceptable cost of both material andprocessing method

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• Melting point -48oC• Boiling point 100.3oC• Density 0.945 g/mL• Heat of polymerization 54.3 kJ/mol

(!! exothermic reaction !!)• Colourless liquid• Immiscible with water but miscible with

organic solvents• Irritant• !! Flammable !!

Properties of MMA

=C

O

CH3

CH3

C=O

CH2

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Properties of poly(methyl methacrylate)

• Transparency and high clarity

• Low absorption of visible and UV light (to 250 nm)

• Density 1.19 g/cm3 (causes polymerizationcontraction/shrinkage app. 22 vol %)

• Compression strength 90 – 100 MPa

• Very rigid - elastic modulus above 2.4 GPa

• Water sorption up to app. 1.0 wt %

• Temperature resistance Tg = 120 – 125oC

• Soluble in organic solvents (MMA, acetone, toluene, etc.)

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Methyl methacrylate polymers in dentistry• First applications for dentures (thermoplastic

compression molding) of molten PMMA powder(too expensive and complicated)

• 1936 – Kulzer: powder/liquid systemPowder – PMMA prepolymer with residuals of DBPLiquid – MMA monomer and additives

- When mixed workable plastic mass(dough) is formed and polymerized inplaster individually fabricated mold– Decreased polymerisation shrinkage

(app. 6 vol %)

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PMMA prepolymer beadsPrepared by suspension polymerization

Average particle size 0.005-0.100 mmAcrylics and Other Synthetic Resins Used in Dentistry

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• Initiation– Decomposition of dibenzoyl peroxide (DBP)

during heating

O Oheat

2

=C

O

CH3

CH3

C=O

CH2 -C

O

CH3

CH3

C=O

CH2.

.O

O

+.O

O

OO

Polymerization scheme of MMAmonomers

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C = concentration of peroxidek = kinetic constantt = time

Thermal decomposition of peroxides

Effect of temperature

The temperature dependence of kinetic constant is given by an Arhenius law :

with : k(T) = kinetic constant at temperature T (K)Ea = Activation energyR = Gas constantT = Temperature in K

The higher T – the higher k and higher conc. of radicals

T=const.

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C=O

C

O

CH3

CH3.

•Chain termination

+-C

O

CH3

CH3

C=O

CH2 . =C

O

CH3

CH3

C=O

CH2n( ) -C

O

CH3

CH3

C=O

CH2 - C

O

CH3

CH3

C=O

CH2( )n.

•Propagation

+

CH3

CH3

C=O

C

O

.

CH3

CH3

C=O

C

O

C=O

-C

O

H2C

CH3

CH3

OO- O

CH2-R2

O

R1-H2C- R1- CH2-R2

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+-C

O

CH3

CH3

C=O

CH2 . =C

O

CH3

CH3

C=O

CH2 =C

O

CH3

CH3

C=O

CH2 - C

O

CH3

CH3

C=O

CH3.

•Chain transfer to MMA monomer

-MX – CH2-CH2. + HO- -OH -MX – CH2-CH3 +HO- -O.

Hydroquinone (HQ) Stable hydroquinoneradical will not

propagate/stop otherchain radical

HO OCH3Methoxy phenol (hydroquinone

monomethyl ether) MEHQ

R R- +

•Chain transfer to phenolic inhibitors

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HO

C(CH3)3

C(CH3)3

CH32,6-di-tert-butyl-4-methylphenol

(Butylated hydroxytoluene - BHT)

Sterically hindered phenols – less efficient but reducedcolour change after polymerization- Synergic mixtures HQ + MEHQ

Oxygen inhibition

MxI + O2 MxIOOH.

Oxygen-inhibited layer on thecomposite/adhesive surfaces

Stable hydroperoxide

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1. To extend monomer shelf life by restricting spontaneous polymerization2. Decrease sensitivity to ambient light3. To prolonge working time

Why polymer inhibitors are added?Usually 0.01 – 0.005 wt %

Note:4-Allyl-2-metoxyfenol

Eugenol = phenol(inhibits polymerisation)

Negative interaction with Zinc oxide-eugenol materials

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Modifications of PMMA for dentalapplications

Cross-linking- improves hardness and stiffness (increases mol. weight)- increases crazing resistance (small cracks originatingat the teeth-denture margin), wear and solvent resistance,but increases brittleness- increases thermal resistance (polymers are easily finished - grinded and polished without melting)

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=C

O

CH3

CH3

C=O

CH2 =C

O

CH3

CH2

C=O

CH2

CH3

CH3

=C

O

C=O

CH2

+ =C

O

CH3

CH3

C=O

CH2+ -C

O

CH3

CH3

C=O

-CH2 -C

O

CH3

CH2

C=O

-CH2

CH3

CH3

-C

O

C=O

-CH2

-C-

O

CH3

CH3

C=O

-CH2

-C-

O

CH3

CH3

C=O

-CH2-C

O

CH3

CH3

C=O

-CH2

Ethylene glycol dimethacrylate (EGDMA)

Example: MMA cross-linked with ethyleneglycol dimethacrylate (EGDMA)

MMACross-link

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• copolymerization (with acrylic andmethacrylic monomers, PVC, PVAc,butadiene) disturbs regular intermolecularorder of a homopolymer

– decreases softening temperature- improves fatique and impact resistance- increase dissolution rate in MMA

• blending of various MMA polymers – increase rate of dissolution in MMA - decreases softening temperature

=CH

O

C=O

CH2

CH3

Methylacrylate

(C) Institute of Medical Biochemistry, 1st Faculty of Medicine, Charles University, and Institute of Dental Research in Prague

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33n-butyl38n-propyl

65ethyl

125methyl

Polymer Tg oCMethacrylate

rigid

Soft, fasterdissolution

Glass transition temperature (softening) ofvarious methacrylic polymers

=C

O

CH3

CH3 (ethyl,

propyl, butyl)

C=O

CH2

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plasticizing – high-boiling compounds swellingthe polymer (phthalates – dibutyl/dioctylphthalates) - to reduce stiffness, hardness and softening point - to prepare flexible polymers (acrylic relining materials – combinations of MMAcopolymers with plasticizers)

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Classification of polymers/resinsaccording to initiation reactions

(activation)– Heat activated (heat cured/heat curing resins) (two component systems)- Chemically activated (self cured/curing, cold

curing, autopolymerizing, fast curing) resins(two component systems)

- Light activated (LC/UV cured) resins (one component system)

NoteDual-cure (a combination of chemically and light activated materials, LC

modified GIC, luting materials)Tri-cure (a combination of acid-base setting reaction, LC and chemical

activation [peroxide/amine] activating system)

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Heat activated resins(denture base resins, resins for artificial acrylic

teeth, crown and bridge polymers)

liquid: MMA, cross-linking agent (app. 1- 6 wt %),inhibitors, additives (plasticizers), regulatorsVolume mixing ratio (powder/liquid): 3-2.5/1.0

Composition: powder: PMMA prepolymer with dibenzoyl peroxide (up to 0.5 - 0.6 wt %)

Regulators – decrease the rateof polymerization heat release

Enable fast curing process

1-methyl-4-(1-methylethenyl)cyclohexene)

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Temperature changes in PMMA duringdenture polymerisation in hot water

MMA boilingpoint

• Too fast

• Standard polymerizationStarts at 60oC

• Directly to 100oC

• Heated to 100oC

MMA vapours release porosity in a denture

Lower temperature decreased riskof porozity

MMA boilingpoint

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Typical porosity of heatactivated denture base resin

Properlypolymerized Too fast heating

Phillips‘ Science of dental materials

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Chemically activated resins

Composition: powder: PMMA prepolymer or copolymer (fine particle

size), dibenzoyl peroxideliquid: MMA, cross-linking agents, inhibitors,accelerators (1-4 wt %), UV absorbers

1. Tertiary aromatic amine

(denture reparations, relining, orthodonticappliances, pouring resins)

2. Barbituric acid derivatives combined with aliphaticamine (lower colour change) and Cu cations

3. Sodium p-toluene-sulfinate (for systemscontaining methacrylic acid), Cu cations

Accelerators

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H3C

Free radical

-N

CH3

CH3

OO

+ -N.

CH3

CH3

+ .+ O -

N,N-dimethyl-p-toluidine (DMPT); N,N-di(hydroxyethyl)-p-toluidine

Compared to heat activated resins:•Lower molecular weight•Higher amount of free monomer 3-5 wt %, heat activated app. up to 1 %•Less colour stability due to oxidation of aromaticamine accelerators

Scheme of DBP decomposition accelerated by tert. amine

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Composition (usually one component):

1. Dimethacrylate resin, light initiating systemcamphorquinone (CQ) - amine, inhibitors

Light activated resins(light cured composite materials, adhesives,

light cured glass-ionomer cements)

2. Filler particles

CQ

Lightactivation

H

H

amine

...

Free radicals

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• BIS-GMA 2,2-bis[4-(2hydroxy-3-metakryloyloxypropoxy) fenyl]propane

(Bowen monomer, 1955)

Typical dimethacrylate resins

higher molecular weight – less polymerizationshrinkage

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Meth. Meth.

Capable of H-bridge formation

high viscosity

H CH3H CH

3

H

H

H

H

H

H

H

H

Lesspolymerizationshrinkage app.

5-6 vol %

Hindered rotation of C-C bond– rigid part of a molecule

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3. Triethylene glycoldimethacrylate TEGDMA (low viscositydiluent)

4. 1,6 hexane dioldimethacrylate

2. Urethane dimethacrylate (UDMA) (2,2,4-trimethylhexamethyle-bis-(2-carbamoyl-oxyethyl)dimethacrylate)

Also of high viscosity

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5. Ethoxylated bis-GMA

n = 1 - 5

Low viscositymonomer

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Other polymers - alternatives toacrylic denture base polymers

~[CO(CH2)4CO-NH(CH2)6NH]n~HO2C-(CH2)4-CO2H + H2N-(CH2)6-NH2 -

adipic acid hexamethylene diamine polyamideNylon 66

Poly(formaldehyde) acetic acid polyacetal

Thermoplastic semicrystalline polymers/injectionmolding at high temperatures

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2-hydroxyethyl methacrylate

N-phenylglycine

4-acryloxyethyl trimelitic acid anhydride

4-methacryloxyethyl trimelitic acidanhydride/acid

Few monomers used in dentaladhesives

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2-methacryloxyethyl phenyl hydrogenphosphate

N-phenylglycine glycidyldimethacrylate

Glycerophosphoric acid dimethycrylate

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Dipentaerythritol pentaacrylate monophosphate

10-methacryoloyloxydecyldihydrogenphosphate

Polyacrylic acid with pendantmethacrylate groups

Itaconic acid

Maleic acid

Polymers and monomers for GIC

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Standards on synthetic polymersand resinous materials

• EN ISO 4049: Dentistry – Polymer-filling, restorative andluting materials

ČSN EN ISO 4049: Stomatologie – Výplňové, rekonstrukční a fixačnípolymerní materiály

• EN ISO 10477: Dentistry – Polymer-based crown and bridgematerials

ČSN EN ISO 10477: Stomatologie – Korunkové a můstkové pryskyřičnémateriály

• EN ISO 1567: Dentistry – Denture base polymers ČSN EN ISO 1567: Stomatologie – Bazální protézové pryskyřice• EN ISO 6874: Dental resin-based pit and fissure sealants ČSN EN ISO 6874: Dentální pryskyřičné materiály pro pečetění jamek a

fisur

(C) Institute of Medical Biochemistry, 1st Faculty of Medicine, Charles University, and Institute of Dental Research in Prague

Page 33: Polymerization Synthetic polymers used in dentistry

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Summary

1. Polymers, characterization, properties (molecular structure)2. Polymerization – addition (free radical, ionic); step-growth-

polycondenzation and polyaddition3. Phases of free radical polymerization4. Properties of MMA, polymerization of PMMA5. Types of activation reactions, inhibitors6. Characteristics of heat, chemical and light activation

systems7. Dimethacrylate resins used in composite materials8. Other monomers and polymers

(C) Institute of Medical Biochemistry, 1st Faculty of Medicine, Charles University, and Institute of Dental Research in Prague


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