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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
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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
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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
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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
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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
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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
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