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Chapter 19. Synthetic Polymers

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1. Characteristics of Polymers

2. Polymer Synthesis

3. Non-degradable Polymers

4. Degradable Polymers



1. Characteristics of Polymers

⑴ Degree of Polymerization (DP)

① Definition: The number of monomer units in a polymer chain

⑵ Average Molecular Weight

① Number-average molecular weight (Mn): Related to freezing point depression, boiling point elevation, vapor pressure lowering, osmotic pressure, etc.


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② Weight-average molecular weight (Mw): Related to viscosity, tensile strength, light scattering, etc.


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③ Z-average molecular weight (Mz): Related to melt elasticity, centrifugation, etc.

④ Viscosity-average molecular weight (Mv): Related to viscosity, etc.

○ Mark-Howink equation


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○ For PLA solutions dissolved in chloroform (25°C): K = 5.45 × 10-4, α = 0.73

⑤ Mz > Mw > Mv > Mn

Polydispersity Index (PDI): Polydispersity index in the general sense

○ Definition: PDI = Mw ÷ Mn. PDI = 1 for monodisperse polymers, PDI > 1 for polydisperse polymers

○ Polymer systems have various molecular weights due to different numbers of monomers, leading to PDI > 1

○ Molecular weight distribution graph types: narrow distribution, broad distribution, bimodal distribution (2 peaks)

○ General PDI can be determined using techniques like GFC, HPLC, GC

⑦ Meaning of PDI in DLS, Zetasizer


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○ σ: The Standard Deviation of the Particle Size Distribution

○ d: The Average Hydrodynamic Particle Size

○ If 0 < PDI < 0.2, the dispersion of the nanoparticles is very good.

○ If highly polydisperse, PDI can exceed 1

○ Zetasizer sets PDI between 0 and 1

⑶ Crystallinity

① Factors: Molecular structure, functional groups, processing temperature, cooling rate

② Measurement methods: X-ray diffraction, infrared spectroscopy, differential scanning calorimetry (DSC) which measures melting heat change

③ Partial Crystallinity

○ Polymers have a mixture of ordered crystalline and disordered amorphous structures

Factor 1: Incomplete crystallinity due to numerous repeating units

Factor 2: High branched structures hinder crystallinity

⑷ Stereoregularity

① Random copolymers (e.g., PLGA): Arrangement like ABBAABBBABABAAAABBB

② Alternating copolymers: Arrangement like ABABABABABABABABABA

③ Block copolymers: Arrangement like AAAAAAAAABBBBBBBBBB

④ Graft copolymers: Arrangement like AAAAAAAAAAAAAAAAAAA

⑸ Glass Transition Temperature (Tg)

① Definition: Temperature at which a polymer solid becomes rubbery

② Cause: Transition temperature for irregular motion of main chain. Only present in amorphous polymers

③ T < Tg: Rigid, glass-like state

④ T > Tg: Rubber-like state



2. Polymer Synthesis

Type 1. Radical Addition Polymerization

① Reaction under catalysts like radical initiators (e.g., benzoyl peroxide) or acids/bases

Example 1: Polyethylene synthesis


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Figure 1. Polyethylene synthesis


Example 2: Polyisoprene synthesis (e.g., natural rubber)


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Figure 2. Polyisoprene synthesis


Example 3: Polystyrene (PS) synthesis

○ Polystyrene has preferred direction in polymerization for maximizing the number of resonance contributors


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Figure 3. Polystyrene synthesis


Example 4: Polymerization of acetone


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Figure 4. Polymerization of acetone


Example 5: Polyethylene oxide synthesis using epoxide ring-opening reaction


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Figure 5. Polyethylene oxide synthesis


Example 6: Polyvinylchloride (PVC) synthesis (CH2CHCl)n

Example 7: Polyacetic acid vinyl synthesis (CH2CHCOOCH3)n

Example 8: Nylon synthesis (first synthetic fiber)


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Figure 6. Nylon 6 synthesis


○ Step A: Ketone Amination Reaction. Faster under weak acid conditions compared to weak alkaline conditions

○ Step B: Beckmann Rearrangement. Ring expansion reaction through rearrangement

○ Step C: Ring-opening polymerization. Initiated by radicals, acids, or bases

Type 2. Dehydration Condensation Polymerization

① Capable of hydrolysis reactions

Example 1: Acetaldehyde polymerization


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Figure 7. Acetaldehyde polymerization


Example 2: Polymerization via acetal formation reaction


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Figure 8. Polymerization via acetal formation reaction


Example 3: Polyester synthesis


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Figure 9. Polyester synthesis


○ glycolic acid → poly(glycolic acid)

○ lactic acid → poly(lactic acid)

○ 3-hydroxybutyric acid → poly(hydroxybutyrate)

Example 4: Polylactide (PLA) synthesis


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Figure 10. Polylactide synthesis


Example 5: Polymerization of amides


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Figure 11. Polymerization of amides


Example 6: Polymerization of ethylene glycol and cyanate


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Figure 12. Polymerization of ethylene glycol and cyanate



3. Non-degradable Polymers

⑴ Polyethylene (PE)

① Low-density polyethylene: Weak properties. Vulnerable to high temperatures

② High-density polyethylene: Used for tubes and catheters

⑵ Polypropylene (PP)

① Has a linear structure

② Good repeated bending properties

③ Resistant to external shocks

④ Applications: Finger joint replacements, disposable syringe body

⑶ Polyamide (Nylon)

① Strong inter-chain hydrogen bonding and high crystallinity → Excellent fiber forming ability

② High absorbency → Water acts as a plasticizer → Degradation of properties due to water and protein enzymes

③ Applications: Surgical sutures

⑷ Polymethyl Methacrylate (PMMA)

① High light transmittance (92%)

② High refractive index (1.49)

③ Large in size

④ High strength and hardness

⑤ Applications: Hard contact lenses, artificial intraocular lenses, dentures, facial prosthetic materials, bone cement

⑸ Polytetrafluoroethylene (Teflon, PTFE)

① A polymer material with carbon-fluorine bonding: C-F bond is strong. Doesn’t decompose even when heated

② High molecular weight, high crystallinity (> 94%), hydrophobic, high density, low coefficient of friction, high heat resistance, stability, porosity

③ Applications: Coating material for frying pans, small diameter artificial blood vessels, catheters

⑹ Polyvinyl Chloride (PVC)

① Hard but flexible with plasticizers

② Long-term plasticizer leaching, toxicity

③ Applications: Tubes used for blood transfusion and dialysis, blood bags

⑺ Polyurethane (PU)

① Stable in the body, blood-compatible, strong, elastic, tough

② Applications: Artificial blood vessels, artificial heart valves, catheters, burn dressings

⑻ Polycarbonate

① Very hard and lightweight

② Excellent mechanical and thermal properties

③ Applications: Heart assist devices, lung assist devices, body of dialysis machines, heart valve actuators

⑼ Polyacetal

① Very hard and lightweight

② Formed by the polymerization of formaldehyde

③ Applications: Joint connection part of artificial hip joints, heart valve actuators

⑽ Polysulfone

① Very hard and lightweight

② Applications: Porous coating material for metallic orthopedic implants, oxygen barrier film

⑾ Hydrogel

① A polymer that swells (30% or more) in water

② Formed by cross-linking of hydrophilic polymers

③ Hydrogel formed by physical cross-linking: Freely undergoes sol-gel transition

④ Hydrogel formed by chemical cross-linking: Once it becomes a gel, cannot transition back to sol

⑤ Representative example: Polyhydroxyethylmethacrylate (PHEMA, polyhydroxyethylmethacrylate)

○ Water absorption is similar to living human tissue

○ Biologically inert, heat resistant, stable, excellent refractive index, high oxygen permeability

○ Applications: Contact lenses



4. Degradable Polymers

⑴ PLA(poly lactic acid): Aliphatic polyester


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Figure 13. Structure of PLA


① Crystallinity: L-PLA is 1.25-1.29 g/㎠

② Melting point: L-PLA is 159 ~ 178 ℃

③ Glass transition temperature

○ L-PLA is 54 ~ 59 ℃, D-PLA is 50 ~ 53 ℃

○ As molecular weight increases, glass transition temperature increases ( because solidification due to increased intermolecular force)

④ Lifespan: 18 ~ 24 months

⑵ PGA(poly(glycolic acid)): Aliphatic polyester


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Figure 14. Structure of PGA


① Molecular weight: 20 ~ 145 kg/mol

② Crystallinity: 1.5 ~ 1.64 g/㎠

③ Melting point: 210 ~ 226 ℃

④ Glass transition temperature: 36 ℃. Semi-crystalline

⑤ Has OH group at α position

⑶ PLGA(poly(lactic-co-glycolic acid))


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Figure 15. Structure of PLGA


① Definition: Copolymer of PLA and PGA

② Comparison between PLGA and PLA


Type PLGA Copolymer PLA Homopolymer
Degradation period Short Long
Structure Amorphous Semi-crystalline
Molecular weight Low High

Table 1. Comparison between PLGA and PLA


③ Physical properties depending on PLGA’s crystallinity


Biodegradable Component Glass Transition Temp. (Tg) (°C) Melting Temp. (Tm) (°C) Tensile Strength (MPa) Elastic Modulus (MPa) Flexural Modulus (MPa) Strain at Yield (%) Strain at Break (%)
Poly(glycolic acid) (MW: 50,000) 35 210 n/a n/a n/a n/a n/a
L-PLA (MW: 50,000) 54 170 28 1200 1400 3.7 6
L-PLA (MW: 300,000) 59 178 48 3000 3250 1.8 2.2
D,L-PLA (MW: 20,000) 50 - n/a n/a n/a n/a n/a
D,L-PLA (MW: 550,000) 53 - 35 2400 2350 3.5 5
PLGA 85:15 50 ~ 55 - - - - - -
PLGA 75:25 50 ~ 55 - - - - - -
PLGA 65:35 45 ~ 50 - - - - - -
PLGA 50:50 45 ~ 50 - - - - - -

Table 2. Physical and mechanical properties of various biodegradable polymers


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Figure 16. Relationship between PGA content and crystallinity


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Figure 17. Relationship between PGA content and melting point

Amorphous PLGA has no melting point


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Figure 18. Change in half-life depending on PLGA’s crystallinity


○ Lifespan comparison: PLA > PGA > PLGA

④ Decomposition time of PLGA

○ Decomposes into lactic acid and glycolic acid by non-specific hydrolysis

○ The closer the weight ratio of PLA and PGA is to 1:1, the shorter the lifespan


Polymer Abbreviation Degradation Time (months)
poly(lactic acid) PLA 18 ~ 24
poly(D,L-lactic acid) PDLA 12 ~ 16
poly(glycolic acid) PGA 2 ~ 4
poly(D,L-lactic-co-glycolic acid, 50:50) PLGA 2
poly(D,L-lactic-co-glycolic acid, 70:30) PLGA 6
poly(D,L-lactic-co-glycolic acid, 85:15) PLGA 10

Table 3. Decomposition period of PLGA depending on composition


Ability to control lifespan is an advantage of PLGA

○ Most protein enzymes do not participate in decomposition

○ Exception: Decomposition of PLGA is accelerated in vitro experiments by microbial-derived proteinase K or lipase

⑤ Decomposition process of PLGA


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Figure 19. Change in device weight and PLGA molecular weight over time


○ 1st. Hydrolysis of ester: Water molecules attack the ester bond in PLGA

○ 2nd. As the polymer breaks, the carboxyl terminus is exposed and molecular weight continues to decrease

○ 3rd. Decomposition rate increases over time: Carboxyl end groups act as a catalyst for ester hydrolysis ( autocatalysis )

○ 4th. Even if the polymer chain breaks, it remains insoluble at high molecular weight and does not diffuse into the solution

○ 5th. After approaching the critical molecular weight (1,000 ~ 1,100), it dissolves → The device weight starts to decrease

○ 6th. As the carboxyl terminus dissolves, pH decreases

⑷ Other biodegradable polymers

① Polycaprolactone(PCL)

② Polyanhydride

③ Polyortho ester

④ Polyamino acid

⑤ Polyhydroxybutyrate(PHB)

⑥ Polyhydroxyvalerate(PHV)

⑦ Polyphosphazene: An inorganic polymer where the main chain is made of nitrogen and phosphorus



Edited: 2020.03.06 18:32

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